Event Loop

Source code: Lib/asyncio/events.py, Lib/asyncio/base_events.py

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(), 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:

asyncio.get_running_loop()

Return the running event loop in the current OS thread.

If there is no running event loop a RuntimeError is raised. This function can only be called from a coroutine or a callback.

New in version 3.7.

asyncio.get_event_loop()

Get the current event loop.

If there is no current event loop set in the current OS thread, the OS thread is main, and set_event_loop() has not yet been called, asyncio will create a new event loop and set it as the current one.

Because this function has rather complex behavior (especially when custom event loop policies are in use), using the get_running_loop() function is preferred to get_event_loop() in coroutines and callbacks.

Consider also using the asyncio.run() function instead of using lower level functions to manually create and close an event loop.

asyncio.set_event_loop(loop)

Set loop as a current event loop for the current OS thread.

asyncio.new_event_loop()

Create a new event loop object.

Note that the behaviour of get_event_loop(), set_event_loop(), and new_event_loop() functions can be altered by setting a custom event loop policy.

Contents

This documentation page contains the following sections:

Event Loop Methods

Event loops have low-level APIs for the following:

Running and stopping the loop

loop.run_until_complete(future)

Run until the future (an instance of Future) has completed.

If the argument is a coroutine object it is implicitly scheduled to run as a asyncio.Task.

Return the Future’s result or raise its exception.

loop.run_forever()

Run the event loop until stop() is called.

If stop() is called before run_forever() 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() is called while run_forever() 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() or run_until_complete() is called.

loop.stop()

Stop the event loop.

loop.is_running()

Return True if the event loop is currently running.

loop.is_closed()

Return True if the event loop was closed.

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.

coroutine loop.shutdown_asyncgens()

Schedule all currently open asynchronous generator objects to close with an aclose() 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() is used.

Example:

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

New in version 3.6.

Scheduling callbacks

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

Schedule the callback callback to be called with args arguments at the next iteration of the event loop.

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

An optional keyword-only context argument allows specifying a custom contextvars.Context for the callback to run in. The current context is used when no context is provided.

An instance of asyncio.Handle is returned, which can be used later to cancel the callback.

This method is not thread-safe.

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

A thread-safe variant of call_soon(). Must be used to schedule callbacks from another thread.

See the concurrency and multithreading section of the documentation.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Note

Most asyncio scheduling functions don’t allow passing keyword arguments. To do that, use functools.partial():

# 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

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.

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 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. If you want the callback to be called with keyword arguments use functools.partial().

An optional keyword-only context argument allows specifying a custom contextvars.Context for the callback to run in. The current context is used when no context is provided.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Changed in version 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.

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().

This method’s behavior is the same as call_later().

An instance of asyncio.TimerHandle is returned which can be used to cancel the callback.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Changed in version 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.

loop.time()

Return the current time, as a float value, according to the event loop’s internal monotonic clock.

Note

Changed in version 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.

See also

The asyncio.sleep() function.

Creating Futures and Tasks

loop.create_future()

Create an asyncio.Future 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).

New in version 3.5.2.

loop.create_task(coro, *, name=None)

Schedule the execution of a Coroutines. Return a Task object.

Third-party event loops can use their own subclass of Task for interoperability. In this case, the result type is a subclass of Task.

If the name argument is provided and not None, it is set as the name of the task using Task.set_name().

Changed in version 3.8: Added the name parameter.

loop.set_task_factory(factory)

Set a task factory that will be used by loop.create_task().

If factory is None the default task factory will be set. Otherwise, factory must be a callable with the signature matching (loop, coro), where loop is a reference to the active event loop, and coro is a coroutine object. The callable must return a asyncio.Future-compatible object.

loop.get_task_factory()

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

Opening network connections

coroutine 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, happy_eyeballs_delay=None, interleave=None)

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

The socket family can be either AF_INET or AF_INET6 depending on host (or the family argument, if provided).

