I/O in Trio
The abstract Stream API
Trio provides a set of abstract base classes that define a standard interface for unidirectional and bidirectional byte streams.
Why is this useful? Because it lets you write generic protocol implementations that can work over arbitrary transports, and easily create complex transport configurations. Here’s some examples:
trio.SocketStream
wraps a raw socket (like a TCP connection over the network), and converts it to the standard stream interface.trio.SSLStream
is a “stream adapter” that can take any object that implements thetrio.abc.Stream
interface, and convert it into an encrypted stream. In Trio the standard way to speak SSL over the network is to wrap anSSLStream
around aSocketStream
.If you spawn a subprocess, you can get a
SendStream
that lets you write to its stdin, and aReceiveStream
that lets you read from its stdout. If for some reason you wanted to speak SSL to a subprocess, you could use aStapledStream
to combine its stdin/stdout into a single bidirectionalStream
, and then wrap that in anSSLStream
:ssl_context = ssl.create_default_context() ssl_context.check_hostname = False s = SSLStream(StapledStream(process.stdin, process.stdout), ssl_context)
It sometimes happens that you want to connect to an HTTPS server, but you have to go through a web proxy… and the proxy also uses HTTPS. So you end up having to do SSL-on-top-of-SSL. In Trio this is trivial – just wrap your first
SSLStream
in a secondSSLStream
:# Get a raw SocketStream connection to the proxy: s0 = await open_tcp_stream("proxy", 443) # Set up SSL connection to proxy: s1 = SSLStream(s0, proxy_ssl_context, server_hostname="proxy") # Request a connection to the website await s1.send_all(b"CONNECT website:443 / HTTP/1.0\r\n\r\n") await check_CONNECT_response(s1) # Set up SSL connection to the real website. Notice that s1 is # already an SSLStream object, and here we're wrapping a second # SSLStream object around it. s2 = SSLStream(s1, website_ssl_context, server_hostname="website") # Make our request await s2.send_all(b"GET /index.html HTTP/1.0\r\n\r\n") ...
The
trio.testing
module provides a set of flexible in-memory stream object implementations, so if you have a protocol implementation to test then you can start two tasks, set up a virtual “socket” connecting them, and then do things like inject random-but-repeatable delays into the connection.
Abstract base classes
Abstract base class |
Inherits from… |
Adds these abstract methods… |
And these concrete methods. |
Example implementations |
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- class trio.abc.AsyncResource
A standard interface for resources that needs to be cleaned up, and where that cleanup may require blocking operations.
This class distinguishes between “graceful” closes, which may perform I/O and thus block, and a “forceful” close, which cannot. For example, cleanly shutting down a TLS-encrypted connection requires sending a “goodbye” message; but if a peer has become non-responsive, then sending this message might block forever, so we may want to just drop the connection instead. Therefore the
aclose()
method is unusual in that it should always close the connection (or at least make its best attempt) even if it fails; failure indicates a failure to achieve grace, not a failure to close the connection.Objects that implement this interface can be used as async context managers, i.e., you can write:
async with create_resource() as some_async_resource: ...
Entering the context manager is synchronous (not a checkpoint); exiting it calls
aclose()
. The default implementations of__aenter__
and__aexit__
should be adequate for all subclasses.- abstractmethod await aclose() None
Close this resource, possibly blocking.
IMPORTANT: This method may block in order to perform a “graceful” shutdown. But, if this fails, then it still must close any underlying resources before returning. An error from this method indicates a failure to achieve grace, not a failure to close the connection.
For example, suppose we call
aclose()
on a TLS-encrypted connection. This requires sending a “goodbye” message; but if the peer has become non-responsive, then our attempt to send this message might block forever, and eventually time out and be cancelled. In this case theaclose()
method onSSLStream
will immediately close the underlying transport stream usingtrio.aclose_forcefully()
before raisingCancelled
.If the resource is already closed, then this method should silently succeed.
Once this method completes, any other pending or future operations on this resource should generally raise
ClosedResourceError
, unless there’s a good reason to do otherwise.See also:
trio.aclose_forcefully()
.
- await trio.aclose_forcefully(resource: AsyncResource) None
Close an async resource or async generator immediately, without blocking to do any graceful cleanup.
AsyncResource
objects guarantee that if theiraclose()
method is cancelled, then they will still close the resource (albeit in a potentially ungraceful fashion).aclose_forcefully()
is a convenience function that exploits this behavior to let you force a resource to be closed without blocking: it works by callingawait resource.aclose()
and then cancelling it immediately.Most users won’t need this, but it may be useful on cleanup paths where you can’t afford to block, or if you want to close a resource and don’t care about handling it gracefully. For example, if
SSLStream
encounters an error and cannot perform its own graceful close, then there’s no point in waiting to gracefully shut down the underlying transport either, so it callsawait aclose_forcefully(self.transport_stream)
.Note that this function is async, and that it acts as a checkpoint, but unlike most async functions it cannot block indefinitely (at least, assuming the underlying resource object is correctly implemented).
- class trio.abc.SendStream
Bases:
AsyncResource
A standard interface for sending data on a byte stream.
The underlying stream may be unidirectional, or bidirectional. If it’s bidirectional, then you probably want to also implement
ReceiveStream
, which makes your object aStream
.SendStream
objects also implement theAsyncResource
interface, so they can be closed by callingaclose()
or using anasync with
block.If you want to send Python objects rather than raw bytes, see
SendChannel
.- abstractmethod await send_all(data: bytes | bytearray | memoryview) None
Sends the given data through the stream, blocking if necessary.
- Parameters:
data (bytes, bytearray, or memoryview) – The data to send.
- Raises:
trio.BusyResourceError – if another task is already executing a
send_all()
,wait_send_all_might_not_block()
, orHalfCloseableStream.send_eof()
on this stream.trio.BrokenResourceError – if something has gone wrong, and the stream is broken.
trio.ClosedResourceError – if you previously closed this stream object, or if another task closes this stream object while
send_all()
is running.
Most low-level operations in Trio provide a guarantee: if they raise
trio.Cancelled
, this means that they had no effect, so the system remains in a known state. This is not true forsend_all()
. If this operation raisestrio.Cancelled
(or any other exception for that matter), then it may have sent some, all, or none of the requested data, and there is no way to know which.
- abstractmethod await wait_send_all_might_not_block() None
Block until it’s possible that
send_all()
might not block.This method may return early: it’s possible that after it returns,
send_all()
will still block. (In the worst case, if no better implementation is available, then it might always return immediately without blocking. It’s nice to do better than that when possible, though.)This method must not return late: if it’s possible for
send_all()
to complete without blocking, then it must return. When implementing it, err on the side of returning early.- Raises:
trio.BusyResourceError – if another task is already executing a
send_all()
,wait_send_all_might_not_block()
, orHalfCloseableStream.send_eof()
on this stream.trio.BrokenResourceError – if something has gone wrong, and the stream is broken.
trio.ClosedResourceError – if you previously closed this stream object, or if another task closes this stream object while
wait_send_all_might_not_block()
is running.
Note
This method is intended to aid in implementing protocols that want to delay choosing which data to send until the last moment. E.g., suppose you’re working on an implementation of a remote display server like VNC, and the network connection is currently backed up so that if you call
send_all()
now then it will sit for 0.5 seconds before actually sending anything. In this case it doesn’t make sense to take a screenshot, then wait 0.5 seconds, and then send it, because the screen will keep changing while you wait; it’s better to wait 0.5 seconds, then take the screenshot, and then send it, because this way the data you deliver will be more up-to-date. Usingwait_send_all_might_not_block()
makes it possible to implement the better strategy.If you use this method, you might also want to read up on
TCP_NOTSENT_LOWAT
.Further reading:
Prioritization Only Works When There’s Pending Data to Prioritize
WWDC 2015: Your App and Next Generation Networks: slides, video and transcript
- class trio.abc.ReceiveStream
Bases:
AsyncResource
A standard interface for receiving data on a byte stream.
The underlying stream may be unidirectional, or bidirectional. If it’s bidirectional, then you probably want to also implement
SendStream
, which makes your object aStream
.ReceiveStream
objects also implement theAsyncResource
interface, so they can be closed by callingaclose()
or using anasync with
block.If you want to receive Python objects rather than raw bytes, see
ReceiveChannel
.ReceiveStream
objects can be used inasync for
loops. Each iteration will produce an arbitrary sized chunk of bytes, like callingreceive_some
with no arguments. Every chunk will contain at least one byte, and the loop automatically exits when reaching end-of-file.- abstractmethod await receive_some(max_bytes: int | None = None) bytes | bytearray
Wait until there is data available on this stream, and then return some of it.
A return value of
b""
(an empty bytestring) indicates that the stream has reached end-of-file. Implementations should be careful that they returnb""
if, and only if, the stream has reached end-of-file!- Parameters:
max_bytes (int) – The maximum number of bytes to return. Must be greater than zero. Optional; if omitted, then the stream object is free to pick a reasonable default.
- Returns:
The data received.
- Return type:
- Raises:
trio.BusyResourceError – if two tasks attempt to call
receive_some()
on the same stream at the same time.trio.BrokenResourceError – if something has gone wrong, and the stream is broken.
trio.ClosedResourceError – if you previously closed this stream object, or if another task closes this stream object while
receive_some()
is running.
- class trio.abc.Stream
Bases:
SendStream
,ReceiveStream
A standard interface for interacting with bidirectional byte streams.
A
Stream
is an object that implements both theSendStream
andReceiveStream
interfaces.If implementing this interface, you should consider whether you can go one step further and implement
HalfCloseableStream
.
- class trio.abc.HalfCloseableStream
Bases:
Stream
This interface extends
Stream
to also allow closing the send part of the stream without closing the receive part.- abstractmethod await send_eof() None
Send an end-of-file indication on this stream, if possible.
