18.5.1. Base Event Loop

The event loop is the central execution device provided by asyncio. It provides multiple facilities, amongst which:

  • Registering, executing and cancelling delayed calls (timeouts).
  • Creating client and server transports for various kinds of communication.
  • Launching subprocesses and the associated transports for communication with an external program.
  • Delegating costly function calls to a pool of threads.
class asyncio.BaseEventLoop

Base class of event loops.

This class is not thread safe.

18.5.1.1. Run an event loop

BaseEventLoop.run_forever()

Run until stop() is called.

BaseEventLoop.run_until_complete(future)

Run until the Future is done.

If the argument is a coroutine object, it is wrapped by async().

Return the Future’s result, or raise its exception.

BaseEventLoop.is_running()

Returns running status of event loop.

BaseEventLoop.stop()

Stop running the event loop.

Every callback scheduled before stop() is called will run. Callbacks scheduled after stop() is called will not run. However, those callbacks will run if run_forever() is called again later.

BaseEventLoop.is_closed()

Returns True if the event loop was closed.

New in version 3.4.2.

BaseEventLoop.close()

Close the event loop. The loop must not be running.

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

This is idempotent and irreversible. No other methods should be called after this one.

18.5.1.2. Calls

Most asyncio functions don’t accept keywords. If you want to pass keywords to your callback, use functools.partial(). For example, loop.call_soon(functools.partial(print, "Hello", flush=True)) will call print("Hello", flush=True).

Note

functools.partial() is better than lambda functions, because asyncio can inspect functools.partial() object to display parameters in debug mode, whereas lambda functions have a poor representation.

BaseEventLoop.call_soon(callback, *args)

Arrange for a callback to be called as soon as possible. The callback is called after call_soon() returns, when control returns to the event loop.

This operates as a FIFO queue, callbacks are called in the order in which they are registered. Each callback will be called exactly once.

Any positional arguments after the callback will be passed to the callback when it is called.

An instance of asyncio.Handle is returned.

Use functools.partial to pass keywords to the callback.

BaseEventLoop.call_soon_threadsafe(callback, *args)

Like call_soon(), but thread safe.

See the concurrency and multithreading section of the documentation.

18.5.1.3. Delayed calls

The event loop has its own internal clock for computing timeouts. Which clock is used depends on the (platform-specific) event loop implementation; ideally it is a monotonic clock. This will generally be a different clock than time.time().

Note

Timeouts (relative delay or absolute when) should not exceed one day.

BaseEventLoop.call_later(delay, callback, *args)

Arrange for the callback to be called after the given delay seconds (either an int or float).

An instance of asyncio.Handle is returned.

callback will be called exactly once per call to call_later(). If two callbacks are scheduled for exactly the same time, it is undefined which will be called first.

The optional positional args will be passed to the callback when it is called. If you want the callback to be called with some named arguments, use a closure or functools.partial().

Use functools.partial to pass keywords to the callback.

BaseEventLoop.call_at(when, callback, *args)

Arrange for the callback to be called at the given absolute timestamp when (an int or float), using the same time reference as BaseEventLoop.time().

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

Use functools.partial to pass keywords to the callback.

BaseEventLoop.time()

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

See also

The asyncio.sleep() function.

18.5.1.4. Coroutines

BaseEventLoop.create_task(coro)

Schedule the execution of a coroutine object: wrap it in a future. 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.

This method was added in Python 3.4.2. Use the async() function to support also older Python versions.

New in version 3.4.2.

18.5.1.5. Creating connections

coroutine BaseEventLoop.create_connection(protocol_factory, host=None, port=None, *, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None)

Create a streaming transport connection to a given Internet host and port: socket family AF_INET or AF_INET6 depending on host (or family if specified), socket type SOCK_STREAM. protocol_factory must be a callable returning a protocol instance.

This method is a coroutine which will try to establish the connection in the background. When successful, the coroutine 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 to represent it.
  2. protocol_factory is called without arguments and must return a protocol instance.
  3. The protocol instance is tied to the transport, and its connection_made() method is called.
  4. The coroutine returns successfully with the (transport, protocol) pair.

The created transport is an implementation-dependent bidirectional stream.

Note

protocol_factory can be any kind of callable, not necessarily a class. For example, if you want to use a pre-created protocol instance, you can pass lambda: my_protocol.

Options allowing to change how the connection is created:

  • 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 context with some unspecified default settings is used.