The socket type will be SOCK_STREAM.

protocol_factory must be a callable returning an asyncio protocol 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 is created for it.

  2. protocol_factory is called without arguments and is expected to return a protocol instance.

  3. The protocol instance is coupled with the transport by calling its connection_made() 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 object, this context is used to create the transport; if ssl is True, a default context returned from ssl.create_default_context() is used.

    See also

    SSL/TLS security considerations

  • 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 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 RFC 8305. 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(). 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 RFC 8305. 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 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.

  • local_addr, if given, is a (local_host, local_port) tuple used to bind the socket to 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).

New in version 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://tools.ietf.org/html/rfc6555

New in version 3.7: The ssl_handshake_timeout parameter.

Changed in version 3.6: The socket option TCP_NODELAY is set by default for all TCP connections.

Changed in version 3.5: Added support for SSL/TLS in ProactorEventLoop.

See also

The open_connection() function is a high-level alternative API. It returns a pair of (StreamReader, StreamWriter) that can be used directly in async/await code.

coroutine loop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

Note

The parameter reuse_address is no longer supported, as using SO_REUSEADDR 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, SO_REUSEPORT is used instead, which specifically prevents processes with differing UIDs from assigning sockets to the same socket address.

Create a datagram connection.

The socket family can be either AF_INET, AF_INET6, or AF_UNIX, depending on host (or the family argument, if provided).

The socket type will be SOCK_DGRAM.

protocol_factory must be a callable returning a protocol 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 to locally. The local_host and local_port are looked up using getaddrinfo().

  • 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().

  • 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 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 SO_REUSEPORT 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 object to be used by the transport. If specified, local_addr and remote_addr should be omitted (must be None).

See UDP echo client protocol and UDP echo server protocol examples.

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

Changed in version 3.8.1: The reuse_address parameter is no longer supported due to security concerns.

Changed in version 3.8: Added support for Windows.

coroutine loop.create_unix_connection(protocol_factory, path=None, *, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None)

Create a Unix connection.

The socket family will be AF_UNIX; socket type will be SOCK_STREAM.

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, bytes, and Path paths are supported.

See the documentation of the loop.create_connection() method for information about arguments to this method.

Availability: Unix.

New in version 3.7: The ssl_handshake_timeout parameter.

Changed in version 3.7: The path parameter can now be a path-like object.

Creating network servers

coroutine 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, ssl_handshake_timeout=None, start_serving=True)

Create a TCP server (socket type SOCK_STREAM) listening on port of the host address.

Returns a Server object.

Arguments:

  • protocol_factory must be a callable returning a protocol 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).

  • family can be set to either socket.AF_INET or AF_INET6 to force the socket to use IPv4 or IPv6. If not set, the family will be determined from host name (defaults to AF_UNSPEC).

  • flags is a bitmask for getaddrinfo().

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

  • backlog is the maximum number of queued connections passed to listen() (defaults to 100).

  • ssl can be set to an SSLContext 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.

  • 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).

  • 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() or Server.serve_forever() to make the server to start accepting connections.

New in version 3.7: Added ssl_handshake_timeout and start_serving parameters.

Changed in version 3.6: The socket option TCP_NODELAY is set by default for all TCP connections.

Changed in version 3.5: Added support for SSL/TLS in ProactorEventLoop.

Changed in version 3.5.1: The host parameter can be a sequence of strings.

See also

The start_server() function is a higher-level alternative API that returns a pair of StreamReader and StreamWriter that can be used in an async/await code.

coroutine loop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, start_serving=True)

Similar to loop.create_server() but works with the AF_UNIX socket family.

path is the name of a Unix domain socket, and is required, unless a sock argument is provided. Abstract Unix sockets, str, bytes, and Path paths are supported.

See the documentation of the loop.create_server() method for information about arguments to this method.

Availability: Unix.

