python_doc_md/Intermediate/library/asyncio-eventloop.md

::: currentmodule
asyncio
:::

# Event Loop {#asyncio-event-loop}

**Source code:** `Lib/asyncio/events.py`{.interpreted-text role="source"}, `Lib/asyncio/base_events.py`{.interpreted-text role="source"}

------------------------------------------------------------------------

**Preface**

The event loop is the core of every asyncio application. Event loops run asynchronous tasks and callbacks, perform network IO operations, and run subprocesses.

Application developers should typically use the high-level asyncio functions, such as `asyncio.run`{.interpreted-text role="func"}, and should rarely need to reference the loop object or call its methods. This section is intended mostly for authors of lower-level code, libraries, and frameworks, who need finer control over the event loop behavior.

**Obtaining the Event Loop**

The following low-level functions can be used to get, set, or create an event loop:

:::: function
get_running_loop()

Return the running event loop in the current OS thread.

Raise a `RuntimeError`{.interpreted-text role="exc"} if there is no running event loop.

This function can only be called from a coroutine or a callback.

::: versionadded
3.7
:::
::::

:::::: function
get_event_loop()

Get the current event loop.

When called from a coroutine or a callback (e.g. scheduled with call_soon or similar API), this function will always return the running event loop.

If there is no running event loop set, the function will return the result of the `get_event_loop_policy().get_event_loop()` call.

Because this function has rather complex behavior (especially when custom event loop policies are in use), using the `get_running_loop`{.interpreted-text role="func"} function is preferred to `get_event_loop`{.interpreted-text role="func"} in coroutines and callbacks.

As noted above, consider using the higher-level `asyncio.run`{.interpreted-text role="func"} function, instead of using these lower level functions to manually create and close an event loop.

::: versionchanged
3.14 Raises a `RuntimeError`{.interpreted-text role="exc"} if there is no current event loop.
:::

:::: note
::: title
Note
:::

The `!asyncio`{.interpreted-text role="mod"} policy system is deprecated and will be removed in Python 3.16; from there on, this function will return the current running event loop if present else it will return the loop set by `set_event_loop`{.interpreted-text role="func"}.
::::
::::::

::: function
set_event_loop(loop)

Set *loop* as the current event loop for the current OS thread.
:::

::: function
new_event_loop()

Create and return a new event loop object.
:::

Note that the behaviour of `get_event_loop`{.interpreted-text role="func"}, `set_event_loop`{.interpreted-text role="func"}, and `new_event_loop`{.interpreted-text role="func"} functions can be altered by `setting a custom event loop policy <asyncio-policies>`{.interpreted-text role="ref"}.

**Contents**

This documentation page contains the following sections:

- The [Event Loop Methods](#event-loop-methods) section is the reference documentation of the event loop APIs;
- The [Callback Handles](#callback-handles) section documents the `Handle`{.interpreted-text role="class"} and `TimerHandle`{.interpreted-text role="class"} instances which are returned from scheduling methods such as `loop.call_soon`{.interpreted-text role="meth"} and `loop.call_later`{.interpreted-text role="meth"};
- The [Server Objects](#server-objects) section documents types returned from event loop methods like `loop.create_server`{.interpreted-text role="meth"};
- The [Event Loop Implementations](#event-loop-implementations) section documents the `SelectorEventLoop`{.interpreted-text role="class"} and `ProactorEventLoop`{.interpreted-text role="class"} classes;
- The [Examples](#examples) section showcases how to work with some event loop APIs.

## Event Loop Methods {#asyncio-event-loop-methods}

Event loops have **low-level** APIs for the following:

::: {.contents depth="1" local=""}
:::

### Running and stopping the loop

::: method
loop.run_until_complete(future)

Run until the *future* (an instance of `Future`{.interpreted-text role="class"}) has completed.

If the argument is a `coroutine object <coroutine>`{.interpreted-text role="ref"} it is implicitly scheduled to run as a `asyncio.Task`{.interpreted-text role="class"}.

Return the Future\'s result or raise its exception.
:::

::: method
loop.run_forever()

Run the event loop until `stop`{.interpreted-text role="meth"} is called.

If `stop`{.interpreted-text role="meth"} is called before `run_forever`{.interpreted-text role="meth"} is called, the loop will poll the I/O selector once with a timeout of zero, run all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exit.

If `stop`{.interpreted-text role="meth"} is called while `run_forever`{.interpreted-text role="meth"} is running, the loop will run the current batch of callbacks and then exit. Note that new callbacks scheduled by callbacks will not run in this case; instead, they will run the next time `run_forever`{.interpreted-text role="meth"} or `run_until_complete`{.interpreted-text role="meth"} is called.
:::

::: method
loop.stop()

Stop the event loop.
:::

::: method
loop.is_running()

Return `True` if the event loop is currently running.
:::

::: method
loop.is_closed()

Return `True` if the event loop was closed.
:::

::: method
loop.close()

Close the event loop.

The loop must not be running when this function is called. Any pending callbacks will be discarded.

This method clears all queues and shuts down the executor, but does not wait for the executor to finish.

This method is idempotent and irreversible. No other methods should be called after the event loop is closed.
:::

:::: {.method async=""}
loop.shutdown_asyncgens()

Schedule all currently open `asynchronous generator`{.interpreted-text role="term"} objects to close with an `~agen.aclose`{.interpreted-text role="meth"} call. After calling this method, the event loop will issue a warning if a new asynchronous generator is iterated. This should be used to reliably finalize all scheduled asynchronous generators.

Note that there is no need to call this function when `asyncio.run`{.interpreted-text role="func"} is used.

Example:

    try:
        loop.run_forever()
    finally:
        loop.run_until_complete(loop.shutdown_asyncgens())
        loop.close()

::: versionadded
3.6
:::
::::

::::::: {.method async=""}
loop.shutdown_default_executor(timeout=None)

Schedule the closure of the default executor and wait for it to join all of the threads in the `~concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="class"}. Once this method has been called, using the default executor with `loop.run_in_executor`{.interpreted-text role="meth"} will raise a `RuntimeError`{.interpreted-text role="exc"}.

The *timeout* parameter specifies the amount of time (in `float`{.interpreted-text role="class"} seconds) the executor will be given to finish joining. With the default, `None`, the executor is allowed an unlimited amount of time.

If the *timeout* is reached, a `RuntimeWarning`{.interpreted-text role="exc"} is emitted and the default executor is terminated without waiting for its threads to finish joining.

:::: note
::: title
Note
:::

Do not call this method when using `asyncio.run`{.interpreted-text role="func"}, as the latter handles default executor shutdown automatically.
::::

::: versionadded
3.9
:::

::: versionchanged
3.12 Added the *timeout* parameter.
:::
:::::::

### Scheduling callbacks

::: method
loop.call_soon(callback, \*args, context=None)

Schedule the *callback* `callback`{.interpreted-text role="term"} to be called with *args* arguments at the next iteration of the event loop.

Return an instance of `asyncio.Handle`{.interpreted-text role="class"}, which can be used later to cancel the callback.

Callbacks are called in the order in which they are registered. Each callback will be called exactly once.

The optional keyword-only *context* argument specifies a custom `contextvars.Context`{.interpreted-text role="class"} for the *callback* to run in. Callbacks use the current context when no *context* is provided.

Unlike `call_soon_threadsafe`{.interpreted-text role="meth"}, this method is not thread-safe.
:::

:::: method
loop.call_soon_threadsafe(callback, \*args, context=None)

A thread-safe variant of `call_soon`{.interpreted-text role="meth"}. When scheduling callbacks from another thread, this function *must* be used, since `call_soon`{.interpreted-text role="meth"} is not thread-safe.

This function is safe to be called from a reentrant context or signal handler, however, it is not safe or fruitful to use the returned handle in such contexts.

Raises `RuntimeError`{.interpreted-text role="exc"} if called on a loop that\'s been closed. This can happen on a secondary thread when the main application is shutting down.

See the `concurrency and multithreading <asyncio-multithreading>`{.interpreted-text role="ref"} section of the documentation.