The difference between
send_eof()
andaclose()
is thatsend_eof()
is a unidirectional end-of-file indication. After you call this method, you shouldn’t try sending any more data on this stream, and your remote peer should receive an end-of-file indication (eventually, after receiving all the data you sent before that). But, they may continue to send data to you, and you can continue to receive it by callingreceive_some()
. You can think of it as callingaclose()
on just theSendStream
“half” of the stream object (and in fact that’s literally howtrio.StapledStream
implements it).Examples:
On a socket, this corresponds to
shutdown(..., SHUT_WR)
(man page).The SSH protocol provides the ability to multiplex bidirectional “channels” on top of a single encrypted connection. A Trio implementation of SSH could expose these channels as
HalfCloseableStream
objects, and callingsend_eof()
would send anSSH_MSG_CHANNEL_EOF
request (see RFC 4254 §5.3).On an SSL/TLS-encrypted connection, the protocol doesn’t provide any way to do a unidirectional shutdown without closing the connection entirely, so
SSLStream
implementsStream
, notHalfCloseableStream
.
If an EOF has already been sent, then this method should silently succeed.
- Raises:
trio.BusyResourceError – if another task is already executing a
send_all()
,wait_send_all_might_not_block()
, orsend_eof()
on this stream.trio.BrokenResourceError – if something has gone wrong, and the stream is broken.
trio.ClosedResourceError – if you previously closed this stream object, or if another task closes this stream object while
send_eof()
is running.
- class trio.abc.Listener
Bases:
AsyncResource
,Generic
[T_resource
]A standard interface for listening for incoming connections.
Listener
objects also implement theAsyncResource
interface, so they can be closed by callingaclose()
or using anasync with
block.- abstractmethod await accept() T_resource
Wait until an incoming connection arrives, and then return it.
- Returns:
An object representing the incoming connection. In practice this is generally some kind of
Stream
, but in principle you could also define aListener
that returned, say, channel objects.- Return type:
- Raises:
trio.BusyResourceError – if two tasks attempt to call
accept()
on the same listener at the same time.trio.ClosedResourceError – if you previously closed this listener object, or if another task closes this listener object while
accept()
is running.
Listeners don’t generally raise
BrokenResourceError
, because for listeners there is no general condition of “the network/remote peer broke the connection” that can be handled in a generic way, like there is for streams. Other errors can occur and be raised fromaccept()
– for example, if you run out of file descriptors then you might get anOSError
with its errno set toEMFILE
.
- class trio.abc.SendChannel
Bases:
AsyncResource
,Generic
[SendType
]A standard interface for sending Python objects to some receiver.
SendChannel
objects also implement theAsyncResource
interface, so they can be closed by callingaclose
or using anasync with
block.If you want to send raw bytes rather than Python objects, see
SendStream
.- abstractmethod await send(value: SendType) None
Attempt to send an object through the channel, blocking if necessary.
- Parameters:
value (object) – The object to send.
- Raises:
trio.BrokenResourceError – if something has gone wrong, and the channel is broken. For example, you may get this if the receiver has already been closed.
trio.ClosedResourceError – if you previously closed this
SendChannel
object, or if another task closes it whilesend()
is running.trio.BusyResourceError – some channels allow multiple tasks to call
send
at the same time, but others don’t. If you try to callsend
simultaneously from multiple tasks on a channel that doesn’t support it, then you can getBusyResourceError
.
- class trio.abc.ReceiveChannel
Bases:
AsyncResource
,Generic
[ReceiveType
]A standard interface for receiving Python objects from some sender.
You can iterate over a
ReceiveChannel
using anasync for
loop:async for value in receive_channel: ...
This is equivalent to calling
receive()
repeatedly. The loop exits without error whenreceive
raisesEndOfChannel
.ReceiveChannel
objects also implement theAsyncResource
interface, so they can be closed by callingaclose
or using anasync with
block.If you want to receive raw bytes rather than Python objects, see
ReceiveStream
.- abstractmethod await receive() ReceiveType
Attempt to receive an incoming object, blocking if necessary.
- Returns:
Whatever object was received.
- Return type:
- Raises:
trio.EndOfChannel – if the sender has been closed cleanly, and no more objects are coming. This is not an error condition.
trio.ClosedResourceError – if you previously closed this
ReceiveChannel
object.trio.BrokenResourceError – if something has gone wrong, and the channel is broken.
trio.BusyResourceError – some channels allow multiple tasks to call
receive
at the same time, but others don’t. If you try to callreceive
simultaneously from multiple tasks on a channel that doesn’t support it, then you can getBusyResourceError
.
- class trio.abc.Channel
Bases:
SendChannel
[T
],ReceiveChannel
[T
]A standard interface for interacting with bidirectional channels.
A
Channel
is an object that implements both theSendChannel
andReceiveChannel
interfaces, so you can both send and receive objects.
Generic stream tools
Trio currently provides a generic helper for writing servers that
listen for connections using one or more
Listener
s, and a generic utility class for working
with streams. And if you want to test code that’s written against the
streams interface, you should also check out Streams in
trio.testing
.
- await trio.serve_listeners(handler: Callable[[StreamT], Awaitable[object]], listeners: list[ListenerT], *, handler_nursery: Nursery | None = None, task_status: TaskStatus[list[ListenerT]] = TASK_STATUS_IGNORED) NoReturn
Listen for incoming connections on
listeners
, and for each one start a task runninghandler(stream)
.Warning
If
handler
raises an exception, then this function doesn’t do anything special to catch it – so by default the exception will propagate out and crash your server. If you don’t want this, then catch exceptions inside yourhandler
, or use ahandler_nursery
object that responds to exceptions in some other way.- Parameters:
handler – An async callable, that will be invoked like
handler_nursery.start_soon(handler, stream)
for each incoming connection.listeners – A list of
Listener
objects.serve_listeners()
takes responsibility for closing them.handler_nursery – The nursery used to start handlers, or any object with a
start_soon
method. IfNone
(the default), thenserve_listeners()
will create a new nursery internally and use that.task_status – This function can be used with
nursery.start
, which will returnlisteners
.
- Returns:
This function never returns unless cancelled.
Resource handling:
If
handler
neglects to close thestream
, then it will be closed usingtrio.aclose_forcefully()
.Error handling:
Most errors coming from
accept()
are allowed to propagate out (crashing the server in the process). However, some errors – those which indicate that the server is temporarily overloaded – are handled specially. These areOSError
s with one of the following errnos:EMFILE
: process is out of file descriptorsENFILE
: system is out of file descriptorsENOBUFS
,ENOMEM
: the kernel hit some sort of memory limitation when trying to create a socket object
When
serve_listeners()
gets one of these errors, then it:Logs the error to the standard library logger
trio.serve_listeners
(level = ERROR, with exception information included). By default this causes it to be printed to stderr.Waits 100 ms before calling
accept
again, in hopes that the system will recover.
- class trio.StapledStream(send_stream: SendStreamT, receive_stream: ReceiveStreamT)
Bases:
HalfCloseableStream
,Generic
[SendStreamT
,ReceiveStreamT
]This class staples together two unidirectional streams to make single bidirectional stream.
- Parameters:
send_stream (SendStream) – The stream to use for sending.
receive_stream (ReceiveStream) – The stream to use for receiving.
Example
A silly way to make a stream that echoes back whatever you write to it:
left, right = trio.testing.memory_stream_pair() echo_stream = StapledStream(SocketStream(left), SocketStream(right)) await echo_stream.send_all(b"x") assert await echo_stream.receive_some() == b"x"
StapledStream
objects implement the methods in theHalfCloseableStream
interface. They also have two additional public attributes:- send_stream
The underlying
SendStream
.send_all()
andwait_send_all_might_not_block()
are delegated to this object.
- receive_stream
The underlying
ReceiveStream
.receive_some()
is delegated to this object.
- await send_all(data: bytes | bytearray | memoryview) None
Calls
self.send_stream.send_all
.
- await send_eof() None
Shuts down the send side of the stream.
If
self.send_stream.send_eof()
exists, then this calls it. Otherwise, this callsself.send_stream.aclose()
.
Sockets and networking
The high-level network interface is built on top of our stream abstraction.
- await trio.open_tcp_stream(host: str | bytes, port: int, *, happy_eyeballs_delay: float | None = 0.25, local_address: str | None = None) SocketStream
Connect to the given host and port over TCP.
If the given
host
has multiple IP addresses associated with it, then we have a problem: which one do we use?One approach would be to attempt to connect to the first one, and then if that fails, attempt to connect to the second one … until we’ve tried all of them. But the problem with this is that if the first IP address is unreachable (for example, because it’s an IPv6 address and our network discards IPv6 packets), then we might end up waiting tens of seconds for the first connection attempt to timeout before we try the second address.
Another approach would be to attempt to connect to all of the addresses at the same time, in parallel, and then use whichever connection succeeds first, abandoning the others. This would be fast, but create a lot of unnecessary load on the network and the remote server.
This function strikes a balance between these two extremes: it works its way through the available addresses one at a time, like the first approach; but, if
happy_eyeballs_delay
seconds have passed and it’s still waiting for an attempt to succeed or fail, then it gets impatient and starts the next connection attempt in parallel. As soon as any one connection attempt succeeds, all the other attempts are cancelled. This avoids unnecessary load because most connections will succeed after just one or two attempts, but if one of the addresses is unreachable then it doesn’t slow us down too much.This is known as a “happy eyeballs” algorithm, and our particular variant is modelled after how Chrome connects to webservers; see RFC 6555 for more details.
- Parameters:
host (str or bytes) – The host to connect to. Can be an IPv4 address, IPv6 address, or a hostname.
port (int) – The port to connect to.
happy_eyeballs_delay (float or None) – How many seconds to wait for each connection attempt to succeed or fail before getting impatient and starting another one in parallel. Set to
None
if you want to limit to only one connection attempt at a time (likesocket.create_connection()
). Default: 0.25 (250 ms).local_address (None or str) –
The local IP address or hostname to use as the source for outgoing connections. If
None
, we let the OS pick the source IP.This is useful in some exotic networking configurations where your host has multiple IP addresses, and you want to force the use of a specific one.
Note that if you pass an IPv4
local_address
, then you won’t be able to connect to IPv6 hosts, and vice-versa. If you want to take advantage of this to force the use of IPv4 or IPv6 without specifying an exact source address, you can use the IPv4 wildcard addresslocal_address="0.0.0.0"
, or the IPv6 wildcard addresslocal_address="::"
.
- Returns:
a
Stream
connected to the given server.- Return type:
- Raises:
OSError – if the connection fails.