  • server_hostname, is only for use together with ssl, and sets or overrides the hostname that the target server’s certificate will be matched against. 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 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.

  • 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 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.

On Windows with ProactorEventLoop, SSL/TLS is not supported.

See also

The open_connection() function can be used to get a pair of (StreamReader, StreamWriter) instead of a protocol.

coroutine BaseEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0)

Create datagram connection: socket family AF_INET or AF_INET6 depending on host (or family if specified), socket type SOCK_DGRAM.

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

See the BaseEventLoop.create_connection() method for parameters.

On Windows with ProactorEventLoop, this method is not supported.

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

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

Create UNIX connection: socket family AF_UNIX, socket type SOCK_STREAM. The AF_UNIX socket family is used to communicate between processes on the same machine efficiently.

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

See the BaseEventLoop.create_connection() method for parameters.

Availability: UNIX.

18.5.1.6. Creating listening connections

coroutine BaseEventLoop.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)

Create a TCP server (socket type SOCK_STREAM) bound to host and port.

Return a Server object, its sockets attribute contains created sockets. Use the Server.close() method to stop the server: close listening sockets.

Parameters:

  • 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 it will be determined from host (defaults to socket.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 should be omitted (must be None).
  • backlog is the maximum number of queued connections passed to listen() (defaults to 100).
  • ssl can be set to an SSLContext to enable SSL 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.

This method is a coroutine.

On Windows with ProactorEventLoop, SSL/TLS is not supported.

See also

The function start_server() creates a (StreamReader, StreamWriter) pair and calls back a function with this pair.

coroutine BaseEventLoop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None)

Similar to BaseEventLoop.create_server(), but specific to the socket family AF_UNIX.

This method is a coroutine.

Availability: UNIX.

18.5.1.7. Watch file descriptors

On Windows with SelectorEventLoop, only socket handles are supported (ex: pipe file descriptors are not supported).

On Windows with ProactorEventLoop, these methods are not supported.

BaseEventLoop.add_reader(fd, callback, *args)

Start watching the file descriptor for read availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

BaseEventLoop.remove_reader(fd)

Stop watching the file descriptor for read availability.

BaseEventLoop.add_writer(fd, callback, *args)

Start watching the file descriptor for write availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

BaseEventLoop.remove_writer(fd)

Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level BaseEventLoop.add_reader() method to register the file descriptor of a socket.

18.5.1.8. Low-level socket operations

coroutine BaseEventLoop.sock_recv(sock, nbytes)

Receive data from the socket. The return value is a bytes object representing the data received. The maximum amount of data to be received at once is specified by nbytes.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

See also

The socket.socket.recv() method.

coroutine BaseEventLoop.sock_sendall(sock, data)

Send data to the socket. The socket must be connected to a remote socket. This method continues to send data from data until either all data has been sent or an error occurs. None is returned on success. On error, an exception is raised, and there is no way to determine how much data, if any, was successfully processed by the receiving end of the connection.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

See also

The socket.socket.sendall() method.

coroutine BaseEventLoop.sock_connect(sock, address)

Connect to a remote socket at address.

The address must be already resolved to avoid the trap of hanging the entire event loop when the address requires doing a DNS lookup. For example, it must be an IP address, not an hostname, for AF_INET and AF_INET6 address families. Use getaddrinfo() to resolve the hostname asynchronously.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

See also

The BaseEventLoop.create_connection() method, the open_connection() function and the socket.socket.connect() method.

coroutine BaseEventLoop.sock_accept(sock)

Accept a connection. 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.

The socket sock must be non-blocking.

This method is a coroutine.

See also

The BaseEventLoop.create_server() method, the start_server() function and the socket.socket.accept() method.

18.5.1.9. Resolve host name

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

This method is a coroutine, similar to socket.getaddrinfo() function but non-blocking.

coroutine BaseEventLoop.getnameinfo(sockaddr, flags=0)

This method is a coroutine, similar to socket.getnameinfo() function but non-blocking.

18.5.1.10. Connect pipes

On Windows with SelectorEventLoop, these methods are not supported. Use ProactorEventLoop to support pipes on Windows.

coroutine BaseEventLoop.connect_read_pipe(protocol_factory, pipe)

Register read pipe in eventloop.

protocol_factory should instantiate object with Protocol interface. pipe is a file-like object. Return pair (transport, protocol), where transport supports the ReadTransport interface.