::: versionchanged
3.7 The *context* keyword-only parameter was added. See `567`{.interpreted-text role="pep"} for more details.
:::
::::

::::: {#asyncio-pass-keywords}
:::: note
::: title
Note
:::

Most `asyncio`{.interpreted-text role="mod"} scheduling functions don\'t allow passing keyword arguments. To do that, use `functools.partial`{.interpreted-text role="func"}:

    # will schedule "print("Hello", flush=True)"
    loop.call_soon(
        functools.partial(print, "Hello", flush=True))

Using partial objects is usually more convenient than using lambdas, as asyncio can render partial objects better in debug and error messages.
::::
:::::

### Scheduling delayed callbacks {#asyncio-delayed-calls}

Event loop provides mechanisms to schedule callback functions to be called at some point in the future. Event loop uses monotonic clocks to track time.

::::::: method
loop.call_later(delay, callback, \*args, context=None)

Schedule *callback* to be called after the given *delay* number of seconds (can be either an int or a float).

An instance of `asyncio.TimerHandle`{.interpreted-text role="class"} is returned which can be used to cancel the callback.

*callback* will be called exactly once. If two callbacks are scheduled for exactly the same time, the order in which they are called is undefined.

The optional positional *args* will be passed to the callback when it is called. Use `functools.partial`{.interpreted-text role="func"} `to pass keyword arguments <asyncio-pass-keywords>`{.interpreted-text role="ref"} to *callback*.

An optional keyword-only *context* argument allows specifying a custom `contextvars.Context`{.interpreted-text role="class"} for the *callback* to run in. The current context is used when no *context* is provided.

:::: note
::: title
Note
:::

For performance, callbacks scheduled with `loop.call_later`{.interpreted-text role="meth"} may run up to one clock-resolution early (see `time.get_clock_info('monotonic').resolution`).
::::

::: versionchanged
3.7 The *context* keyword-only parameter was added. See `567`{.interpreted-text role="pep"} for more details.
:::

::: versionchanged
3.8 In Python 3.7 and earlier with the default event loop implementation, the *delay* could not exceed one day. This has been fixed in Python 3.8.
:::
:::::::

::::::: method
loop.call_at(when, callback, \*args, context=None)

Schedule *callback* to be called at the given absolute timestamp *when* (an int or a float), using the same time reference as `loop.time`{.interpreted-text role="meth"}.

This method\'s behavior is the same as `call_later`{.interpreted-text role="meth"}.

An instance of `asyncio.TimerHandle`{.interpreted-text role="class"} is returned which can be used to cancel the callback.

:::: note
::: title
Note
:::

For performance, callbacks scheduled with `loop.call_at`{.interpreted-text role="meth"} may run up to one clock-resolution early (see `time.get_clock_info('monotonic').resolution`).
::::

::: versionchanged
3.7 The *context* keyword-only parameter was added. See `567`{.interpreted-text role="pep"} for more details.
:::

::: versionchanged
3.8 In Python 3.7 and earlier with the default event loop implementation, the difference between *when* and the current time could not exceed one day. This has been fixed in Python 3.8.
:::
:::::::

::: method
loop.time()

Return the current time, as a `float`{.interpreted-text role="class"} value, according to the event loop\'s internal monotonic clock.
:::

:::: note
::: title
Note
:::

.. versionchanged:: 3.8 In Python 3.7 and earlier timeouts (relative *delay* or absolute *when*) should not exceed one day. This has been fixed in Python 3.8.
::::

::: seealso
The `asyncio.sleep`{.interpreted-text role="func"} function.
:::

### Creating Futures and Tasks

:::: method
loop.create_future()

Create an `asyncio.Future`{.interpreted-text role="class"} object attached to the event loop.

This is the preferred way to create Futures in asyncio. This lets third-party event loops provide alternative implementations of the Future object (with better performance or instrumentation).

::: versionadded
3.5.2
:::
::::

:::::::: method
loop.create_task(coro, *, name=None, context=None, eager_start=None,*\*kwargs)

Schedule the execution of `coroutine <coroutine>`{.interpreted-text role="ref"} *coro*. Return a `Task`{.interpreted-text role="class"} object.

Third-party event loops can use their own subclass of `Task`{.interpreted-text role="class"} for interoperability. In this case, the result type is a subclass of `Task`{.interpreted-text role="class"}.

The full function signature is largely the same as that of the `Task`{.interpreted-text role="class"} constructor (or factory) - all of the keyword arguments to this function are passed through to that interface.

If the *name* argument is provided and not `None`, it is set as the name of the task using `Task.set_name`{.interpreted-text role="meth"}.

An optional keyword-only *context* argument allows specifying a custom `contextvars.Context`{.interpreted-text role="class"} for the *coro* to run in. The current context copy is created when no *context* is provided.

An optional keyword-only *eager_start* argument allows specifying if the task should execute eagerly during the call to create_task, or be scheduled later. If *eager_start* is not passed the mode set by `loop.set_task_factory`{.interpreted-text role="meth"} will be used.

::: versionchanged
3.8 Added the *name* parameter.
:::

::: versionchanged
3.11 Added the *context* parameter.
:::

::: versionchanged
3.13.3 Added `kwargs` which passes on arbitrary extra parameters, including `name` and `context`.
:::

::: versionchanged
3.13.4 Rolled back the change that passes on *name* and *context* (if it is None), while still passing on other arbitrary keyword arguments (to avoid breaking backwards compatibility with 3.13.3).
:::

::: versionchanged
3.14 All *kwargs* are now passed on. The *eager_start* parameter works with eager task factories.
:::
::::::::

:::::: method
loop.set_task_factory(factory)

Set a task factory that will be used by `loop.create_task`{.interpreted-text role="meth"}.

If *factory* is `None` the default task factory will be set. Otherwise, *factory* must be a *callable* with the signature matching `(loop, coro, **kwargs)`, where *loop* is a reference to the active event loop, and *coro* is a coroutine object. The callable must pass on all *kwargs*, and return a `asyncio.Task`{.interpreted-text role="class"}-compatible object.

::: versionchanged
3.13.3 Required that all *kwargs* are passed on to `asyncio.Task`{.interpreted-text role="class"}.
:::

:::: versionchanged
3.13.4 *name* is no longer passed to task factories. *context* is no longer passed to task factories if it is `None`.

::: versionchanged
3.14 *name* and *context* are now unconditionally passed on to task factories again.
:::
::::
::::::

::: method
loop.get_task_factory()

Return a task factory or `None` if the default one is in use.
:::

### Opening network connections

:::::::::: {.method async=""}
loop.create_connection(protocol_factory, host=None, port=None, \*, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, happy_eyeballs_delay=None, interleave=None, all_errors=False)

Open a streaming transport connection to a given address specified by *host* and *port*.

The socket family can be either `~socket.AF_INET`{.interpreted-text role="py:const"} or `~socket.AF_INET6`{.interpreted-text role="py:const"} depending on *host* (or the *family* argument, if provided).

The socket type will be `~socket.SOCK_STREAM`{.interpreted-text role="py:const"}.

*protocol_factory* must be a callable returning an `asyncio protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

This method will try to establish the connection in the background. When successful, it returns a `(transport, protocol)` pair.

The chronological synopsis of the underlying operation is as follows:

1.  The connection is established and a `transport <asyncio-transport>`{.interpreted-text role="ref"} is created for it.
2.  *protocol_factory* is called without arguments and is expected to return a `protocol <asyncio-protocol>`{.interpreted-text role="ref"} instance.
3.  The protocol instance is coupled with the transport by calling its `~BaseProtocol.connection_made`{.interpreted-text role="meth"} method.
4.  A `(transport, protocol)` tuple is returned on success.

The created transport is an implementation-dependent bidirectional stream.

Other arguments:

- *ssl*: if given and not false, a SSL/TLS transport is created (by default a plain TCP transport is created). If *ssl* is a `ssl.SSLContext`{.interpreted-text role="class"} object, this context is used to create the transport; if *ssl* is `True`{.interpreted-text role="const"}, a default context returned from `ssl.create_default_context`{.interpreted-text role="func"} is used.

  ::: seealso
  `SSL/TLS security considerations <ssl-security>`{.interpreted-text role="ref"}
  :::

- *server_hostname* sets or overrides the hostname that the target server\'s certificate will be matched against. Should only be passed if *ssl* is not `None`. By default the value of the *host* argument is used. If *host* is empty, there is no default and you must pass a value for *server_hostname*. If *server_hostname* is an empty string, hostname matching is disabled (which is a serious security risk, allowing for potential man-in-the-middle attacks).