See also
open_ssl_over_tcp_stream
- await trio.serve_tcp(handler: Callable[[trio.SocketStream], Awaitable[object]], port: int, *, host: str | bytes | None = None, backlog: int | None = None, handler_nursery: trio.Nursery | None = None, task_status: TaskStatus[list[trio.SocketListener]] = TASK_STATUS_IGNORED) None
Listen for incoming TCP connections, and for each one start a task running
handler(stream)
.This is a thin convenience wrapper around
open_tcp_listeners()
andserve_listeners()
– see them for full details.Warning
If
handler
raises an exception, then this function doesn’t do anything special to catch it – so by default the exception will propagate out and crash your server. If you don’t want this, then catch exceptions inside yourhandler
, or use ahandler_nursery
object that responds to exceptions in some other way.When used with
nursery.start
you get back the newly opened listeners. So, for example, if you want to start a server in your test suite and then connect to it to check that it’s working properly, you can use something like:from trio import SocketListener, SocketStream from trio.testing import open_stream_to_socket_listener async with trio.open_nursery() as nursery: listeners: list[SocketListener] = await nursery.start(serve_tcp, handler, 0) client_stream: SocketStream = await open_stream_to_socket_listener(listeners[0]) # Then send and receive data on 'client_stream', for example: await client_stream.send_all(b"GET / HTTP/1.0\r\n\r\n")
This avoids several common pitfalls:
It lets the kernel pick a random open port, so your test suite doesn’t depend on any particular port being open.
It waits for the server to be accepting connections on that port before
start
returns, so there’s no race condition where the incoming connection arrives before the server is ready.It uses the Listener object to find out which port was picked, so it can connect to the right place.
- Parameters:
handler – The handler to start for each incoming connection. Passed to
serve_listeners()
.port – The port to listen on. Use 0 to let the kernel pick an open port. Passed to
open_tcp_listeners()
.host (str, bytes, or None) – The host interface to listen on; use
None
to bind to the wildcard address. Passed toopen_tcp_listeners()
.backlog – The listen backlog, or None to have a good default picked. Passed to
open_tcp_listeners()
.handler_nursery – The nursery to start handlers in, or None to use an internal nursery. Passed to
serve_listeners()
.task_status – This function can be used with
nursery.start
.
- Returns:
This function only returns when cancelled.
- await trio.open_ssl_over_tcp_stream(host: str | bytes, port: int, *, https_compatible: bool = False, ssl_context: ssl.SSLContext | None = None, happy_eyeballs_delay: float | None = 0.25) trio.SSLStream[SocketStream]
Make a TLS-encrypted Connection to the given host and port over TCP.
This is a convenience wrapper that calls
open_tcp_stream()
and wraps the result in anSSLStream
.This function does not perform the TLS handshake; you can do it manually by calling
do_handshake()
, or else it will be performed automatically the first time you send or receive data.- Parameters:
host (bytes or str) – The host to connect to. We require the server to have a TLS certificate valid for this hostname.
port (int) – The port to connect to.
https_compatible (bool) – Set this to True if you’re connecting to a web server. See
SSLStream
for details. Default: False.ssl_context (
SSLContext
or None) – The SSL context to use. If None (the default),ssl.create_default_context()
will be called to create a context.happy_eyeballs_delay (float) – See
open_tcp_stream()
.
- Returns:
the encrypted connection to the server.
- Return type:
- await trio.serve_ssl_over_tcp(handler: Callable[[trio.SSLStream[SocketStream]], Awaitable[object]], port: int, ssl_context: ssl.SSLContext, *, host: str | bytes | None = None, https_compatible: bool = False, backlog: int | None = None, handler_nursery: trio.Nursery | None = None, task_status: trio.TaskStatus[list[trio.SSLListener[SocketStream]]] = TASK_STATUS_IGNORED) NoReturn
Listen for incoming TCP connections, and for each one start a task running
handler(stream)
.This is a thin convenience wrapper around
open_ssl_over_tcp_listeners()
andserve_listeners()
– see them for full details.Warning
If
handler
raises an exception, then this function doesn’t do anything special to catch it – so by default the exception will propagate out and crash your server. If you don’t want this, then catch exceptions inside yourhandler
, or use ahandler_nursery
object that responds to exceptions in some other way.When used with
nursery.start
you get back the newly opened listeners. See the documentation forserve_tcp()
for an example where this is useful.- Parameters:
handler – The handler to start for each incoming connection. Passed to
serve_listeners()
.port (int) – The port to listen on. Use 0 to let the kernel pick an open port. Ultimately passed to
open_tcp_listeners()
.ssl_context (SSLContext) – The SSL context to use for all incoming connections. Passed to
open_ssl_over_tcp_listeners()
.host (str, bytes, or None) – The address to bind to; use
None
to bind to the wildcard address. Ultimately passed toopen_tcp_listeners()
.https_compatible (bool) – Set this to True if you want to use “HTTPS-style” TLS. See
SSLStream
for details.handler_nursery – The nursery to start handlers in, or None to use an internal nursery. Passed to
serve_listeners()
.task_status – This function can be used with
nursery.start
.
- Returns:
This function only returns when cancelled.
- await trio.open_unix_socket(filename: str | bytes | PathLike[str] | PathLike[bytes]) SocketStream
Opens a connection to the specified Unix domain socket.
You must have read/write permission on the specified file to connect.
- Parameters:
filename (str or bytes) – The filename to open the connection to.
- Returns:
a
Stream
connected to the given file.- Return type:
- Raises:
OSError – If the socket file could not be connected to.
RuntimeError – If AF_UNIX sockets are not supported.
- class trio.SocketStream(socket: SocketType)
Bases:
HalfCloseableStream
An implementation of the
trio.abc.HalfCloseableStream
interface based on a raw network socket.- Parameters:
socket – The Trio socket object to wrap. Must have type
SOCK_STREAM
, and be connected.
By default for TCP sockets,
SocketStream
enablesTCP_NODELAY
, and (on platforms where it’s supported) enablesTCP_NOTSENT_LOWAT
with a reasonable buffer size (currently 16 KiB) – see issue #72 for discussion. You can of course override these defaults by callingsetsockopt()
.Once a
SocketStream
object is constructed, it implements the fulltrio.abc.HalfCloseableStream
interface. In addition, it provides a few extra features:- socket
The Trio socket object that this stream wraps.
- getsockopt(level: int, option: int, buffersize: int = 0) int | bytes
Check the current value of an option on the underlying socket.
See
socket.socket.getsockopt()
for details.
- await send_all(data: bytes | bytearray | memoryview) None
- class trio.SocketListener(socket: SocketType)
Bases:
Listener
[SocketStream
]A
Listener
that uses a listening socket to accept incoming connections asSocketStream
objects.- Parameters:
socket – The Trio socket object to wrap. Must have type
SOCK_STREAM
, and be listening.
Note that the
SocketListener
“takes ownership” of the given socket; closing theSocketListener
will also close the socket.- socket
The Trio socket object that this stream wraps.
- await accept() SocketStream
Accept an incoming connection.
- Returns:
- Raises:
OSError – if the underlying call to
accept
raises an unexpected error.ClosedResourceError – if you already closed the socket.
This method handles routine errors like
ECONNABORTED
, but passes other errors on to its caller. In particular, it does not make any special effort to handle resource exhaustion errors likeEMFILE
,ENFILE
,ENOBUFS
,ENOMEM
.
- await trio.open_tcp_listeners(port: int, *, host: str | bytes | None = None, backlog: int | None = None) list[SocketListener]
Create
SocketListener
objects to listen for TCP connections.- Parameters:
port (int) –
The port to listen on.
If you use 0 as your port, then the kernel will automatically pick an arbitrary open port. But be careful: if you use this feature when binding to multiple IP addresses, then each IP address will get its own random port, and the returned listeners will probably be listening on different ports. In particular, this will happen if you use
host=None
– which is the default – because in this caseopen_tcp_listeners()
will bind to both the IPv4 wildcard address (0.0.0.0
) and also the IPv6 wildcard address (::
).The local interface to bind to. This is passed to
getaddrinfo()
with theAI_PASSIVE
flag set.If you want to bind to the wildcard address on both IPv4 and IPv6, in order to accept connections on all available interfaces, then pass
None
. This is the default.If you have a specific interface you want to bind to, pass its IP address or hostname here. If a hostname resolves to multiple IP addresses, this function will open one listener on each of them.
If you want to use only IPv4, or only IPv6, but want to accept on all interfaces, pass the family-specific wildcard address:
"0.0.0.0"
for IPv4-only and"::"
for IPv6-only.backlog (int or None) – The listen backlog to use. If you leave this as
None
then Trio will pick a good default. (Currently: whatever your system has configured as the maximum backlog.)
- Returns:
list of
SocketListener
- Raises:
- await trio.open_ssl_over_tcp_listeners(port: int, ssl_context: ssl.SSLContext, *, host: str | bytes | None = None, https_compatible: bool = False, backlog: int | None = None) list[trio.SSLListener[SocketStream]]
Start listening for SSL/TLS-encrypted TCP connections to the given port.
- Parameters:
port (int) – The port to listen on. See
open_tcp_listeners()
.ssl_context (SSLContext) – The SSL context to use for all incoming connections.
host (str, bytes, or None) – The address to bind to; use
None
to bind to the wildcard address. Seeopen_tcp_listeners()
.backlog (int or None) – See
open_tcp_listeners()
for details.
SSL / TLS support
Trio provides SSL/TLS support based on the standard library ssl
module. Trio’s SSLStream
and SSLListener
take their
configuration from a ssl.SSLContext
, which you can create
using ssl.create_default_context()
and customize using the
other constants and functions in the ssl
module.
Warning
Avoid instantiating ssl.SSLContext
directly.
A newly constructed SSLContext
has less secure
defaults than one returned by ssl.create_default_context()
.
Instead of using ssl.SSLContext.wrap_socket()
, you
create a SSLStream
:
- class trio.SSLStream(transport_stream: T_Stream, ssl_context: SSLContext, *, server_hostname: str | bytes | None = None, server_side: bool = False, https_compatible: bool = False)
Bases:
Stream
,Generic
[T_Stream
]Encrypted communication using SSL/TLS.