New in version 3.7: The ssl_handshake_timeout and start_serving parameters.

Changed in version 3.7: The path parameter can now be a Path object.

coroutine loop.connect_accepted_socket(protocol_factory, sock, *, ssl=None, ssl_handshake_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 implementation.

  • sock is a preexisting socket object returned from socket.accept.

  • ssl can be set to an SSLContext 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).

Returns a (transport, protocol) pair.

New in version 3.7: The ssl_handshake_timeout parameter.

New in version 3.5.3.

Transferring files

coroutine 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() 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() 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 if the system does not support the sendfile syscall and fallback is False.

New in version 3.7.

TLS Upgrade

coroutine loop.start_tls(transport, protocol, sslcontext, *, server_side=False, server_hostname=None, ssl_handshake_timeout=None)

Upgrade an existing transport-based connection to TLS.

Return a new transport instance, that the protocol must start using immediately after the await. The transport instance passed to the start_tls method should never be used again.

Parameters:

  • transport and protocol instances that methods like create_server() and create_connection() return.

  • sslcontext: a configured instance of SSLContext.

  • server_side pass True when a server-side connection is being upgraded (like the one created by create_server()).

  • 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).

New in version 3.7.

Watching file descriptors

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.

loop.remove_reader(fd)

Stop monitoring the fd file descriptor for read availability.

loop.add_writer(fd, callback, *args)

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

Use functools.partial() to pass keyword arguments to callback.

loop.remove_writer(fd)

Stop monitoring the fd file descriptor for write availability.

See also Platform Support 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() and loop.create_server() are faster than implementations that work with sockets directly. However, there are some use cases when performance is not critical, and working with socket objects directly is more convenient.

coroutine loop.sock_recv(sock, nbytes)

Receive up to nbytes from sock. Asynchronous version of socket.recv().

Return the received data as a bytes object.

sock must be a non-blocking socket.

Changed in version 3.7: Even though this method was always documented as a coroutine method, releases before Python 3.7 returned a Future. Since Python 3.7 this is an async def method.

coroutine loop.sock_recv_into(sock, buf)

Receive data from sock into the buf buffer. Modeled after the blocking socket.recv_into() method.

Return the number of bytes written to the buffer.

sock must be a non-blocking socket.

New in version 3.7.

coroutine loop.sock_sendall(sock, data)

Send data to the sock socket. Asynchronous version of socket.sendall().

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.

Changed in version 3.7: Even though the method was always documented as a coroutine method, before Python 3.7 it returned an Future. Since Python 3.7, this is an async def method.

coroutine loop.sock_connect(sock, address)

Connect sock to a remote socket at address.

Asynchronous version of socket.connect().

sock must be a non-blocking socket.

Changed in version 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(). If not, loop.getaddrinfo() will be used to resolve the address.

See also

loop.create_connection() and asyncio.open_connection().

coroutine loop.sock_accept(sock)

Accept a connection. Modeled after the blocking socket.accept() 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.

Changed in version 3.7: Even though the method was always documented as a coroutine method, before Python 3.7 it returned a Future. Since Python 3.7, this is an async def method.

See also

loop.create_server() and start_server().

coroutine loop.sock_sendfile(sock, file, offset=0, count=None, *, fallback=True)

Send a file using high-performance os.sendfile if possible. Return the total number of bytes sent.

Asynchronous version of socket.sendfile().

sock must be a non-blocking socket.SOCK_STREAM socket.

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() 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 if the system does not support sendfile syscall and fallback is False.

sock must be a non-blocking socket.

New in version 3.7.

DNS

coroutine loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

Asynchronous version of socket.getaddrinfo().

coroutine loop.getnameinfo(sockaddr, flags=0)

Asynchronous version of socket.getnameinfo().

Changed in version 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 objects. Starting with Python 3.7 both methods are coroutines.

Working with pipes

coroutine 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 implementation.

pipe is a file-like object.