- *family*, *proto*, *flags* are the optional address family, protocol and flags to be passed through to getaddrinfo() for *host* resolution. If given, these should all be integers from the corresponding `socket`{.interpreted-text role="mod"} module constants.

- *happy_eyeballs_delay*, if given, enables Happy Eyeballs for this connection. It should be a floating-point number representing the amount of time in seconds to wait for a connection attempt to complete, before starting the next attempt in parallel. This is the \"Connection Attempt Delay\" as defined in `8305`{.interpreted-text role="rfc"}. A sensible default value recommended by the RFC is `0.25` (250 milliseconds).

- *interleave* controls address reordering when a host name resolves to multiple IP addresses. If `0` or unspecified, no reordering is done, and addresses are tried in the order returned by `getaddrinfo`{.interpreted-text role="meth"}. If a positive integer is specified, the addresses are interleaved by address family, and the given integer is interpreted as \"First Address Family Count\" as defined in `8305`{.interpreted-text role="rfc"}. The default is `0` if *happy_eyeballs_delay* is not specified, and `1` if it is.

- *sock*, if given, should be an existing, already connected `socket.socket`{.interpreted-text role="class"} object to be used by the transport. If *sock* is given, none of *host*, *port*, *family*, *proto*, *flags*, *happy_eyeballs_delay*, *interleave* and *local_addr* should be specified.

  :::: note
  ::: title
  Note
  :::

  The *sock* argument transfers ownership of the socket to the transport created. To close the socket, call the transport\'s `~asyncio.BaseTransport.close`{.interpreted-text role="meth"} method.
  ::::

- *local_addr*, if given, is a `(local_host, local_port)` tuple used to bind the socket locally. The *local_host* and *local_port* are looked up using `getaddrinfo()`, similarly to *host* and *port*.

- *ssl_handshake_timeout* is (for a TLS connection) the time in seconds to wait for the TLS handshake to complete before aborting the connection. `60.0` seconds if `None` (default).

- *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown to complete before aborting the connection. `30.0` seconds if `None` (default).

- *all_errors* determines what exceptions are raised when a connection cannot be created. By default, only a single `Exception` is raised: the first exception if there is only one or all errors have same message, or a single `OSError` with the error messages combined. When `all_errors` is `True`, an `ExceptionGroup` will be raised containing all exceptions (even if there is only one).

::: versionchanged
3.5

Added support for SSL/TLS in `ProactorEventLoop`{.interpreted-text role="class"}.
:::

::: versionchanged
3.6

The socket option `socket.TCP_NODELAY <socket-unix-constants>`{.interpreted-text role="ref"} is set by default for all TCP connections.
:::

::: versionchanged
3.7

Added the *ssl_handshake_timeout* parameter.
:::

::: versionchanged
3.8

Added the *happy_eyeballs_delay* and *interleave* parameters.

Happy Eyeballs Algorithm: Success with Dual-Stack Hosts. When a server\'s IPv4 path and protocol are working, but the server\'s IPv6 path and protocol are not working, a dual-stack client application experiences significant connection delay compared to an IPv4-only client. This is undesirable because it causes the dual-stack client to have a worse user experience. This document specifies requirements for algorithms that reduce this user-visible delay and provides an algorithm.

For more information: <https://datatracker.ietf.org/doc/html/rfc6555>
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::

::: versionchanged
3.12 *all_errors* was added.
:::

::: seealso
The `open_connection`{.interpreted-text role="func"} function is a high-level alternative API. It returns a pair of (`StreamReader`{.interpreted-text role="class"}, `StreamWriter`{.interpreted-text role="class"}) that can be used directly in async/await code.
:::
::::::::::

::::::: {.method async=""}
loop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, \*, family=0, proto=0, flags=0, reuse_port=None, allow_broadcast=None, sock=None)

Create a datagram connection.

The socket family can be either `~socket.AF_INET`{.interpreted-text role="py:const"}, `~socket.AF_INET6`{.interpreted-text role="py:const"}, or `~socket.AF_UNIX`{.interpreted-text role="py:const"}, depending on *host* (or the *family* argument, if provided).

The socket type will be `~socket.SOCK_DGRAM`{.interpreted-text role="py:const"}.

*protocol_factory* must be a callable returning a `protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

A tuple of `(transport, protocol)` is returned on success.

Other arguments:

- *local_addr*, if given, is a `(local_host, local_port)` tuple used to bind the socket locally. The *local_host* and *local_port* are looked up using `getaddrinfo`{.interpreted-text role="meth"}.

  :::: note
  ::: title
  Note
  :::

  On Windows, when using the proactor event loop with `local_addr=None`, an `OSError`{.interpreted-text role="exc"} with `!errno.WSAEINVAL`{.interpreted-text role="attr"} will be raised when running it.
  ::::

- *remote_addr*, if given, is a `(remote_host, remote_port)` tuple used to connect the socket to a remote address. The *remote_host* and *remote_port* are looked up using `getaddrinfo`{.interpreted-text role="meth"}.

- *family*, *proto*, *flags* are the optional address family, protocol and flags to be passed through to `getaddrinfo`{.interpreted-text role="meth"} for *host* resolution. If given, these should all be integers from the corresponding `socket`{.interpreted-text role="mod"} module constants.

- *reuse_port* tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows and some Unixes. If the `socket.SO_REUSEPORT <socket-unix-constants>`{.interpreted-text role="ref"} constant is not defined then this capability is unsupported.

- *allow_broadcast* tells the kernel to allow this endpoint to send messages to the broadcast address.

- *sock* can optionally be specified in order to use a preexisting, already connected, `socket.socket`{.interpreted-text role="class"} object to be used by the transport. If specified, *local_addr* and *remote_addr* should be omitted (must be `None`{.interpreted-text role="const"}).

  :::: note
  ::: title
  Note
  :::

  The *sock* argument transfers ownership of the socket to the transport created. To close the socket, call the transport\'s `~asyncio.BaseTransport.close`{.interpreted-text role="meth"} method.
  ::::

See `UDP echo client protocol <asyncio-udp-echo-client-protocol>`{.interpreted-text role="ref"} and `UDP echo server protocol <asyncio-udp-echo-server-protocol>`{.interpreted-text role="ref"} examples.

::: versionchanged
3.4.4 The *family*, *proto*, *flags*, *reuse_address*, *reuse_port*, *allow_broadcast*, and *sock* parameters were added.
:::

::: versionchanged
3.8 Added support for Windows.
:::

::: versionchanged
3.8.1 The *reuse_address* parameter is no longer supported, as using `socket.SO_REUSEADDR <socket-unix-constants>`{.interpreted-text role="ref"} poses a significant security concern for UDP. Explicitly passing `reuse_address=True` will raise an exception.

When multiple processes with differing UIDs assign sockets to an identical UDP socket address with `SO_REUSEADDR`, incoming packets can become randomly distributed among the sockets.

For supported platforms, *reuse_port* can be used as a replacement for similar functionality. With *reuse_port*, `socket.SO_REUSEPORT <socket-unix-constants>`{.interpreted-text role="ref"} is used instead, which specifically prevents processes with differing UIDs from assigning sockets to the same socket address.
:::

::: versionchanged
3.11 The *reuse_address* parameter, disabled since Python 3.8.1, 3.7.6 and 3.6.10, has been entirely removed.
:::
:::::::

:::::: {.method async=""}
loop.create_unix_connection(protocol_factory, path=None, \*, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None)

Create a Unix connection.

The socket family will be `~socket.AF_UNIX`{.interpreted-text role="py:const"}; socket type will be `~socket.SOCK_STREAM`{.interpreted-text role="py:const"}.

A tuple of `(transport, protocol)` is returned on success.

*path* is the name of a Unix domain socket and is required, unless a *sock* parameter is specified. Abstract Unix sockets, `str`{.interpreted-text role="class"}, `bytes`{.interpreted-text role="class"}, and `~pathlib.Path`{.interpreted-text role="class"} paths are supported.

See the documentation of the `loop.create_connection`{.interpreted-text role="meth"} method for information about arguments to this method.

::: availability
Unix.
:::

::: versionchanged
3.7 Added the *ssl_handshake_timeout* parameter. The *path* parameter can now be a `path-like object`{.interpreted-text role="term"}.
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::
::::::

### Creating network servers

:::::::::: {#loop_create_server}
::::::::: {.method async=""}
loop.create_server(protocol_factory, host=None, port=None, \*, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None, keep_alive=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, start_serving=True)

Create a TCP server (socket type `~socket.SOCK_STREAM`{.interpreted-text role="const"}) listening on *port* of the *host* address.