SSLStream
wraps an arbitraryStream
, and allows you to perform encrypted communication over it using the usualStream
interface. You pass regular data tosend_all()
, then it encrypts it and sends the encrypted data on the underlyingStream
;receive_some()
takes encrypted data out of the underlyingStream
and decrypts it before returning it.You should read the standard library’s
ssl
documentation carefully before attempting to use this class, and probably other general documentation on SSL/TLS as well. SSL/TLS is subtle and quick to anger. Really. I’m not kidding.- Parameters:
transport_stream (Stream) – The stream used to transport encrypted data. Required.
ssl_context (SSLContext) – The
SSLContext
used for this connection. Required. Usually created by callingssl.create_default_context()
.server_hostname (str, bytes, or None) – The name of the server being connected to. Used for SNI and for validating the server’s certificate (if hostname checking is enabled). This is effectively mandatory for clients, and actually mandatory if
ssl_context.check_hostname
isTrue
.server_side (bool) – Whether this stream is acting as a client or server. Defaults to False, i.e. client mode.
https_compatible (bool) –
There are two versions of SSL/TLS commonly encountered in the wild: the standard version, and the version used for HTTPS (HTTP-over-SSL/TLS).
Standard-compliant SSL/TLS implementations always send a cryptographically signed
close_notify
message before closing the connection. This is important because if the underlying transport were simply closed, then there wouldn’t be any way for the other side to know whether the connection was intentionally closed by the peer that they negotiated a cryptographic connection to, or by some man-in-the-middle attacker who can’t manipulate the cryptographic stream, but can manipulate the transport layer (a so-called “truncation attack”).However, this part of the standard is widely ignored by real-world HTTPS implementations, which means that if you want to interoperate with them, then you NEED to ignore it too.
Fortunately this isn’t as bad as it sounds, because the HTTP protocol already includes its own equivalent of
close_notify
, so doing this again at the SSL/TLS level is redundant. But not all protocols do! Therefore, by default Trio implements the safer standard-compliant version (https_compatible=False
). But if you’re speaking HTTPS or some other protocol whereclose_notify
s are commonly skipped, then you should sethttps_compatible=True
; with this setting, Trio will neither expect nor sendclose_notify
messages.If you have code that was written to use
ssl.SSLSocket
and now you’re porting it to Trio, then it may be useful to know that a difference betweenSSLStream
andssl.SSLSocket
is thatSSLSocket
implements thehttps_compatible=True
behavior by default.
- transport_stream
The underlying transport stream that was passed to
__init__
. An example of when this would be useful is if you’re usingSSLStream
over aSocketStream
and want to call theSocketStream
’ssetsockopt()
method.- Type:
Internally, this class is implemented using an instance of
ssl.SSLObject
, and all ofSSLObject
’s methods and attributes are re-exported as methods and attributes on this class. However, there is one difference:SSLObject
has several methods that return information about the encrypted connection, likecipher()
orselected_alpn_protocol()
. If you call them before the handshake, when they can’t possibly return useful data, thenssl.SSLObject
returns None, buttrio.SSLStream
raisesNeedHandshakeError
.This also means that if you register a SNI callback using
sni_callback
, then the first argument your callback receives will be assl.SSLObject
.- await aclose() None
Gracefully shut down this connection, and close the underlying transport.
If
https_compatible
is False (the default), then this attempts to first send aclose_notify
and then close the underlying stream by calling itsaclose()
method.If
https_compatible
is set to True, then this simply closes the underlying stream and marks this stream as closed.
- await do_handshake() None
Ensure that the initial handshake has completed.
The SSL protocol requires an initial handshake to exchange certificates, select cryptographic keys, and so forth, before any actual data can be sent or received. You don’t have to call this method; if you don’t, then
SSLStream
will automatically perform the handshake as needed, the first time you try to send or receive data. But if you want to trigger it manually – for example, because you want to look at the peer’s certificate before you start talking to them – then you can call this method.If the initial handshake is already in progress in another task, this waits for it to complete and then returns.
If the initial handshake has already completed, this returns immediately without doing anything (except executing a checkpoint).
Warning
If this method is cancelled, then it may leave the
SSLStream
in an unusable state. If this happens then any future attempt to use the object will raisetrio.BrokenResourceError
.
- await receive_some(max_bytes: int | None = None) bytes | bytearray
Read some data from the underlying transport, decrypt it, and return it.
See
trio.abc.ReceiveStream.receive_some()
for details.Warning
If this method is cancelled while the initial handshake or a renegotiation are in progress, then it may leave the
SSLStream
in an unusable state. If this happens then any future attempt to use the object will raisetrio.BrokenResourceError
.
- await send_all(data: bytes | bytearray | memoryview) None
Encrypt some data and then send it on the underlying transport.
See
trio.abc.SendStream.send_all()
for details.Warning
If this method is cancelled, then it may leave the
SSLStream
in an unusable state. If this happens then any attempt to use the object will raisetrio.BrokenResourceError
.
- await unwrap() tuple[Stream, bytes | bytearray]
Cleanly close down the SSL/TLS encryption layer, allowing the underlying stream to be used for unencrypted communication.
You almost certainly don’t need this.
- Returns:
A pair
(transport_stream, trailing_bytes)
, wheretransport_stream
is the underlying transport stream, andtrailing_bytes
is a byte string. SinceSSLStream
doesn’t necessarily know where the end of the encrypted data will be, it can happen that it accidentally reads too much from the underlying stream.trailing_bytes
contains this extra data; you should process it as if it was returned from a call totransport_stream.receive_some(...)
.
And if you’re implementing a server, you can use SSLListener
:
- class trio.SSLListener(transport_listener: Listener[T_Stream], ssl_context: SSLContext, *, https_compatible: bool = False)
Bases:
Listener
[SSLStream
[T_Stream
]]A
Listener
for SSL/TLS-encrypted servers.SSLListener
wraps around another Listener, and converts all incoming connections to encrypted connections by wrapping them in aSSLStream
.- Parameters:
transport_listener (Listener) – The listener whose incoming connections will be wrapped in
SSLStream
.ssl_context (SSLContext) – The
SSLContext
that will be used for incoming connections.
- transport_listener
The underlying listener that was passed to
__init__
.- Type:
- await accept() SSLStream[T_Stream]
Accept the next connection and wrap it in an
SSLStream
.See
trio.abc.Listener.accept()
for details.
Some methods on SSLStream
raise NeedHandshakeError
if
you call them before the handshake completes:
Datagram TLS support
Trio also has support for Datagram TLS (DTLS), which is like TLS but for unreliable UDP connections. This can be useful for applications where TCP’s reliable in-order delivery is problematic, like teleconferencing, latency-sensitive games, and VPNs.
Currently, using DTLS with Trio requires PyOpenSSL. We hope to
eventually allow the use of the stdlib ssl
module as well, but
unfortunately that’s not yet possible.
Warning
Note that PyOpenSSL is in many ways lower-level than the
ssl
module – in particular, it currently HAS NO BUILT-IN
MECHANISM TO VALIDATE CERTIFICATES. We strongly recommend that
you use the service-identity library to validate
hostnames and certificates.
- class trio.DTLSEndpoint(socket: SocketType, *, incoming_packets_buffer: int = 10)
A DTLS endpoint.
A single UDP socket can handle arbitrarily many DTLS connections simultaneously, acting as a client or server as needed. A
DTLSEndpoint
object holds a UDP socket and manages these connections, which are represented asDTLSChannel
objects.- Parameters:
socket – (trio.socket.SocketType): A
SOCK_DGRAM
socket. If you want to accept incoming connections in server mode, then you should probably bind the socket to some known port.incoming_packets_buffer (int) – Each
DTLSChannel
using this socket has its own buffer that holds incoming packets until you callreceive
to read them. This lets you adjust the size of this buffer.statistics
lets you check if the buffer has overflowed.
- socket
- incoming_packets_buffer
Both constructor arguments are also exposed as attributes, in case you need to access them later.
- connect(address: tuple[str, int], ssl_context: OpenSSL.SSL.Context) DTLSChannel
Initiate an outgoing DTLS connection.
Notice that this is a synchronous method. That’s because it doesn’t actually initiate any I/O – it just sets up a
DTLSChannel
object. The actual handshake doesn’t occur until you start using theDTLSChannel
. This gives you a chance to do further configuration first, like setting MTU etc.- Parameters:
address – The address to connect to. Usually a (host, port) tuple, like
("127.0.0.1", 12345)
.ssl_context (OpenSSL.SSL.Context) – The PyOpenSSL context object to use for this connection.
- Returns:
DTLSChannel
- await serve(ssl_context: OpenSSL.SSL.Context, async_fn: Callable[[DTLSChannel, Unpack[PosArgsT]], Awaitable[object]], *args: Unpack[PosArgsT], task_status: trio.TaskStatus[None] = TASK_STATUS_IGNORED) None
Listen for incoming connections, and spawn a handler for each using an internal nursery.
Similar to
serve_tcp
, this function never returns until cancelled, or theDTLSEndpoint
is closed and all handlers have exited.Usage commonly looks like:
async def handler(dtls_channel): ... async with trio.open_nursery() as nursery: await nursery.start(dtls_endpoint.serve, ssl_context, handler) # ... do other things here ...
The
dtls_channel
passed into the handler function has already performed the “cookie exchange” part of the DTLS handshake, so the peer address is trustworthy. But the actual cryptographic handshake doesn’t happen until you start using it, giving you a chance for any last minute configuration, and the option to catch and handle handshake errors.- Parameters:
ssl_context (OpenSSL.SSL.Context) – The PyOpenSSL context object to use for incoming connections.
async_fn – The handler function that will be invoked for each incoming connection.
*args – Additional arguments to pass to the handler function.
- class trio.DTLSChannel(*args: object, **kwargs: object)
-
A DTLS connection.
This class has no public constructor – you get instances by calling
DTLSEndpoint.serve
orconnect
.- endpoint
The
DTLSEndpoint
that this connection is using.
- peer_address
The IP/port of the remote peer that this connection is associated with.
- await do_handshake(*, initial_retransmit_timeout: float = 1.0) None
Perform the handshake.