Return pair (transport, protocol), where transport supports the ReadTransport interface and protocol is an object instantiated by the protocol_factory.

With SelectorEventLoop event loop, the pipe is set to non-blocking mode.

coroutine 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 implementation.

pipe is file-like object.

Return pair (transport, protocol), where transport supports WriteTransport interface and protocol is an object instantiated by the protocol_factory.

With SelectorEventLoop event loop, the pipe is set to non-blocking mode.

Note

SelectorEventLoop does not support the above methods on Windows. Use ProactorEventLoop instead for Windows.

See also

The loop.subprocess_exec() and loop.subprocess_shell() methods.

Unix signals

loop.add_signal_handler(signum, callback, *args)

Set callback as the handler for the signum signal.

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(), a callback registered with this function is allowed to interact with the event loop.

Raise ValueError if the signal number is invalid or uncatchable. Raise RuntimeError if there is a problem setting up the handler.

Use functools.partial() to pass keyword arguments to callback.

Like signal.signal(), this function must be invoked in the main thread.

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.

See also

The signal module.

Executing code in thread or process pools

awaitable loop.run_in_executor(executor, func, *args)

Arrange for func to be called in the specified executor.

The executor argument should be an concurrent.futures.Executor instance. The default executor is used if executor is None.

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)

asyncio.run(main())

This method returns a asyncio.Future object.

Use functools.partial() to pass keyword arguments to func.

Changed in version 3.5.3: loop.run_in_executor() no longer configures the max_workers of the thread pool executor it creates, instead leaving it up to the thread pool executor (ThreadPoolExecutor) to set the default.

loop.set_default_executor(executor)

Set executor as the default executor used by run_in_executor(). executor should be an instance of ThreadPoolExecutor.

Deprecated since version 3.8: Using an executor that is not an instance of ThreadPoolExecutor is deprecated and will trigger an error in Python 3.9.

executor must be an instance of concurrent.futures.ThreadPoolExecutor.

Error Handling API

Allows customizing how exceptions are handled in the event loop.

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() documentation for details about context).

loop.get_exception_handler()

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

New in version 3.5.2.

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().

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 instance;

  • ‘handle’ (optional): asyncio.Handle instance;

  • ‘protocol’ (optional): Protocol instance;

  • ‘transport’ (optional): Transport instance;

  • ‘socket’ (optional): socket.socket instance.

Note

This method should not be overloaded in subclassed event loops. For custom exception handling, use the set_exception_handler() method.

Enabling debug mode

loop.get_debug()

Get the debug mode (bool) of the event loop.

The default value is True if the environment variable PYTHONASYNCIODEBUG is set to a non-empty string, False otherwise.

loop.set_debug(enabled: bool)

Set the debug mode of the event loop.

Changed in version 3.7: The new -X dev command line option can now also be used to enable the debug mode.

See also

The debug mode of asyncio.

Running Subprocesses

Methods described in this subsections are low-level. In regular async/await code consider using the high-level asyncio.create_subprocess_shell() and asyncio.create_subprocess_exec() convenience functions instead.

Note

The default asyncio event loop on Windows does not support subprocesses. See Subprocess Support on Windows for details.

coroutine 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:

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 class called with shell=False and the list of strings passed as the first argument; however, where Popen 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 class.

Other parameters:

  • stdin can be any of these:

    • a file-like object representing a pipe to be connected to the subprocess’s standard input stream using connect_write_pipe()

    • the subprocess.PIPE 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 constant which indicates that the special os.devnull file will be used

  • stdout can be any of these:

    • a file-like object representing a pipe to be connected to the subprocess’s standard output stream using connect_write_pipe()

    • the subprocess.PIPE 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 constant which indicates that the special os.devnull file will be used

  • stderr can be any of these:

    • a file-like object representing a pipe to be connected to the subprocess’s standard error stream using connect_write_pipe()

    • the subprocess.PIPE 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 constant which indicates that the special os.devnull file will be used

    • the subprocess.STDOUT constant which will connect the standard error stream to the process’ standard output stream

  • All other keyword arguments are passed to subprocess.Popen 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() can be used to convert the bytes returned from the stream to text.