Returns a `Server`{.interpreted-text role="class"} object.

Arguments:

- *protocol_factory* must be a callable returning a `protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

- The *host* parameter can be set to several types which determine where the server would be listening:

  - If *host* is a string, the TCP server is bound to a single network interface specified by *host*.
  - If *host* is a sequence of strings, the TCP server is bound to all network interfaces specified by the sequence.
  - If *host* is an empty string or `None`, all interfaces are assumed and a list of multiple sockets will be returned (most likely one for IPv4 and another one for IPv6).

- The *port* parameter can be set to specify which port the server should listen on. If `0` or `None` (the default), a random unused port will be selected (note that if *host* resolves to multiple network interfaces, a different random port will be selected for each interface).

- *family* can be set to either `socket.AF_INET`{.interpreted-text role="const"} or `~socket.AF_INET6`{.interpreted-text role="const"} to force the socket to use IPv4 or IPv6. If not set, the *family* will be determined from host name (defaults to `~socket.AF_UNSPEC`{.interpreted-text role="const"}).

- *flags* is a bitmask for `getaddrinfo`{.interpreted-text role="meth"}.

- *sock* can optionally be specified in order to use a preexisting socket object. If specified, *host* and *port* must not be specified.

  :::: note
  ::: title
  Note
  :::

  The *sock* argument transfers ownership of the socket to the server created. To close the socket, call the server\'s `~asyncio.Server.close`{.interpreted-text role="meth"} method.
  ::::

- *backlog* is the maximum number of queued connections passed to `~socket.socket.listen`{.interpreted-text role="meth"} (defaults to 100).

- *ssl* can be set to an `~ssl.SSLContext`{.interpreted-text role="class"} instance to enable TLS over the accepted connections.

- *reuse_address* tells the kernel to reuse a local socket in `TIME_WAIT` state, without waiting for its natural timeout to expire. If not specified will automatically be set to `True` on Unix.

- *reuse_port* tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows.

- *keep_alive* set to `True` keeps connections active by enabling the periodic transmission of messages.

::: versionchanged
3.13

Added the *keep_alive* parameter.
:::

- *ssl_handshake_timeout* is (for a TLS server) the time in seconds to wait for the TLS handshake to complete before aborting the connection. `60.0` seconds if `None` (default).
- *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown to complete before aborting the connection. `30.0` seconds if `None` (default).
- *start_serving* set to `True` (the default) causes the created server to start accepting connections immediately. When set to `False`, the user should await on `Server.start_serving`{.interpreted-text role="meth"} or `Server.serve_forever`{.interpreted-text role="meth"} to make the server to start accepting connections.

::: versionchanged
3.5

Added support for SSL/TLS in `ProactorEventLoop`{.interpreted-text role="class"}.
:::

::: versionchanged
3.5.1

The *host* parameter can be a sequence of strings.
:::

::: versionchanged
3.6

Added *ssl_handshake_timeout* and *start_serving* parameters. The socket option `socket.TCP_NODELAY <socket-unix-constants>`{.interpreted-text role="ref"} is set by default for all TCP connections.
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::

::: seealso
The `start_server`{.interpreted-text role="func"} function is a higher-level alternative API that returns a pair of `StreamReader`{.interpreted-text role="class"} and `StreamWriter`{.interpreted-text role="class"} that can be used in an async/await code.
:::
:::::::::
::::::::::

::::::: {.method async=""}
loop.create_unix_server(protocol_factory, path=None, \*, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, start_serving=True, cleanup_socket=True)

Similar to `loop.create_server`{.interpreted-text role="meth"} but works with the `~socket.AF_UNIX`{.interpreted-text role="py:const"} socket family.

*path* is the name of a Unix domain socket, and is required, unless a *sock* argument is provided. Abstract Unix sockets, `str`{.interpreted-text role="class"}, `bytes`{.interpreted-text role="class"}, and `~pathlib.Path`{.interpreted-text role="class"} paths are supported.

If *cleanup_socket* is true then the Unix socket will automatically be removed from the filesystem when the server is closed, unless the socket has been replaced after the server has been created.

See the documentation of the `loop.create_server`{.interpreted-text role="meth"} method for information about arguments to this method.

::: availability
Unix.
:::

::: versionchanged
3.7

Added the *ssl_handshake_timeout* and *start_serving* parameters. The *path* parameter can now be a `~pathlib.Path`{.interpreted-text role="class"} object.
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::

::: versionchanged
3.13

Added the *cleanup_socket* parameter.
:::
:::::::

:::::: {.method async=""}
loop.connect_accepted_socket(protocol_factory, sock, \*, ssl=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None)

Wrap an already accepted connection into a transport/protocol pair.

This method can be used by servers that accept connections outside of asyncio but that use asyncio to handle them.

Parameters:

- *protocol_factory* must be a callable returning a `protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

- *sock* is a preexisting socket object returned from `socket.accept <socket.socket.accept>`{.interpreted-text role="meth"}.

  :::: note
  ::: title
  Note
  :::

  The *sock* argument transfers ownership of the socket to the transport created. To close the socket, call the transport\'s `~asyncio.BaseTransport.close`{.interpreted-text role="meth"} method.
  ::::

- *ssl* can be set to an `~ssl.SSLContext`{.interpreted-text role="class"} to enable SSL over the accepted connections.

- *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to wait for the SSL handshake to complete before aborting the connection. `60.0` seconds if `None` (default).

- *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown to complete before aborting the connection. `30.0` seconds if `None` (default).

Returns a `(transport, protocol)` pair.

::: versionadded
3.5.3
:::

::: versionchanged
3.7

Added the *ssl_handshake_timeout* parameter.
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::
::::::

### Transferring files

:::: {.method async=""}
loop.sendfile(transport, file, offset=0, count=None, \*, fallback=True)

Send a *file* over a *transport*. Return the total number of bytes sent.

The method uses high-performance `os.sendfile`{.interpreted-text role="meth"} if available.

*file* must be a regular file object opened in binary mode.

*offset* tells from where to start reading the file. If specified, *count* is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is always updated, even when this method raises an error, and `file.tell() <io.IOBase.tell>`{.interpreted-text role="meth"} can be used to obtain the actual number of bytes sent.

*fallback* set to `True` makes asyncio to manually read and send the file when the platform does not support the sendfile system call (e.g. Windows or SSL socket on Unix).

Raise `SendfileNotAvailableError`{.interpreted-text role="exc"} if the system does not support the *sendfile* syscall and *fallback* is `False`.

::: versionadded
3.7
:::
::::

### TLS Upgrade

::::: {.method async=""}
loop.start_tls(transport, protocol, sslcontext, \*, server_side=False, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None)

Upgrade an existing transport-based connection to TLS.

Create a TLS coder/decoder instance and insert it between the *transport* and the *protocol*. The coder/decoder implements both *transport*-facing protocol and *protocol*-facing transport.

Return the created two-interface instance. After *await*, the *protocol* must stop using the original *transport* and communicate with the returned object only because the coder caches *protocol*-side data and sporadically exchanges extra TLS session packets with *transport*.

In some situations (e.g. when the passed transport is already closing) this may return `None`.

Parameters:

- *transport* and *protocol* instances that methods like `~loop.create_server`{.interpreted-text role="meth"} and `~loop.create_connection`{.interpreted-text role="meth"} return.
- *sslcontext*: a configured instance of `~ssl.SSLContext`{.interpreted-text role="class"}.
- *server_side* pass `True` when a server-side connection is being upgraded (like the one created by `~loop.create_server`{.interpreted-text role="meth"}).
- *server_hostname*: sets or overrides the host name that the target server\'s certificate will be matched against.
- *ssl_handshake_timeout* is (for a TLS connection) the time in seconds to wait for the TLS handshake to complete before aborting the connection. `60.0` seconds if `None` (default).
- *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown to complete before aborting the connection. `30.0` seconds if `None` (default).

::: versionadded
3.7
:::

::: versionchanged
3.11

Added the *ssl_shutdown_timeout* parameter.
:::
:::::

### Watching file descriptors

::: method
loop.add_reader(fd, callback, \*args)

Start monitoring the *fd* file descriptor for read availability and invoke *callback* with the specified arguments once *fd* is available for reading.