Calling this is optional – if you don’t, then it will be automatically called the first time you call
send
orreceive
. But calling it explicitly can be useful in case you want to control the retransmit timeout, use a cancel scope to place an overall timeout on the handshake, or catch errors from the handshake specifically.It’s safe to call this multiple times, or call it simultaneously from multiple tasks – the first call will perform the handshake, and the rest will be no-ops.
- Parameters:
initial_retransmit_timeout (float) –
Since UDP is an unreliable protocol, it’s possible that some of the packets we send during the handshake will get lost. To handle this, DTLS uses a timer to automatically retransmit handshake packets that don’t receive a response. This lets you set the timeout we use to detect packet loss. Ideally, it should be set to ~1.5 times the round-trip time to your peer, but 1 second is a reasonable default. There’s some useful guidance here.
This is the initial timeout, because if packets keep being lost then Trio will automatically back off to longer values, to avoid overloading the network.
- await receive() bytes
Fetch the next packet of data from this connection’s peer, waiting if necessary.
This is safe to call from multiple tasks simultaneously, in case you have some reason to do that. And more importantly, it’s cancellation-safe, meaning that cancelling a call to
receive
will never cause a packet to be lost or corrupt the underlying connection.
- close() None
Close this connection.
DTLSChannel
s don’t actually own any OS-level resources – the socket is owned by theDTLSEndpoint
, not the individual connections. So you don’t really have to call this. But it will interrupt any other tasks callingreceive
with aClosedResourceError
, and cause future attempts to use this connection to fail.You can also use this object as a synchronous or asynchronous context manager.
- await aclose() None
Close this connection, but asynchronously.
This is included to satisfy the
trio.abc.Channel
contract. It’s identical toclose
, but async.
- set_ciphertext_mtu(new_mtu: int) None
Tells Trio the largest amount of data that can be sent in a single packet to this peer.
Trio doesn’t actually enforce this limit – if you pass a huge packet to
send
, then we’ll dutifully encrypt it and attempt to send it. But calling this method does have two useful effects:If called before the handshake is performed, then Trio will automatically fragment handshake messages to fit within the given MTU. It also might fragment them even smaller, if it detects signs of packet loss, so setting this should never be necessary to make a successful connection. But, the packet loss detection only happens after multiple timeouts have expired, so if you have reason to believe that a smaller MTU is required, then you can set this to skip those timeouts and establish the connection more quickly.
It changes the value returned from
get_cleartext_mtu
. So if you have some kind of estimate of the network-level MTU, then you can use this to figure out how much overhead DTLS will need for hashes/padding/etc., and how much space you have left for your application data.
The MTU here is measuring the largest UDP payload you think can be sent, the amount of encrypted data that can be handed to the operating system in a single call to
send
. It should not include IP/UDP headers. Note that OS estimates of the MTU often are link-layer MTUs, so you have to subtract off 28 bytes on IPv4 and 48 bytes on IPv6 to get the ciphertext MTU.By default, Trio assumes an MTU of 1472 bytes on IPv4, and 1452 bytes on IPv6, which correspond to the common Ethernet MTU of 1500 bytes after accounting for IP/UDP overhead.
- get_cleartext_mtu() int
Returns the largest number of bytes that you can pass in a single call to
send
while still fitting within the network-level MTU.See
set_ciphertext_mtu
for more details.
- statistics() DTLSChannelStatistics
Returns a
DTLSChannelStatistics
object with statistics about this connection.
- class trio.DTLSChannelStatistics(incoming_packets_dropped_in_trio: int)
Currently this has only one attribute:
incoming_packets_dropped_in_trio
(int
): Gives a count of the number of incoming packets from this peer that Trio successfully received from the network, but then got dropped because the internal channel buffer was full. If this is non-zero, then you might want to callreceive
more often, or use a largerincoming_packets_buffer
, or just not worry about it because your UDP-based protocol should be able to handle the occasional lost packet, right?
Low-level networking with trio.socket
The trio.socket
module provides Trio’s basic low-level
networking API. If you’re doing ordinary things with stream-oriented
connections over IPv4/IPv6/Unix domain sockets, then you probably want
to stick to the high-level API described above. If you want to use
UDP, or exotic address families like AF_BLUETOOTH
, or otherwise
get direct access to all the quirky bits of your system’s networking
API, then you’re in the right place.
Top-level exports
Generally, the API exposed by trio.socket
mirrors that of the
standard library socket
module. Most constants (like
SOL_SOCKET
) and simple utilities (like inet_aton()
)
are simply re-exported unchanged. But there are also some differences,
which are described here.
First, Trio provides analogues to all the standard library functions that return socket objects; their interface is identical, except that they’re modified to return Trio socket objects instead:
- trio.socket.socket(family=-1, type=-1, proto=-1, fileno=None)
Create a new Trio socket, like
socket.socket
.This function’s behavior can be customized using
set_custom_socket_factory()
.
- trio.socket.socketpair(family=None, type=SocketKind.SOCK_STREAM, proto=0)
Like
socket.socketpair()
, but returns a pair of Trio socket objects.
- trio.socket.fromfd(fd, family, type, proto=0)
Like
socket.fromfd()
, but returns a Trio socket object.
Like
socket.fromshare()
, but returns a Trio socket object.
In addition, there is a new function to directly convert a standard library socket into a Trio socket:
- trio.socket.from_stdlib_socket(sock: socket) SocketType
Convert a standard library
socket.socket
object into a Trio socket object.
Unlike socket.socket
, trio.socket.socket()
is a
function, not a class; if you want to check whether an object is a
Trio socket, use isinstance(obj, trio.socket.SocketType)
.
For name lookup, Trio provides the standard functions, but with some changes:
- await trio.socket.getaddrinfo(host: bytes | str | None, port: bytes | str | int | None, family: int = 0, type: int = 0, proto: int = 0, flags: int = 0) list[tuple[AddressFamily, SocketKind, int, str, tuple[str, int] | tuple[str, int, int, int]]]
Look up a numeric address given a name.
Arguments and return values are identical to
socket.getaddrinfo()
, except that this version is async.Also,
trio.socket.getaddrinfo()
correctly uses IDNA 2008 to process non-ASCII domain names. (socket.getaddrinfo()
uses IDNA 2003, which can give the wrong result in some cases and cause you to connect to a different host than the one you intended; see bpo-17305.)This function’s behavior can be customized using
set_custom_hostname_resolver()
.
- await trio.socket.getnameinfo(sockaddr: tuple[str, int] | tuple[str, int, int, int], flags: int) tuple[str, str]
Look up a name given a numeric address.
Arguments and return values are identical to
socket.getnameinfo()
, except that this version is async.This function’s behavior can be customized using
set_custom_hostname_resolver()
.
- await trio.socket.getprotobyname(name: str) int
Look up a protocol number by name. (Rarely used.)
Like
socket.getprotobyname()
, but async.
Trio intentionally DOES NOT include some obsolete, redundant, or broken features:
gethostbyname()
,gethostbyname_ex()
,gethostbyaddr()
: obsolete; usegetaddrinfo()
andgetnameinfo()
instead.getservbyport()
: obsolete and buggy; instead, do:_, service_name = await getnameinfo(('127.0.0.1', port), NI_NUMERICHOST)
getservbyname()
: obsolete and buggy; instead, do:await getaddrinfo(None, service_name)
getfqdn()
: obsolete; usegetaddrinfo()
with theAI_CANONNAME
flag.getdefaulttimeout()
,setdefaulttimeout()
: instead, use Trio’s standard support for Cancellation and timeouts.On Windows,
SO_REUSEADDR
is not exported, because it’s a trap: the name is the same as UnixSO_REUSEADDR
, but the semantics are different and extremely broken. In the very rare cases where you actually wantSO_REUSEADDR
on Windows, then it can still be accessed from the standard library’ssocket
module.
Socket objects
- class trio.socket.SocketType
Note
trio.socket.SocketType
is an abstract class and cannot be instantiated directly; you get concrete socket objects by calling constructors liketrio.socket.socket()
. However, you can use it to check if an object is a Trio socket viaisinstance(obj, trio.socket.SocketType)
.Trio socket objects are overall very similar to the standard library socket objects, with a few important differences:
First, and most obviously, everything is made “Trio-style”: blocking methods become async methods, and the following attributes are not supported:
setblocking()
: Trio sockets always act like blocking sockets; if you need to read/write from multiple sockets at once, then create multiple tasks.settimeout()
: see Cancellation and timeouts instead.makefile()
: Python’s file-like API is synchronous, so it can’t be implemented on top of an async socket.sendall()
: Could be supported, but you’re better off using the higher-levelSocketStream
, and specifically itssend_all()
method, which also does additional error checking.
In addition, the following methods are similar to the equivalents in
socket.socket
, but have some Trio-specific quirks:- await connect()
Connect the socket to a remote address.
Similar to
socket.socket.connect()
, except async.Warning
Due to limitations of the underlying operating system APIs, it is not always possible to properly cancel a connection attempt once it has begun. If
connect()
is cancelled, and is unable to abort the connection attempt, then it will:forcibly close the socket to prevent accidental reuse
raise
Cancelled
.
tl;dr: if
connect()
is cancelled then the socket is left in an unknown state – possibly open, and possibly closed. The only reasonable thing to do is to close it.
- is_readable()
Check whether the socket is readable or not.
- sendfile()
We also keep track of an extra bit of state, because it turns out to be useful for
trio.SocketStream
:- did_shutdown_SHUT_WR
This
bool
attribute is True if you’ve calledsock.shutdown(SHUT_WR)
orsock.shutdown(SHUT_RDWR)
, and False otherwise.
The following methods are identical to their equivalents in
socket.socket
, except async, and the ones that take address arguments require pre-resolved addresses:recvmsg()
(if available)recvmsg_into()
(if available)sendmsg()
(if available)
All methods and attributes not mentioned above are identical to their equivalents in
socket.socket
:
Asynchronous filesystem I/O
Trio provides built-in facilities for performing asynchronous filesystem operations like reading or renaming a file. Generally, we recommend that you use these instead of Python’s normal synchronous file APIs. But the tradeoffs here are somewhat subtle: sometimes people switch to async I/O, and then they’re surprised and confused when they find it doesn’t speed up their program. The next section explains the theory behind async file I/O, to help you better understand your code’s behavior. Or, if you just want to get started, you can jump down to the API overview.