See the constructor of the subprocess.Popen class for documentation on other arguments.

Returns a pair of (transport, protocol), where transport conforms to the asyncio.SubprocessTransport base class and protocol is an object instantiated by the protocol_factory.

coroutine 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 or a bytes string encoded to the filesystem encoding, using the platform’s “shell” syntax.

This is similar to the standard library subprocess.Popen class called with shell=True.

The protocol_factory must be a callable returning a subclass of the SubprocessProtocol class.

See subprocess_exec() for more details about the remaining arguments.

Returns a pair of (transport, protocol), where transport conforms to the SubprocessTransport base class and protocol is an object instantiated by the protocol_factory.

Note

It is the application’s responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid shell injection vulnerabilities. The shlex.quote() 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

class asyncio.Handle

A callback wrapper object returned by loop.call_soon(), loop.call_soon_threadsafe().

cancel()

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

cancelled()

Return True if the callback was cancelled.

New in version 3.7.

class asyncio.TimerHandle

A callback wrapper object returned by loop.call_later(), and loop.call_at().

This class is a subclass of Handle.

when()

Return a scheduled callback time as float seconds.

The time is an absolute timestamp, using the same time reference as loop.time().

New in version 3.7.

Server Objects

Server objects are created by loop.create_server(), loop.create_unix_server(), start_server(), and start_unix_server() functions.

Do not instantiate the class directly.

class asyncio.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.

Changed in version 3.7: Server object is an asynchronous context manager since Python 3.7.

close()

Stop serving: close listening sockets and set the sockets attribute to None.

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

The server is closed asynchronously, use the wait_closed() coroutine to wait until the server is closed.

get_loop()

Return the event loop associated with the server object.

New in version 3.7.

coroutine start_serving()

Start accepting connections.

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

The start_serving keyword-only parameter to loop.create_server() and asyncio.start_server() allows creating a Server object that is not accepting connections initially. In this case Server.start_serving(), or Server.serve_forever() can be used to make the Server start accepting connections.

New in version 3.7.

coroutine 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))

New in version 3.7.

is_serving()

Return True if the server is accepting new connections.

New in version 3.7.

coroutine wait_closed()

Wait until the close() method completes.

sockets

List of socket.socket objects the server is listening on.

Changed in version 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 ships with two different event loop implementations: SelectorEventLoop and ProactorEventLoop.

By default asyncio is configured to use SelectorEventLoop on Unix and ProactorEventLoop on Windows.

class asyncio.SelectorEventLoop

An event loop based on the selectors 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

selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)

Availability: Unix, Windows.

class asyncio.ProactorEventLoop

An event loop for Windows that uses “I/O Completion Ports” (IOCP).

Availability: Windows.

See also

MSDN documentation on I/O Completion Ports.

class asyncio.AbstractEventLoop

Abstract base class for asyncio-compliant event loops.

The Event Loop Methods 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() and loop.call_soon(). Modern asyncio applications rarely need to be written this way; consider using the high-level functions like asyncio.run().

Hello World with call_soon()

An example using the loop.call_soon() 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.get_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()

See also

A similar Hello World example created with a coroutine and the run() function.

Display the current date with call_later()

An example of a callback displaying the current date every second. The callback uses the loop.call_later() 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.get_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()

See also

A similar current date example created with a coroutine and the run() function.

Watch a file descriptor for read events

Wait until a file descriptor received some data using the loop.add_reader() 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.get_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()

See also

Set signal handlers for SIGINT and SIGTERM

(This signals example only works on Unix.)

Register handlers for signals SIGINT and SIGTERM using the loop.add_signal_handler() 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())