Any preexisting callback registered for *fd* is cancelled and replaced by *callback*.
:::

::: method
loop.remove_reader(fd)

Stop monitoring the *fd* file descriptor for read availability. Returns `True` if *fd* was previously being monitored for reads.
:::

::: method
loop.add_writer(fd, callback, \*args)

Start monitoring the *fd* file descriptor for write availability and invoke *callback* with the specified arguments *args* once *fd* is available for writing.

Any preexisting callback registered for *fd* is cancelled and replaced by *callback*.

Use `functools.partial`{.interpreted-text role="func"} `to pass keyword arguments
<asyncio-pass-keywords>`{.interpreted-text role="ref"} to *callback*.
:::

::: method
loop.remove_writer(fd)

Stop monitoring the *fd* file descriptor for write availability. Returns `True` if *fd* was previously being monitored for writes.
:::

See also `Platform Support <asyncio-platform-support>`{.interpreted-text role="ref"} section for some limitations of these methods.

### Working with socket objects directly

In general, protocol implementations that use transport-based APIs such as `loop.create_connection`{.interpreted-text role="meth"} and `loop.create_server`{.interpreted-text role="meth"} are faster than implementations that work with sockets directly. However, there are some use cases when performance is not critical, and working with `~socket.socket`{.interpreted-text role="class"} objects directly is more convenient.

:::: {.method async=""}
loop.sock_recv(sock, nbytes)

Receive up to *nbytes* from *sock*. Asynchronous version of `socket.recv() <socket.socket.recv>`{.interpreted-text role="meth"}.

Return the received data as a bytes object.

*sock* must be a non-blocking socket.

::: versionchanged
3.7 Even though this method was always documented as a coroutine method, releases before Python 3.7 returned a `Future`{.interpreted-text role="class"}. Since Python 3.7 this is an `async def` method.
:::
::::

:::: {.method async=""}
loop.sock_recv_into(sock, buf)

Receive data from *sock* into the *buf* buffer. Modeled after the blocking `socket.recv_into() <socket.socket.recv_into>`{.interpreted-text role="meth"} method.

Return the number of bytes written to the buffer.

*sock* must be a non-blocking socket.

::: versionadded
3.7
:::
::::

:::: {.method async=""}
loop.sock_recvfrom(sock, bufsize)

Receive a datagram of up to *bufsize* from *sock*. Asynchronous version of `socket.recvfrom() <socket.socket.recvfrom>`{.interpreted-text role="meth"}.

Return a tuple of (received data, remote address).

*sock* must be a non-blocking socket.

::: versionadded
3.11
:::
::::

:::: {.method async=""}
loop.sock_recvfrom_into(sock, buf, nbytes=0)

Receive a datagram of up to *nbytes* from *sock* into *buf*. Asynchronous version of `socket.recvfrom_into() <socket.socket.recvfrom_into>`{.interpreted-text role="meth"}.

Return a tuple of (number of bytes received, remote address).

*sock* must be a non-blocking socket.

::: versionadded
3.11
:::
::::

:::: {.method async=""}
loop.sock_sendall(sock, data)

Send *data* to the *sock* socket. Asynchronous version of `socket.sendall() <socket.socket.sendall>`{.interpreted-text role="meth"}.

This method continues to send to the socket until either all data in *data* has been sent or an error occurs. `None` is returned on success. On error, an exception is raised. Additionally, there is no way to determine how much data, if any, was successfully processed by the receiving end of the connection.

*sock* must be a non-blocking socket.

::: versionchanged
3.7 Even though the method was always documented as a coroutine method, before Python 3.7 it returned a `Future`{.interpreted-text role="class"}. Since Python 3.7, this is an `async def` method.
:::
::::

:::: {.method async=""}
loop.sock_sendto(sock, data, address)

Send a datagram from *sock* to *address*. Asynchronous version of `socket.sendto() <socket.socket.sendto>`{.interpreted-text role="meth"}.

Return the number of bytes sent.

*sock* must be a non-blocking socket.

::: versionadded
3.11
:::
::::

::::: {.method async=""}
loop.sock_connect(sock, address)

Connect *sock* to a remote socket at *address*.

Asynchronous version of `socket.connect() <socket.socket.connect>`{.interpreted-text role="meth"}.

*sock* must be a non-blocking socket.

::: versionchanged
3.5.2 `address` no longer needs to be resolved. `sock_connect` will try to check if the *address* is already resolved by calling `socket.inet_pton`{.interpreted-text role="func"}. If not, `loop.getaddrinfo`{.interpreted-text role="meth"} will be used to resolve the *address*.
:::

::: seealso
`loop.create_connection`{.interpreted-text role="meth"} and `asyncio.open_connection() <open_connection>`{.interpreted-text role="func"}.
:::
:::::

::::: {.method async=""}
loop.sock_accept(sock)

Accept a connection. Modeled after the blocking `socket.accept() <socket.socket.accept>`{.interpreted-text role="meth"} method.

The socket must be bound to an address and listening for connections. The return value is a pair `(conn, address)` where *conn* is a *new* socket object usable to send and receive data on the connection, and *address* is the address bound to the socket on the other end of the connection.

*sock* must be a non-blocking socket.

::: versionchanged
3.7 Even though the method was always documented as a coroutine method, before Python 3.7 it returned a `Future`{.interpreted-text role="class"}. Since Python 3.7, this is an `async def` method.
:::

::: seealso
`loop.create_server`{.interpreted-text role="meth"} and `start_server`{.interpreted-text role="func"}.
:::
:::::

:::: {.method async=""}
loop.sock_sendfile(sock, file, offset=0, count=None, \*, fallback=True)

Send a file using high-performance `os.sendfile`{.interpreted-text role="mod"} if possible. Return the total number of bytes sent.

Asynchronous version of `socket.sendfile() <socket.socket.sendfile>`{.interpreted-text role="meth"}.

*sock* must be a non-blocking `socket.SOCK_STREAM`{.interpreted-text role="const"} `~socket.socket`{.interpreted-text role="class"}.

*file* must be a regular file object open in binary mode.

*offset* tells from where to start reading the file. If specified, *count* is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is always updated, even when this method raises an error, and `file.tell() <io.IOBase.tell>`{.interpreted-text role="meth"} can be used to obtain the actual number of bytes sent.

*fallback*, when set to `True`, makes asyncio manually read and send the file when the platform does not support the sendfile syscall (e.g. Windows or SSL socket on Unix).

Raise `SendfileNotAvailableError`{.interpreted-text role="exc"} if the system does not support *sendfile* syscall and *fallback* is `False`.

*sock* must be a non-blocking socket.

::: versionadded
3.7
:::
::::

### DNS

::: {.method async=""}
loop.getaddrinfo(host, port, \*, family=0, type=0, proto=0, flags=0)

Asynchronous version of `socket.getaddrinfo`{.interpreted-text role="meth"}.
:::

::: {.method async=""}
loop.getnameinfo(sockaddr, flags=0)

Asynchronous version of `socket.getnameinfo`{.interpreted-text role="meth"}.
:::

:::: note
::: title
Note
:::

Both *getaddrinfo* and *getnameinfo* internally utilize their synchronous versions through the loop\'s default thread pool executor. When this executor is saturated, these methods may experience delays, which higher-level networking libraries may report as increased timeouts. To mitigate this, consider using a custom executor for other user tasks, or setting a default executor with a larger number of workers.
::::

::: versionchanged
3.7 Both *getaddrinfo* and *getnameinfo* methods were always documented to return a coroutine, but prior to Python 3.7 they were, in fact, returning `asyncio.Future`{.interpreted-text role="class"} objects. Starting with Python 3.7 both methods are coroutines.
:::

### Working with pipes

::: {.method async=""}
loop.connect_read_pipe(protocol_factory, pipe)

Register the read end of *pipe* in the event loop.

*protocol_factory* must be a callable returning an `asyncio protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

*pipe* is a `file-like object <file object>`{.interpreted-text role="term"}.

Return pair `(transport, protocol)`, where *transport* supports the `ReadTransport`{.interpreted-text role="class"} interface and *protocol* is an object instantiated by the *protocol_factory*.