Background: Why is async file I/O useful? The answer may surprise you
Many people expect that switching from synchronous file I/O to async file I/O will always make their program faster. This is not true! If we just look at total throughput, then async file I/O might be faster, slower, or about the same, and it depends in a complicated way on things like your exact patterns of disk access, or how much RAM you have. The main motivation for async file I/O is not to improve throughput, but to reduce the frequency of latency glitches.
To understand why, you need to know two things.
First, right now no mainstream operating system offers a generic,
reliable, native API for async file or filesystem operations, so we
have to fake it by using threads (specifically,
trio.to_thread.run_sync()
). This is cheap but isn’t free: on a
typical PC, dispatching to a worker thread adds something like ~100 µs
of overhead to each operation. (“µs” is pronounced “microseconds”, and
there are 1,000,000 µs in a second. Note that all the numbers here are
going to be rough orders of magnitude to give you a sense of scale; if
you need precise numbers for your environment, measure!)
And second, the cost of a disk operation is incredibly
bimodal. Sometimes, the data you need is already cached in RAM, and
then accessing it is very, very fast – calling io.FileIO
's
read
method on a cached file takes on the order of ~1 µs. But when
the data isn’t cached, then accessing it is much, much slower: the
average is ~100 µs for SSDs and ~10,000 µs for spinning disks, and if
you look at tail latencies then for both types of storage you’ll see
cases where occasionally some operation will be 10x or 100x slower
than average. And that’s assuming your program is the only thing
trying to use that disk – if you’re on some oversold cloud VM fighting
for I/O with other tenants then who knows what will happen. And some
operations can require multiple disk accesses.
Putting these together: if your data is in RAM then it should be clear that using a thread is a terrible idea – if you add 100 µs of overhead to a 1 µs operation, then that’s a 100x slowdown! On the other hand, if your data’s on a spinning disk, then using a thread is great – instead of blocking the main thread and all tasks for 10,000 µs, we only block them for 100 µs and can spend the rest of that time running other tasks to get useful work done, which can effectively be a 100x speedup.
But here’s the problem: for any individual I/O operation, there’s no way to know in advance whether it’s going to be one of the fast ones or one of the slow ones, so you can’t pick and choose. When you switch to async file I/O, it makes all the fast operations slower, and all the slow operations faster. Is that a win? In terms of overall speed, it’s hard to say: it depends what kind of disks you’re using and your kernel’s disk cache hit rate, which in turn depends on your file access patterns, how much spare RAM you have, the load on your service, … all kinds of things. If the answer is important to you, then there’s no substitute for measuring your code’s actual behavior in your actual deployment environment. But what we can say is that async disk I/O makes performance much more predictable across a wider range of runtime conditions.
If you’re not sure what to do, then we recommend that you use async disk I/O by default, because it makes your code more robust when conditions are bad, especially with regards to tail latencies; this improves the chances that what your users see matches what you saw in testing. Blocking the main thread stops all tasks from running for that time. 10,000 µs is 10 ms, and it doesn’t take many 10 ms glitches to start adding up to real money; async disk I/O can help prevent those. Just don’t expect it to be magic, and be aware of the tradeoffs.
API overview
If you want to perform general filesystem operations like creating and
listing directories, renaming files, or checking file metadata – or if
you just want a friendly way to work with filesystem paths – then you
want trio.Path
. It’s an asyncified replacement for the
standard library’s pathlib.Path
, and provides the same
comprehensive set of operations.
For reading and writing to files and file-like objects, Trio also
provides a mechanism for wrapping any synchronous file-like object
into an asynchronous interface. If you have a trio.Path
object you can get one of these by calling its open()
method; or if you know the file’s name you can open it directly with
trio.open_file()
. Alternatively, if you already have an open
file-like object, you can wrap it with trio.wrap_file()
– one
case where this is especially useful is to wrap io.BytesIO
or
io.StringIO
when writing tests.
Asynchronous path objects
- class trio.Path(*args: str | os.PathLike[str])
An async
pathlib.Path
that executes blocking methods intrio.to_thread.run_sync()
.Instantiating
Path
returns a concrete platform-specific subclass, one ofPosixPath
orWindowsPath
.- await absolute()
Like
absolute()
, but async.Return an absolute version of this path by prepending the current working directory. No normalization or symlink resolution is performed.
Use resolve() to get the canonical path to a file.
- property anchor
The concatenation of the drive and root, or ‘’.
- as_posix()
Return the string representation of the path with forward (/) slashes.
- as_uri()
Return the path as a ‘file’ URI.
- await chmod(mode, *, follow_symlinks=True)
Like
chmod()
, but async.Change the permissions of the path, like os.chmod().
- classmethod await cwd()
Like
cwd()
, but async.Return a new path pointing to the current working directory (as returned by os.getcwd()).
- property drive
The drive prefix (letter or UNC path), if any.
- await expanduser()
Like
expanduser()
, but async.Return a new path with expanded ~ and ~user constructs (as returned by os.path.expanduser)
- await glob(pattern)
Like
glob()
, but async.Iterate over this subtree and yield all existing files (of any kind, including directories) matching the given relative pattern.
This is an async method that returns a synchronous iterator, so you use it like:
for subpath in await mypath.glob(): ...
Note
The iterator is loaded into memory immediately during the initial call (see issue #501 for discussion).
- await hardlink_to(target)
Like
hardlink_to()
, but async.Make this path a hard link pointing to the same file as target.
Note the order of arguments (self, target) is the reverse of os.link’s.
- classmethod await home()
Like
home()
, but async.Return a new path pointing to the user’s home directory (as returned by os.path.expanduser(‘~’)).
- is_absolute()
True if the path is absolute (has both a root and, if applicable, a drive).
- await is_block_device()
Like
is_block_device()
, but async.Whether this path is a block device.
- await is_char_device()
Like
is_char_device()
, but async.Whether this path is a character device.
- await is_file()
Like
is_file()
, but async.Whether this path is a regular file (also True for symlinks pointing to regular files).
- await is_mount()
Like
is_mount()
, but async.Check if this path is a POSIX mount point
- is_relative_to(*other)
Return True if the path is relative to another path or False.
- is_reserved()
Return True if the path contains one of the special names reserved by the system, if any.
- await is_socket()
Like
is_socket()
, but async.Whether this path is a socket.
- await is_symlink()
Like
is_symlink()
, but async.Whether this path is a symbolic link.
- await iterdir()
Like
iterdir()
, but async.Iterate over the files in this directory. Does not yield any result for the special paths ‘.’ and ‘..’.
This is an async method that returns a synchronous iterator, so you use it like:
for subpath in await mypath.iterdir(): ...
Note
The iterator is loaded into memory immediately during the initial call (see issue #501 for discussion).
- joinpath(*args)
Combine this path with one or several arguments, and return a new path representing either a subpath (if all arguments are relative paths) or a totally different path (if one of the arguments is anchored).
- await lchmod(mode)
Like
lchmod()
, but async.Like chmod(), except if the path points to a symlink, the symlink’s permissions are changed, rather than its target’s.
- await link_to(target)
Like
link_to()
, but async.Make the target path a hard link pointing to this path.
Note this function does not make this path a hard link to target, despite the implication of the function and argument names. The order of arguments (target, link) is the reverse of Path.symlink_to, but matches that of os.link.
Deprecated since Python 3.10 and scheduled for removal in Python 3.12. Use
hardlink_to()
instead.
- await lstat()
Like
lstat()
, but async.Like stat(), except if the path points to a symlink, the symlink’s status information is returned, rather than its target’s.
- match(path_pattern)
Return True if this path matches the given pattern.
- await mkdir(mode=511, parents=False, exist_ok=False)
Like
mkdir()
, but async.Create a new directory at this given path.
- property name
The final path component, if any.
- await open(mode='r', buffering=-1, encoding=None, errors=None, newline=None)
Like
open()
, but async.Open the file pointed by this path and return a file object, as the built-in open() function does.
- property parent
The logical parent of the path.
- property parents
A sequence of this path’s logical parents.
- property parts
An object providing sequence-like access to the components in the filesystem path.
- await read_bytes()
Like
read_bytes()
, but async.Open the file in bytes mode, read it, and close the file.
- await read_text(encoding=None, errors=None)
Like
read_text()
, but async.Open the file in text mode, read it, and close the file.
- await readlink()
Like
readlink()
, but async.Return the path to which the symbolic link points.
- relative_to(*other)
Return the relative path to another path identified by the passed arguments. If the operation is not possible (because this is not a subpath of the other path), raise ValueError.
- await rename(target)
Like
rename()
, but async.Rename this path to the target path.
The target path may be absolute or relative. Relative paths are interpreted relative to the current working directory, not the directory of the Path object.
Returns the new Path instance pointing to the target path.
- await replace(target)
Like
replace()
, but async.Rename this path to the target path, overwriting if that path exists.
The target path may be absolute or relative. Relative paths are interpreted relative to the current working directory, not the directory of the Path object.
Returns the new Path instance pointing to the target path.
- await resolve(strict=False)
Like
resolve()
, but async.Make the path absolute, resolving all symlinks on the way and also normalizing it.
- await rglob(pattern)
Like
rglob()
, but async.Recursively yield all existing files (of any kind, including directories) matching the given relative pattern, anywhere in this subtree.
This is an async method that returns a synchronous iterator, so you use it like:
for subpath in await mypath.rglob(): ...
Note
The iterator is loaded into memory immediately during the initial call (see issue #501 for discussion).
- property root
The root of the path, if any.
- await samefile(other_path)
Like
samefile()
, but async.Return whether other_path is the same or not as this file (as returned by os.path.samefile()).
- await stat(*, follow_symlinks=True)
Like
stat()
, but async.Return the result of the stat() system call on this path, like os.stat() does.
- property stem
The final path component, minus its last suffix.
- property suffix
The final component’s last suffix, if any.
This includes the leading period. For example: ‘.txt’
- property suffixes
A list of the final component’s suffixes, if any.