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

This method is a coroutine.

coroutine BaseEventLoop.connect_write_pipe(protocol_factory, pipe)

Register write pipe in eventloop.

protocol_factory should instantiate object with BaseProtocol interface. pipe is file-like object. Return pair (transport, protocol), where transport supports WriteTransport interface.

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

This method is a coroutine.

18.5.1.11. UNIX signals

Availability: UNIX only.

BaseEventLoop.add_signal_handler(signum, callback, *args)

Add a handler for a signal.

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 keywords to the callback.

BaseEventLoop.remove_signal_handler(sig)

Remove a handler for a signal.

Return True if a signal handler was removed, False if not.

See also

The signal module.

18.5.1.12. Executor

Call a function in an Executor (pool of threads or pool of processes). By default, an event loop uses a thread pool executor (ThreadPoolExecutor).

coroutine BaseEventLoop.run_in_executor(executor, callback, *args)

Arrange for a callback to be called in the specified executor.

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

Use functools.partial to pass keywords to the callback.

This method is a coroutine.

BaseEventLoop.set_default_executor(executor)

Set the default executor used by run_in_executor().

18.5.1.13. Error Handling API

Allows to customize how exceptions are handled in the event loop.

BaseEventLoop.set_exception_handler(handler)

Set handler as the new event loop exception handler.

If handler is None, the default exception handler will be set.

If handler is a callable object, it should have a matching signature to (loop, context), where loop will be a reference to the active event loop, context will be a dict object (see call_exception_handler() documentation for details about context).

BaseEventLoop.default_exception_handler(context)

Default exception handler.

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

context parameter has the same meaning as in call_exception_handler().

BaseEventLoop.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 later):

  • ‘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

Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use set_exception_handler() method.

18.5.1.14. Debug mode

BaseEventLoop.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.

New in version 3.4.2.

BaseEventLoop.set_debug(enabled: bool)

Set the debug mode of the event loop.

New in version 3.4.2.

See also

The debug mode of asyncio.

18.5.1.15. Server

class asyncio.Server

Server listening on sockets.

Object created by the BaseEventLoop.create_server() method and the start_server() function. Don’t instantiate the class directly.

close()

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

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

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

coroutine wait_closed()

Wait until the close() method completes.

This method is a coroutine.

sockets

List of socket.socket objects the server is listening to, or None if the server is closed.

18.5.1.16. Handle

class asyncio.Handle

A callback wrapper object returned by BaseEventLoop.call_soon(), BaseEventLoop.call_soon_threadsafe(), BaseEventLoop.call_later(), and BaseEventLoop.call_at().

cancel()

Cancel the call.

18.5.1.17. Event loop examples

18.5.1.17.1. Hello World with call_soon()

Example using the BaseEventLoop.call_soon() method to schedule a callback. The callback displays "Hello World" and then stops the event loop:

import asyncio

def hello_world(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()
loop.run_forever()
loop.close()

See also

The Hello World coroutine example uses a coroutine.

18.5.1.17.2. Display the current date with call_later()

Example of callback displaying the current date every second. The callback uses the BaseEventLoop.call_later() method to reschedule itself during 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()
loop.run_forever()
loop.close()

See also

The coroutine displaying the current date example uses a coroutine.

18.5.1.17.3. Watch a file descriptor for read events

Wait until a file descriptor received some data using the BaseEventLoop.add_reader() method and then close the event loop:

import asyncio
try:
    from socket import socketpair
except ImportError:
    from asyncio.windows_utils 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())

# Run the event loop
loop.run_forever()

# We are done, close sockets and the event loop
rsock.close()
wsock.close()
loop.close()

See also

The register an open socket to wait for data using a protocol example uses a low-level protocol created by the BaseEventLoop.create_connection() method.

The register an open socket to wait for data using streams example uses high-level streams created by the open_connection() function in a coroutine.

18.5.1.17.4. Set signal handlers for SIGINT and SIGTERM

Register handlers for signals SIGINT and SIGTERM using the BaseEventLoop.add_signal_handler() method:

import asyncio
import functools
import os
import signal

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

loop = asyncio.get_event_loop()
for signame in ('SIGINT', 'SIGTERM'):
    loop.add_signal_handler(getattr(signal, signame),
                            functools.partial(ask_exit, signame))

print("Event loop running forever, press CTRL+c to interrupt.")
print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
try:
    loop.run_forever()
finally:
    loop.close()

This example only works on UNIX.