With `SelectorEventLoop`{.interpreted-text role="class"} event loop, the *pipe* is set to non-blocking mode.
:::

::: {.method async=""}
loop.connect_write_pipe(protocol_factory, pipe)

Register the write end of *pipe* in the event loop.

*protocol_factory* must be a callable returning an `asyncio protocol <asyncio-protocol>`{.interpreted-text role="ref"} implementation.

*pipe* is `file-like object <file object>`{.interpreted-text role="term"}.

Return pair `(transport, protocol)`, where *transport* supports `WriteTransport`{.interpreted-text role="class"} interface and *protocol* is an object instantiated by the *protocol_factory*.

With `SelectorEventLoop`{.interpreted-text role="class"} event loop, the *pipe* is set to non-blocking mode.
:::

:::: note
::: title
Note
:::

`SelectorEventLoop`{.interpreted-text role="class"} does not support the above methods on Windows. Use `ProactorEventLoop`{.interpreted-text role="class"} instead for Windows.
::::

::: seealso
The `loop.subprocess_exec`{.interpreted-text role="meth"} and `loop.subprocess_shell`{.interpreted-text role="meth"} methods.
:::

### Unix signals

:::: {#loop_add_signal_handler}
::: method
loop.add_signal_handler(signum, callback, \*args)

Set *callback* as the handler for the *signum* signal, passing *args* as positional arguments.

The callback will be invoked by *loop*, along with other queued callbacks and runnable coroutines of that event loop. Unlike signal handlers registered using `signal.signal`{.interpreted-text role="func"}, a callback registered with this function is allowed to interact with the event loop.

Raise `ValueError`{.interpreted-text role="exc"} if the signal number is invalid or uncatchable. Raise `RuntimeError`{.interpreted-text role="exc"} if there is a problem setting up the handler.

Use `functools.partial`{.interpreted-text role="func"} `to pass keyword arguments
<asyncio-pass-keywords>`{.interpreted-text role="ref"} to *callback*.

Like `signal.signal`{.interpreted-text role="func"}, this function must be invoked in the main thread.
:::
::::

:::: method
loop.remove_signal_handler(sig)

Remove the handler for the *sig* signal.

Return `True` if the signal handler was removed, or `False` if no handler was set for the given signal.

::: availability
Unix.
:::
::::

::: seealso
The `signal`{.interpreted-text role="mod"} module.
:::

### Executing code in thread or process pools

:::: awaitablemethod
loop.run_in_executor(executor, func, \*args)

Arrange for *func* to be called in the specified executor passing *args* as positional arguments.

The *executor* argument should be an `concurrent.futures.Executor`{.interpreted-text role="class"} instance. The default executor is used if *executor* is `None`. The default executor can be set by `loop.set_default_executor`{.interpreted-text role="meth"}, otherwise, a `concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="class"} will be lazy-initialized and used by `run_in_executor`{.interpreted-text role="func"} if needed.

Example:

    import asyncio
    import concurrent.futures

    def blocking_io():
        # File operations (such as logging) can block the
        # event loop: run them in a thread pool.
        with open('/dev/urandom', 'rb') as f:
            return f.read(100)

    def cpu_bound():
        # CPU-bound operations will block the event loop:
        # in general it is preferable to run them in a
        # process pool.
        return sum(i * i for i in range(10 ** 7))

    async def main():
        loop = asyncio.get_running_loop()

        ## Options:

        # 1. Run in the default loop's executor:
        result = await loop.run_in_executor(
            None, blocking_io)
        print('default thread pool', result)

        # 2. Run in a custom thread pool:
        with concurrent.futures.ThreadPoolExecutor() as pool:
            result = await loop.run_in_executor(
                pool, blocking_io)
            print('custom thread pool', result)

        # 3. Run in a custom process pool:
        with concurrent.futures.ProcessPoolExecutor() as pool:
            result = await loop.run_in_executor(
                pool, cpu_bound)
            print('custom process pool', result)

        # 4. Run in a custom interpreter pool:
        with concurrent.futures.InterpreterPoolExecutor() as pool:
            result = await loop.run_in_executor(
                pool, cpu_bound)
            print('custom interpreter pool', result)

    if __name__ == '__main__':
        asyncio.run(main())

Note that the entry point guard (`if __name__ == '__main__'`) is required for option 3 due to the peculiarities of `multiprocessing`{.interpreted-text role="mod"}, which is used by `~concurrent.futures.ProcessPoolExecutor`{.interpreted-text role="class"}. See `Safe importing of main module <multiprocessing-safe-main-import>`{.interpreted-text role="ref"}.

This method returns a `asyncio.Future`{.interpreted-text role="class"} object.

Use `functools.partial`{.interpreted-text role="func"} `to pass keyword arguments
<asyncio-pass-keywords>`{.interpreted-text role="ref"} to *func*.

::: versionchanged
3.5.3 `loop.run_in_executor`{.interpreted-text role="meth"} no longer configures the `max_workers` of the thread pool executor it creates, instead leaving it up to the thread pool executor (`~concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="class"}) to set the default.
:::
::::

:::: method
loop.set_default_executor(executor)

Set *executor* as the default executor used by `run_in_executor`{.interpreted-text role="meth"}. *executor* must be an instance of `~concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="class"}, which includes `~concurrent.futures.InterpreterPoolExecutor`{.interpreted-text role="class"}.

::: versionchanged
3.11 *executor* must be an instance of `~concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="class"}.
:::
::::

### Error Handling API

Allows customizing how exceptions are handled in the event loop.

:::: method
loop.set_exception_handler(handler)

Set *handler* as the new event loop exception handler.

If *handler* is `None`, the default exception handler will be set. Otherwise, *handler* must be a callable with the signature matching `(loop, context)`, where `loop` is a reference to the active event loop, and `context` is a `dict` object containing the details of the exception (see `call_exception_handler`{.interpreted-text role="meth"} documentation for details about context).

If the handler is called on behalf of a `~asyncio.Task`{.interpreted-text role="class"} or `~asyncio.Handle`{.interpreted-text role="class"}, it is run in the `contextvars.Context`{.interpreted-text role="class"} of that task or callback handle.

::: versionchanged
3.12

The handler may be called in the `~contextvars.Context`{.interpreted-text role="class"} of the task or handle where the exception originated.
:::
::::

:::: method
loop.get_exception_handler()

Return the current exception handler, or `None` if no custom exception handler was set.

::: versionadded
3.5.2
:::
::::

::: method
loop.default_exception_handler(context)

Default exception handler.

This is called when an exception occurs and no exception handler is set. This can be called by a custom exception handler that wants to defer to the default handler behavior.

*context* parameter has the same meaning as in `call_exception_handler`{.interpreted-text role="meth"}.
:::

::::: method
loop.call_exception_handler(context)

Call the current event loop exception handler.

*context* is a `dict` object containing the following keys (new keys may be introduced in future Python versions):

- \'message\': Error message;

- \'exception\' (optional): Exception object;

- \'future\' (optional): `asyncio.Future`{.interpreted-text role="class"} instance;

- \'task\' (optional): `asyncio.Task`{.interpreted-text role="class"} instance;

- \'handle\' (optional): `asyncio.Handle`{.interpreted-text role="class"} instance;

- \'protocol\' (optional): `Protocol <asyncio-protocol>`{.interpreted-text role="ref"} instance;

- \'transport\' (optional): `Transport <asyncio-transport>`{.interpreted-text role="ref"} instance;

- \'socket\' (optional): `socket.socket`{.interpreted-text role="class"} instance;

- \'source_traceback\' (optional): Traceback of the source;

- \'handle_traceback\' (optional): Traceback of the handle;

- 

  \'asyncgen\' (optional): Asynchronous generator that caused

  :   the exception.

:::: note
::: title
Note
:::

This method should not be overloaded in subclassed event loops. For custom exception handling, use the `set_exception_handler`{.interpreted-text role="meth"} method.
::::
:::::

### Enabling debug mode

::: method
loop.get_debug()

Get the debug mode (`bool`{.interpreted-text role="class"}) of the event loop.

The default value is `True` if the environment variable `PYTHONASYNCIODEBUG`{.interpreted-text role="envvar"} is set to a non-empty string, `False` otherwise.
:::

:::: method
loop.set_debug(enabled: bool)

Set the debug mode of the event loop.