These include the leading periods. For example: [‘.tar’, ‘.gz’]
- await symlink_to(target, target_is_directory=False)
Like
symlink_to()
, but async.Make this path a symlink pointing to the target path. Note the order of arguments (link, target) is the reverse of os.symlink.
- await touch(mode=438, exist_ok=True)
Like
touch()
, but async.Create this file with the given access mode, if it doesn’t exist.
- await unlink(missing_ok=False)
Like
unlink()
, but async.Remove this file or link. If the path is a directory, use rmdir() instead.
- with_name(name)
Return a new path with the file name changed.
- with_stem(stem)
Return a new path with the stem changed.
- with_suffix(suffix)
Return a new path with the file suffix changed. If the path has no suffix, add given suffix. If the given suffix is an empty string, remove the suffix from the path.
- await write_bytes(data)
Like
write_bytes()
, but async.Open the file in bytes mode, write to it, and close the file.
- await write_text(data, encoding=None, errors=None, newline=None)
Like
write_text()
, but async.Open the file in text mode, write to it, and close the file.
- class trio.PosixPath(*args: str | os.PathLike[str])
An async
pathlib.PosixPath
that executes blocking methods intrio.to_thread.run_sync()
.
- class trio.WindowsPath(*args: str | os.PathLike[str])
An async
pathlib.WindowsPath
that executes blocking methods intrio.to_thread.run_sync()
.
Asynchronous file objects
- await trio.open_file(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=None, opener=None)
Asynchronous version of
open()
.- Returns:
Example:
async with await trio.open_file(filename) as f: async for line in f: pass assert f.closed
See also
- trio.wrap_file(file)
This wraps any file object in a wrapper that provides an asynchronous file object interface.
- Parameters:
file – a file object
- Returns:
An asynchronous file object that wraps
file
Example:
async_file = trio.wrap_file(StringIO('asdf')) assert await async_file.read() == 'asdf'
- Asynchronous file interface
Trio’s asynchronous file objects have an interface that automatically adapts to the object being wrapped. Intuitively, you can mostly treat them like a regular file object, except adding an
await
in front of any of methods that do I/O. The definition of file object is a little vague in Python though, so here are the details:Synchronous attributes/methods: if any of the following attributes or methods are present, then they’re re-exported unchanged:
closed
,encoding
,errors
,fileno
,isatty
,newlines
,readable
,seekable
,writable
,buffer
,raw
,line_buffering
,closefd
,name
,mode
,getvalue
,getbuffer
.Async methods: if any of the following methods are present, then they’re re-exported as an async method:
flush
,read
,read1
,readall
,readinto
,readline
,readlines
,seek
,tell
,truncate
,write
,writelines
,readinto1
,peek
,detach
.
Special notes:
Async file objects implement Trio’s
AsyncResource
interface: you close them by callingaclose()
instead ofclose
(!!), and they can be used as async context managers. Like allaclose()
methods, theaclose
method on async file objects is guaranteed to close the file before returning, even if it is cancelled or otherwise raises an error.Using the same async file object from multiple tasks simultaneously: because the async methods on async file objects are implemented using threads, it’s only safe to call two of them at the same time from different tasks IF the underlying synchronous file object is thread-safe. You should consult the documentation for the object you’re wrapping. For objects returned from
trio.open_file()
ortrio.Path.open()
, it depends on whether you open the file in binary mode or text mode: binary mode files are task-safe/thread-safe, text mode files are not.Async file objects can be used as async iterators to iterate over the lines of the file:
async with await trio.open_file(...) as f: async for line in f: print(line)
The
detach
method, if present, returns an async file object.
This should include all the attributes exposed by classes in
io
. But if you’re wrapping an object that has other attributes that aren’t on the list above, then you can access them via the.wrapped
attribute:- wrapped
The underlying synchronous file object.
Spawning subprocesses
Trio provides support for spawning other programs as subprocesses, communicating with them via pipes, sending them signals, and waiting for them to exit.
Most of the time, this is done through our high-level interface,
trio.run_process
. It lets you either run a process to completion
while optionally capturing the output, or else run it in a background
task and interact with it while it’s running:
- await trio.run_process(command: str | bytes | os.PathLike | Sequence[str | bytes | os.PathLike], *, stdin: bytes | bytearray | memoryview | int | HasFileno | None = b'', capture_stdout: bool = False, capture_stderr: bool = False, check: bool = True, deliver_cancel: Callable[[Process], Awaitable[object]] | None = None, task_status: TaskStatus[Process] = TASK_STATUS_IGNORED, **options: object) subprocess.CompletedProcess[bytes]
Run
command
in a subprocess and wait for it to complete.This function can be called in two different ways.
One option is a direct call, like:
completed_process_info = await trio.run_process(...)
In this case, it returns a
subprocess.CompletedProcess
instance describing the results. Use this if you want to treat a process like a function call.The other option is to run it as a task using
Nursery.start
– the enhanced version ofstart_soon
that lets a task pass back a value during startup:process = await nursery.start(trio.run_process, ...)
In this case,
start
returns aProcess
object that you can use to interact with the process while it’s running. Use this if you want to treat a process like a background task.Either way,
run_process
makes sure that the process has exited before returning, handles cancellation, optionally checks for errors, and provides some convenient shorthands for dealing with the child’s input/output.Input:
run_process
supports all the samestdin=
arguments assubprocess.Popen
. In addition, if you simply want to pass in some fixed data, you can pass a plainbytes
object, andrun_process
will take care of setting up a pipe, feeding in the data you gave, and then sending end-of-file. The default isb""
, which means that the child will receive an empty stdin. If you want the child to instead read from the parent’s stdin, usestdin=None
.Output: By default, any output produced by the subprocess is passed through to the standard output and error streams of the parent Trio process.
When calling
run_process
directly, you can capture the subprocess’s output by passingcapture_stdout=True
to capture the subprocess’s standard output, and/orcapture_stderr=True
to capture its standard error. Captured data is collected up by Trio into an in-memory buffer, and then provided as thestdout
and/orstderr
attributes of the returnedCompletedProcess
object. The value for any stream that was not captured will beNone
.If you want to capture both stdout and stderr while keeping them separate, pass
capture_stdout=True, capture_stderr=True
.If you want to capture both stdout and stderr but mixed together in the order they were printed, use:
capture_stdout=True, stderr=subprocess.STDOUT
. This directs the child’s stderr into its stdout, so the combined output will be available in thestdout
attribute.If you’re using
await nursery.start(trio.run_process, ...)
and want to capture the subprocess’s output for further processing, then usestdout=subprocess.PIPE
and then make sure to read the data out of theProcess.stdout
stream. If you want to capture stderr separately, usestderr=subprocess.PIPE
. If you want to capture both, but mixed together in the correct order, usestdout=subprocess.PIPE, stderr=subprocess.STDOUT
.Error checking: If the subprocess exits with a nonzero status code, indicating failure,
run_process()
raises asubprocess.CalledProcessError
exception rather than returning normally. The captured outputs are still available as thestdout
andstderr
attributes of that exception. To disable this behavior, so thatrun_process()
returns normally even if the subprocess exits abnormally, passcheck=False
.Note that this can make the
capture_stdout
andcapture_stderr
arguments useful even when startingrun_process
as a task: if you only care about the output if the process fails, then you can enable capturing and then read the output off of theCalledProcessError
.Cancellation: If cancelled,
run_process
sends a termination request to the subprocess, then waits for it to fully exit. Thedeliver_cancel
argument lets you control how the process is terminated.Note
run_process
is intentionally similar to the standard librarysubprocess.run
, but some of the defaults are different. Specifically, we default to:check=True
, because “errors should never pass silently / unless explicitly silenced”.stdin=b""
, because it produces less-confusing results if a subprocess unexpectedly tries to read from stdin.
To get the
subprocess.run
semantics, usecheck=False, stdin=None
.- Parameters:
command (list or str) – The command to run. Typically this is a sequence of strings such as
['ls', '-l', 'directory with spaces']
, where the first element names the executable to invoke and the other elements specify its arguments. Withshell=True
in the**options
, or on Windows,command
may alternatively be a string, which will be parsed following platform-dependent quoting rules.stdin (
bytes
, subprocess.PIPE, file descriptor, or None) –The bytes to provide to the subprocess on its standard input stream, or
None
if the subprocess’s standard input should come from the same place as the parent Trio process’s standard input. As is the case with thesubprocess
module, you can also pass a file descriptor or an object with afileno()
method, in which case the subprocess’s standard input will come from that file.When starting
run_process
as a background task, you can also usestdin=subprocess.PIPE
, in which caseProcess.stdin
will be aSendStream
that you can use to send data to the child.capture_stdout (bool) – If true, capture the bytes that the subprocess writes to its standard output stream and return them in the
stdout
attribute of the returnedsubprocess.CompletedProcess
orsubprocess.CalledProcessError
.capture_stderr (bool) – If true, capture the bytes that the subprocess writes to its standard error stream and return them in the
stderr
attribute of the returnedCompletedProcess
orsubprocess.CalledProcessError
.check (bool) – If false, don’t validate that the subprocess exits successfully. You should be sure to check the
returncode
attribute of the returned object if you passcheck=False
, so that errors don’t pass silently.deliver_cancel (async function or None) –
If
run_process
is cancelled, then it needs to kill the child process. There are multiple ways to do this, so we let you customize it.If you pass None (the default), then the behavior depends on the platform:
On Windows, Trio calls
TerminateProcess
, which should kill the process immediately.On Unix-likes, the default behavior is to send a
SIGTERM
, wait 5 seconds, and send aSIGKILL
.
Alternatively, you can customize this behavior by passing in an arbitrary async function, which will be called with the
Process
object as an argument. For example, the default Unix behavior could be implemented like this:async def my_deliver_cancel(process): process.send_signal(signal.SIGTERM) await trio.sleep(5) process.send_signal(signal.SIGKILL)
When the process actually exits, the
deliver_cancel
function will automatically be cancelled – so if the process exits afterSIGTERM
, then we’ll never reach theSIGKILL
.In any case,
run_process
will always wait for the child process to exit before raisingCancelled
.**options –
run_process()
also accepts any general subprocess options and passes them on to theProcess
constructor. This includes thestdout
andstderr
options, which provide additional redirection possibilities such asstderr=subprocess.STDOUT
,stdout=subprocess.DEVNULL
, or file descriptors.