::: versionchanged
3.7

The new `Python Development Mode <devmode>`{.interpreted-text role="ref"} can now also be used to enable the debug mode.
:::
::::

::: attribute
loop.slow_callback_duration

This attribute can be used to set the minimum execution duration in seconds that is considered \"slow\". When debug mode is enabled, \"slow\" callbacks are logged.

Default value is 100 milliseconds.
:::

::: seealso
The `debug mode of asyncio <asyncio-debug-mode>`{.interpreted-text role="ref"}.
:::

### Running Subprocesses

Methods described in this subsections are low-level. In regular async/await code consider using the high-level `asyncio.create_subprocess_shell`{.interpreted-text role="func"} and `asyncio.create_subprocess_exec`{.interpreted-text role="func"} convenience functions instead.

:::: note
::: title
Note
:::

On Windows, the default event loop `ProactorEventLoop`{.interpreted-text role="class"} supports subprocesses, whereas `SelectorEventLoop`{.interpreted-text role="class"} does not. See `Subprocess Support on Windows <asyncio-windows-subprocess>`{.interpreted-text role="ref"} for details.
::::

:::: {#loop_subprocess_exec}
::: {.method async=""}
loop.subprocess_exec(protocol_factory, *args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,*\*kwargs)

Create a subprocess from one or more string arguments specified by *args*.

*args* must be a list of strings represented by:

- `str`{.interpreted-text role="class"};
- or `bytes`{.interpreted-text role="class"}, encoded to the `filesystem encoding <filesystem-encoding>`{.interpreted-text role="ref"}.

The first string specifies the program executable, and the remaining strings specify the arguments. Together, string arguments form the `argv` of the program.

This is similar to the standard library `subprocess.Popen`{.interpreted-text role="class"} class called with `shell=False` and the list of strings passed as the first argument; however, where `~subprocess.Popen`{.interpreted-text role="class"} takes a single argument which is list of strings, *subprocess_exec* takes multiple string arguments.

The *protocol_factory* must be a callable returning a subclass of the `asyncio.SubprocessProtocol`{.interpreted-text role="class"} class.

Other parameters:

- *stdin* can be any of these:

  - a file-like object
  - an existing file descriptor (a positive integer), for example those created with `os.pipe`{.interpreted-text role="meth"}
  - the `subprocess.PIPE`{.interpreted-text role="const"} constant (default) which will create a new pipe and connect it,
  - the value `None` which will make the subprocess inherit the file descriptor from this process
  - the `subprocess.DEVNULL`{.interpreted-text role="const"} constant which indicates that the special `os.devnull`{.interpreted-text role="data"} file will be used

- *stdout* can be any of these:

  - a file-like object
  - the `subprocess.PIPE`{.interpreted-text role="const"} constant (default) which will create a new pipe and connect it,
  - the value `None` which will make the subprocess inherit the file descriptor from this process
  - the `subprocess.DEVNULL`{.interpreted-text role="const"} constant which indicates that the special `os.devnull`{.interpreted-text role="data"} file will be used

- *stderr* can be any of these:

  - a file-like object
  - the `subprocess.PIPE`{.interpreted-text role="const"} constant (default) which will create a new pipe and connect it,
  - the value `None` which will make the subprocess inherit the file descriptor from this process
  - the `subprocess.DEVNULL`{.interpreted-text role="const"} constant which indicates that the special `os.devnull`{.interpreted-text role="data"} file will be used
  - the `subprocess.STDOUT`{.interpreted-text role="const"} constant which will connect the standard error stream to the process\' standard output stream

- All other keyword arguments are passed to `subprocess.Popen`{.interpreted-text role="class"} without interpretation, except for *bufsize*, *universal_newlines*, *shell*, *text*, *encoding* and *errors*, which should not be specified at all.

  The `asyncio` subprocess API does not support decoding the streams as text. `bytes.decode`{.interpreted-text role="func"} can be used to convert the bytes returned from the stream to text.

If a file-like object passed as *stdin*, *stdout* or *stderr* represents a pipe, then the other side of this pipe should be registered with `~loop.connect_write_pipe`{.interpreted-text role="meth"} or `~loop.connect_read_pipe`{.interpreted-text role="meth"} for use with the event loop.

See the constructor of the `subprocess.Popen`{.interpreted-text role="class"} class for documentation on other arguments.

Returns a pair of `(transport, protocol)`, where *transport* conforms to the `asyncio.SubprocessTransport`{.interpreted-text role="class"} base class and *protocol* is an object instantiated by the *protocol_factory*.

If the transport is closed or is garbage collected, the child process is killed if it is still running.
:::
::::

::: {.method async=""}
loop.subprocess_shell(protocol_factory, cmd, *, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,*\*kwargs)

Create a subprocess from *cmd*, which can be a `str`{.interpreted-text role="class"} or a `bytes`{.interpreted-text role="class"} string encoded to the `filesystem encoding <filesystem-encoding>`{.interpreted-text role="ref"}, using the platform\'s \"shell\" syntax.

This is similar to the standard library `subprocess.Popen`{.interpreted-text role="class"} class called with `shell=True`.

The *protocol_factory* must be a callable returning a subclass of the `SubprocessProtocol`{.interpreted-text role="class"} class.

See `~loop.subprocess_exec`{.interpreted-text role="meth"} for more details about the remaining arguments.

Returns a pair of `(transport, protocol)`, where *transport* conforms to the `SubprocessTransport`{.interpreted-text role="class"} base class and *protocol* is an object instantiated by the *protocol_factory*.

If the transport is closed or is garbage collected, the child process is killed if it is still running.
:::

:::: note
::: title
Note
:::

It is the application\'s responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid [shell injection](https://en.wikipedia.org/wiki/Shell_injection#Shell_injection) vulnerabilities. The `shlex.quote`{.interpreted-text role="func"} function can be used to properly escape whitespace and special characters in strings that are going to be used to construct shell commands.
::::

## Callback Handles

:::::::: Handle
A callback wrapper object returned by `loop.call_soon`{.interpreted-text role="meth"}, `loop.call_soon_threadsafe`{.interpreted-text role="meth"}.

:::: method
get_context()

Return the `contextvars.Context`{.interpreted-text role="class"} object associated with the handle.

::: versionadded
3.12
:::
::::

::: method
cancel()

Cancel the callback. If the callback has already been canceled or executed, this method has no effect.
:::

:::: method
cancelled()

Return `True` if the callback was cancelled.

::: versionadded
3.7
:::
::::
::::::::

::::: TimerHandle
A callback wrapper object returned by `loop.call_later`{.interpreted-text role="meth"}, and `loop.call_at`{.interpreted-text role="meth"}.

This class is a subclass of `Handle`{.interpreted-text role="class"}.

:::: method
when()

Return a scheduled callback time as `float`{.interpreted-text role="class"} seconds.

The time is an absolute timestamp, using the same time reference as `loop.time`{.interpreted-text role="meth"}.

::: versionadded
3.7
:::
::::
:::::

## Server Objects

Server objects are created by `loop.create_server`{.interpreted-text role="meth"}, `loop.create_unix_server`{.interpreted-text role="meth"}, `start_server`{.interpreted-text role="func"}, and `start_unix_server`{.interpreted-text role="func"} functions.

Do not instantiate the `Server`{.interpreted-text role="class"} class directly.

::::::::::::::::::::: Server
*Server* objects are asynchronous context managers. When used in an `async with` statement, it\'s guaranteed that the Server object is closed and not accepting new connections when the `async with` statement is completed:

    srv = await loop.create_server(...)

    async with srv:
        # some code

    # At this point, srv is closed and no longer accepts new connections.

::: versionchanged
3.7 Server object is an asynchronous context manager since Python 3.7.
:::

::: versionchanged
3.11 This class was exposed publicly as `asyncio.Server` in Python 3.9.11, 3.10.3 and 3.11.
:::

::: method
close()

Stop serving: close listening sockets and set the `sockets`{.interpreted-text role="attr"} attribute to `None`.

The sockets that represent existing incoming client connections are left open.

The server is closed asynchronously; use the `wait_closed`{.interpreted-text role="meth"} coroutine to wait until the server is closed (and no more connections are active).
:::

:::: method
close_clients()

Close all existing incoming client connections.

Calls `~asyncio.BaseTransport.close`{.interpreted-text role="meth"} on all associated transports.