- Returns:
When called normally – a
subprocess.CompletedProcess
instance describing the return code and outputs.When called via
Nursery.start
– atrio.Process
instance.- Raises:
UnicodeError – if
stdin
is specified as a Unicode string, rather than bytesValueError – if multiple redirections are specified for the same stream, e.g., both
capture_stdout=True
andstdout=subprocess.DEVNULL
subprocess.CalledProcessError – if
check=False
is not passed and the process exits with a nonzero exit statusOSError – if an error is encountered starting or communicating with the process
ExceptionGroup – if exceptions occur in
deliver_cancel
, or when exceptions occur when communicating with the subprocess. If strict_exception_groups is set to false in the global context, which is deprecated, then single exceptions will be collapsed.
Note
The child process runs in the same process group as the parent Trio process, so a Ctrl+C will be delivered simultaneously to both parent and child. If you don’t want this behavior, consult your platform’s documentation for starting child processes in a different process group.
- class trio._subprocess.HasFileno(Protocol)
Represents any file-like object that has a file descriptor.
- class trio.Process
A child process. Like
subprocess.Popen
, but async.This class has no public constructor. The most common way to get a
Process
object is to combineNursery.start
withrun_process
:process_object = await nursery.start(run_process, ...)
This way,
run_process
supervises the process and makes sure that it is cleaned up properly, while optionally checking the return value, feeding it input, and so on.If you need more control – for example, because you want to spawn a child process that outlives your program – then another option is to use
trio.lowlevel.open_process
:process_object = await trio.lowlevel.open_process(...)
- args
The
command
passed at construction time, specifying the process to execute and its arguments.
- stdin
A stream connected to the child’s standard input stream: when you write bytes here, they become available for the child to read. Only available if the
Process
was constructed usingstdin=PIPE
; otherwise this will be None.- Type:
trio.abc.SendStream or None
- stdout
A stream connected to the child’s standard output stream: when the child writes to standard output, the written bytes become available for you to read here. Only available if the
Process
was constructed usingstdout=PIPE
; otherwise this will be None.- Type:
trio.abc.ReceiveStream or None
- stderr
A stream connected to the child’s standard error stream: when the child writes to standard error, the written bytes become available for you to read here. Only available if the
Process
was constructed usingstderr=PIPE
; otherwise this will be None.- Type:
trio.abc.ReceiveStream or None
- stdio
A stream that sends data to the child’s standard input and receives from the child’s standard output. Only available if both
stdin
andstdout
are available; otherwise this will be None.- Type:
trio.StapledStream or None
- returncode
The exit status of the process (an integer), or
None
if it’s still running.By convention, a return code of zero indicates success. On UNIX, negative values indicate termination due to a signal, e.g., -11 if terminated by signal 11 (
SIGSEGV
). On Windows, a process that exits due to a call toProcess.terminate()
will have an exit status of 1.Unlike the standard library
subprocess.Popen.returncode
, you don’t have to callpoll
orwait
to update this attribute; it’s automatically updated as needed, and will always give you the latest information.
- await wait() int
Block until the process exits.
- Returns:
The exit status of the process; see
returncode
.
- poll() int | None
Returns the exit status of the process (an integer), or
None
if it’s still running.Note that on Trio (unlike the standard library
subprocess.Popen
),process.poll()
andprocess.returncode
always give the same result. Seereturncode
for more details. This method is only included to make it easier to port code fromsubprocess
.
- kill() None
Immediately terminate the process.
On UNIX, this is equivalent to
send_signal(signal.SIGKILL)
. On Windows, it callsTerminateProcess
. In both cases, the process cannot prevent itself from being killed, but the termination will be delivered asynchronously; usewait()
if you want to ensure the process is actually dead before proceeding.
- terminate() None
Terminate the process, politely if possible.
On UNIX, this is equivalent to
send_signal(signal.SIGTERM)
; by convention this requests graceful termination, but a misbehaving or buggy process might ignore it. On Windows,terminate()
forcibly terminates the process in the same manner askill()
.
- send_signal(sig: signal.Signals | int) None
Send signal
sig
to the process.On UNIX,
sig
may be any signal defined in thesignal
module, such assignal.SIGINT
orsignal.SIGTERM
. On Windows, it may be anything accepted by the standard librarysubprocess.Popen.send_signal()
.
Note
communicate()
is not provided as a method onProcess
objects; callrun_process()
normally for simple capturing, or write the loop yourself if you have unusual needs.communicate()
has quite unusual cancellation behavior in the standard library (on some platforms it spawns a background thread which continues to read from the child process even after the timeout has expired) and we wanted to provide an interface with fewer surprises.
If trio.run_process
is too limiting, we also offer a low-level API,
trio.lowlevel.open_process
. For example, if you want to spawn a
child process that will outlive the parent process and be
orphaned, then run_process
can’t do that, but
open_process
can.
Options for starting subprocesses
All of Trio’s subprocess APIs accept the numerous keyword arguments used
by the standard subprocess
module to control the environment in
which a process starts and the mechanisms used for communicating with
it. These may be passed wherever you see **options
in the
documentation below. See the full list
or just the frequently used ones
in the subprocess
documentation. (You may need to import
subprocess
in order to access constants such as PIPE
or
DEVNULL
.)
Currently, Trio always uses unbuffered byte streams for communicating
with a process, so it does not support the encoding
, errors
,
universal_newlines
(alias text
), and bufsize
options.
Quoting: more than you wanted to know
The command to run and its arguments usually must be passed to Trio’s
subprocess APIs as a sequence of strings, where the first element in
the sequence specifies the command to run and the remaining elements
specify its arguments, one argument per element. This form is used
because it avoids potential quoting pitfalls; for example, you can run
["cp", "-f", source_file, dest_file]
without worrying about
whether source_file
or dest_file
contains spaces.
If you only run subprocesses without shell=True
and on UNIX,
that’s all you need to know about specifying the command. If you use
shell=True
or run on Windows, you probably should read the
rest of this section to be aware of potential pitfalls.
With shell=True
on UNIX, you must specify the command as a single
string, which will be passed to the shell as if you’d entered it at an
interactive prompt. The advantage of this option is that it lets you
use shell features like pipes and redirection without writing code to
handle them. For example, you can write Process("ls | grep
some_string", shell=True)
. The disadvantage is that you must
account for the shell’s quoting rules, generally by wrapping in
shlex.quote()
any argument that might contain spaces, quotes, or
other shell metacharacters. If you don’t do that, your safe-looking
f"ls | grep {some_string}"
might end in disaster when invoked with
some_string = "foo; rm -rf /"
.
On Windows, the fundamental API for process spawning (the
CreateProcess()
system call) takes a string, not a list, and it’s
actually up to the child process to decide how it wants to split that
string into individual arguments. Since the C language specifies that
main()
should take a list of arguments, most programs you
encounter will follow the rules used by the Microsoft C/C++ runtime.
subprocess.Popen
, and thus also Trio, uses these rules
when it converts an argument sequence to a string, and they
are documented
alongside the subprocess
module. There is no documented
Python standard library function that can directly perform that
conversion, so even on Windows, you almost always want to pass an
argument sequence rather than a string. But if the program you’re
spawning doesn’t split its command line back into individual arguments
in the standard way, you might need to pass a string to work around this.
(Or you might just be out of luck: as far as I can tell, there’s simply
no way to pass an argument containing a double-quote to a Windows
batch file.)
On Windows with shell=True
, things get even more chaotic. Now
there are two separate sets of quoting rules applied, one by the
Windows command shell CMD.EXE
and one by the process being
spawned, and they’re different. (And there’s no shlex.quote()
to save you: it uses UNIX-style quoting rules, even on Windows.) Most
special characters interpreted by the shell &<>()^|
are not
treated as special if the shell thinks they’re inside double quotes,
but %FOO%
environment variable substitutions still are, and the
shell doesn’t provide any way to write a double quote inside a
double-quoted string. Outside double quotes, any character (including
a double quote) can be escaped using a leading ^
. But since a
pipeline is processed by running each command in the pipeline in a
subshell, multiple layers of escaping can be needed:
echo ^^^&x | find "x" | find "x" # prints: &x
And if you combine pipelines with () grouping, you can need even more levels of escaping:
(echo ^^^^^^^&x | find "x") | find "x" # prints: &x
Since process creation takes a single arguments string, CMD.EXE
's
quoting does not influence word splitting, and double quotes are not
removed during CMD.EXE’s expansion pass. Double quotes are troublesome
because CMD.EXE handles them differently from the MSVC runtime rules; in:
prog.exe "foo \"bar\" baz"
the program will see one argument foo "bar" baz
but CMD.EXE thinks
bar\
is not quoted while foo \
and baz
are. All of this
makes it a formidable task to reliably interpolate anything into a
shell=True
command line on Windows, and Trio falls back on the
subprocess
behavior: If you pass a sequence with
shell=True
, it’s quoted in the same way as a sequence with
shell=False
, and had better not contain any shell metacharacters
you weren’t planning on.
Further reading:
Signals
- with trio.open_signal_receiver(*signals: signal.Signals | int) Generator[AsyncIterator[int], None, None] as signal_aiter
A context manager for catching signals.
Entering this context manager starts listening for the given signals and returns an async iterator; exiting the context manager stops listening.
The async iterator blocks until a signal arrives, and then yields it.
Note that if you leave the
with
block while the iterator has unextracted signals still pending inside it, then they will be re-delivered using Python’s regular signal handling logic. This avoids a race condition when signals arrives just before we exit thewith
block.- Parameters:
signals – the signals to listen for.
- Raises:
TypeError – if no signals were provided.
RuntimeError – if you try to use this anywhere except Python’s main thread. (This is a Python limitation.)
Example
A common convention for Unix daemons is that they should reload their configuration when they receive a
SIGHUP
. Here’s a sketch of what that might look like usingopen_signal_receiver()
:with trio.open_signal_receiver(signal.SIGHUP) as signal_aiter: async for signum in signal_aiter: assert signum == signal.SIGHUP reload_configuration()