`close`{.interpreted-text role="meth"} should be called before `close_clients`{.interpreted-text role="meth"} when closing the server to avoid races with new clients connecting.

::: versionadded
3.13
:::
::::

:::: method
abort_clients()

Close all existing incoming client connections immediately, without waiting for pending operations to complete.

Calls `~asyncio.WriteTransport.abort`{.interpreted-text role="meth"} on all associated transports.

`close`{.interpreted-text role="meth"} should be called before `abort_clients`{.interpreted-text role="meth"} when closing the server to avoid races with new clients connecting.

::: versionadded
3.13
:::
::::

:::: method
get_loop()

Return the event loop associated with the server object.

::: versionadded
3.7
:::
::::

:::: {.method async=""}
start_serving()

Start accepting connections.

This method is idempotent, so it can be called when the server is already serving.

The *start_serving* keyword-only parameter to `loop.create_server`{.interpreted-text role="meth"} and `asyncio.start_server`{.interpreted-text role="meth"} allows creating a Server object that is not accepting connections initially. In this case `Server.start_serving()`, or `Server.serve_forever`{.interpreted-text role="meth"} can be used to make the Server start accepting connections.

::: versionadded
3.7
:::
::::

:::: {.method async=""}
serve_forever()

Start accepting connections until the coroutine is cancelled. Cancellation of `serve_forever` task causes the server to be closed.

This method can be called if the server is already accepting connections. Only one `serve_forever` task can exist per one *Server* object.

Example:

    async def client_connected(reader, writer):
        # Communicate with the client with
        # reader/writer streams.  For example:
        await reader.readline()

    async def main(host, port):
        srv = await asyncio.start_server(
            client_connected, host, port)
        await srv.serve_forever()

    asyncio.run(main('127.0.0.1', 0))

::: versionadded
3.7
:::
::::

:::: method
is_serving()

Return `True` if the server is accepting new connections.

::: versionadded
3.7
:::
::::

::: {.method async=""}
wait_closed()

Wait until the `close`{.interpreted-text role="meth"} method completes and all active connections have finished.
:::

:::: attribute
sockets

List of socket-like objects, `asyncio.trsock.TransportSocket`, which the server is listening on.

::: versionchanged
3.7 Prior to Python 3.7 `Server.sockets` used to return an internal list of server sockets directly. In 3.7 a copy of that list is returned.
:::
::::
:::::::::::::::::::::

## Event Loop Implementations[]{#asyncio-event-loops} {#asyncio-event-loop-implementations}

asyncio ships with two different event loop implementations: `SelectorEventLoop`{.interpreted-text role="class"} and `ProactorEventLoop`{.interpreted-text role="class"}.

By default asyncio is configured to use `EventLoop`{.interpreted-text role="class"}.

:::: SelectorEventLoop
A subclass of `AbstractEventLoop`{.interpreted-text role="class"} based on the `selectors`{.interpreted-text role="mod"} module.

Uses the most efficient *selector* available for the given platform. It is also possible to manually configure the exact selector implementation to be used:

    import asyncio
    import selectors

    async def main():
       ...

    loop_factory = lambda: asyncio.SelectorEventLoop(selectors.SelectSelector())
    asyncio.run(main(), loop_factory=loop_factory)

::: availability
Unix, Windows.
:::
::::

::::: ProactorEventLoop
A subclass of `AbstractEventLoop`{.interpreted-text role="class"} for Windows that uses \"I/O Completion Ports\" (IOCP).

::: availability
Windows.
:::

::: seealso
[MSDN documentation on I/O Completion Ports](https://learn.microsoft.com/windows/win32/fileio/i-o-completion-ports).
:::
:::::

:::: EventLoop
An alias to the most efficient available subclass of `AbstractEventLoop`{.interpreted-text role="class"} for the given platform.

It is an alias to `SelectorEventLoop`{.interpreted-text role="class"} on Unix and `ProactorEventLoop`{.interpreted-text role="class"} on Windows.

::: versionadded
3.13
:::
::::

::: AbstractEventLoop
Abstract base class for asyncio-compliant event loops.

The `asyncio-event-loop-methods`{.interpreted-text role="ref"} section lists all methods that an alternative implementation of `AbstractEventLoop` should have defined.
:::

## Examples

Note that all examples in this section **purposefully** show how to use the low-level event loop APIs, such as `loop.run_forever`{.interpreted-text role="meth"} and `loop.call_soon`{.interpreted-text role="meth"}. Modern asyncio applications rarely need to be written this way; consider using the high-level functions like `asyncio.run`{.interpreted-text role="func"}.

### Hello World with call_soon() {#asyncio_example_lowlevel_helloworld}

An example using the `loop.call_soon`{.interpreted-text role="meth"} method to schedule a callback. The callback displays `"Hello World"` and then stops the event loop:

    import asyncio

    def hello_world(loop):
        """A callback to print 'Hello World' and stop the event loop"""
        print('Hello World')
        loop.stop()

    loop = asyncio.new_event_loop()

    # Schedule a call to hello_world()
    loop.call_soon(hello_world, loop)

    # Blocking call interrupted by loop.stop()
    try:
        loop.run_forever()
    finally:
        loop.close()

::: seealso
A similar `Hello World <coroutine>`{.interpreted-text role="ref"} example created with a coroutine and the `run`{.interpreted-text role="func"} function.
:::

### Display the current date with call_later() {#asyncio_example_call_later}

An example of a callback displaying the current date every second. The callback uses the `loop.call_later`{.interpreted-text role="meth"} method to reschedule itself after 5 seconds, and then stops the event loop:

    import asyncio
    import datetime

    def display_date(end_time, loop):
        print(datetime.datetime.now())
        if (loop.time() + 1.0) < end_time:
            loop.call_later(1, display_date, end_time, loop)
        else:
            loop.stop()

    loop = asyncio.new_event_loop()

    # Schedule the first call to display_date()
    end_time = loop.time() + 5.0
    loop.call_soon(display_date, end_time, loop)

    # Blocking call interrupted by loop.stop()
    try:
        loop.run_forever()
    finally:
        loop.close()

::: seealso
A similar `current date <asyncio_example_sleep>`{.interpreted-text role="ref"} example created with a coroutine and the `run`{.interpreted-text role="func"} function.
:::

### Watch a file descriptor for read events {#asyncio_example_watch_fd}

Wait until a file descriptor received some data using the `loop.add_reader`{.interpreted-text role="meth"} method and then close the event loop:

    import asyncio
    from socket import socketpair

    # Create a pair of connected file descriptors
    rsock, wsock = socketpair()

    loop = asyncio.new_event_loop()

    def reader():
        data = rsock.recv(100)
        print("Received:", data.decode())

        # We are done: unregister the file descriptor
        loop.remove_reader(rsock)

        # Stop the event loop
        loop.stop()

    # Register the file descriptor for read event
    loop.add_reader(rsock, reader)

    # Simulate the reception of data from the network
    loop.call_soon(wsock.send, 'abc'.encode())

    try:
        # Run the event loop
        loop.run_forever()
    finally:
        # We are done. Close sockets and the event loop.
        rsock.close()
        wsock.close()
        loop.close()

::: seealso
- A similar `example <asyncio_example_create_connection>`{.interpreted-text role="ref"} using transports, protocols, and the `loop.create_connection`{.interpreted-text role="meth"} method.
- Another similar `example <asyncio_example_create_connection-streams>`{.interpreted-text role="ref"} using the high-level `asyncio.open_connection`{.interpreted-text role="func"} function and streams.
:::

### Set signal handlers for SIGINT and SIGTERM {#asyncio_example_unix_signals}

(This `signals` example only works on Unix.)

Register handlers for signals `~signal.SIGINT`{.interpreted-text role="const"} and `~signal.SIGTERM`{.interpreted-text role="const"} using the `loop.add_signal_handler`{.interpreted-text role="meth"} method:

    import asyncio
    import functools
    import os
    import signal

    def ask_exit(signame, loop):
        print("got signal %s: exit" % signame)
        loop.stop()

    async def main():
        loop = asyncio.get_running_loop()

        for signame in {'SIGINT', 'SIGTERM'}:
            loop.add_signal_handler(
                getattr(signal, signame),
                functools.partial(ask_exit, signame, loop))

        await asyncio.sleep(3600)

    print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
    print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")

    asyncio.run(main())