The event loop is the central execution device provided by asyncio. It provides multiple facilities, amongst which:
Event loop management is abstracted with a policy pattern, to provide maximal flexibility for custom platforms and frameworks. Throughout the execution of a process, a single global policy object manages the event loops available to the process based on the calling context. A policy is an object implementing the AbstractEventLoopPolicy interface.
For most users of asyncio, policies never have to be dealt with explicitly, since the default global policy is sufficient.
The default policy defines context as the current thread, and manages an event loop per thread that interacts with asyncio. The module-level functions get_event_loop() and set_event_loop() provide convenient access to event loops managed by the default policy.
The following functions are convenient shortcuts to accessing the methods of the global policy. Note that this provides access to the default policy, unless an alternative policy was set by calling set_event_loop_policy() earlier in the execution of the process.
Equivalent to calling get_event_loop_policy().get_event_loop().
Equivalent to calling get_event_loop_policy().set_event_loop(loop).
Equivalent to calling get_event_loop_policy().new_event_loop().
An event loop policy must implement the following interface:
Get the event loop for current context. Returns an event loop object implementing BaseEventLoop interface, or raises an exception in case no event loop has been set for the current context and the current policy does not specify to create one. It should never return None.
Set the event loop of the current context to loop.
Create and return a new event loop object according to this policy’s rules. If there’s need to set this loop as the event loop of the current context, set_event_loop() must be called explicitly.
Get the current event loop policy.
Set the current event loop policy. If policy is None, the default policy is restored.
Run until the Future is done.
If the argument is a coroutine, it is wrapped in a Task.
Return the Future’s result, or raise its exception.
Returns running status of event loop.
Stop running the event loop.
Every callback scheduled before stop() is called will run. Callback scheduled after stop() is called won’t. However, those callbacks will run if run_forever() is called again later.
Close the event loop. The loop should 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.
Arrange for a callback to be called as soon as possible.
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.
Like call_soon(), but thread safe.
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().
Arrange for the callback to be called after the given delay seconds (either an int or float).
A “handle” is returned: an opaque object with a cancel() method that can be used to cancel the call.
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().
Arrange for the callback to be called at the given absolute timestamp when (an int or float), using the same time reference as time().
This method’s behavior is the same as call_later().
Return the current time, as a float value, according to the event loop’s internal clock.
See also
The asyncio.sleep() function.
Create a streaming transport connection to a given Internet host and port. protocol_factory must be a callable returning a protocol instance.
This method returns a coroutine object 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:
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:
See also
The open_connection() function can be used to get a pair of (StreamReader, StreamWriter) instead of a protocol.
A coroutine function which creates a TCP server bound to host and port.
The return value is a AbstractServer object which can be used to stop the service.
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 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 AF_UNSPEC).
flags is a bitmask for getaddrinfo().
sock can optionally be specified in order to use a preexisting socket object.
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 returns a coroutine object.
See also
The function start_server() creates a (StreamReader, StreamWriter) pair and calls back a function with this pair.
Create datagram connection.
This method returns a coroutine object.
Start watching the file descriptor for read availability and then call the callback with specified arguments.
Stop watching the file descriptor for read availability.
Start watching the file descriptor for write availability and then call the callback with specified arguments.
Stop watching the file descriptor for write availability.
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.
This method returns a coroutine object.
See also
The socket.socket.recv() method.
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 sent.
This method returns a coroutine object.
See also
The socket.socket.sendall() method.
Connect to a remote socket at address.
This method returns a coroutine object.
See also
The BaseEventLoop.create_connection() method, the open_connection() function and the socket.socket.connect() method.
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.
This method returns a coroutine object.
See also
The BaseEventLoop.create_server() method, the start_server() function and the socket.socket.accept() method.
Similar to the socket.getaddrinfo() function, but return a coroutine object.
Similar to the socket.getnameinfo() function, but return a coroutine object.
Run subprocesses asynchronously using the subprocess module.
Note
On Windows, the default event loop uses selectors.SelectSelector which only supports sockets. The ProactorEventLoop should be used to support subprocesses.
Note
On Mac OS X older than 10.9 (Mavericks), selectors.KqueueSelector does not support character devices like PTY, whereas it is used by the default event loop. The SelectorEventLoop can be used with SelectSelector or PollSelector to handle character devices on Mac OS X 10.6 (Snow Leopard) and later.
XXX
This method returns a coroutine object.
See the constructor of the subprocess.Popen class for parameters.
XXX
This method returns a coroutine object.
See the constructor of the subprocess.Popen class for parameters.
Register read pipe in eventloop.
protocol_factory should instantiate object with Protocol interface. pipe is file-like object already switched to nonblocking. Return pair (transport, protocol), where transport support ReadTransport interface.
This method returns a coroutine object.
Register write pipe in eventloop.
protocol_factory should instantiate object with BaseProtocol interface. Pipe is file-like object already switched to nonblocking. Return pair (transport, protocol), where transport support WriteTransport interface.
This method returns a coroutine object.
See also
The create_subprocess_exec() and create_subprocess_shell() functions.
Availability: UNIX only.
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.
Remove a handler for a signal.
Return True if a signal handler was removed, False if not.
See also
The signal module.
Call a function in an Executor (pool of threads or pool of processes). By default, an event loop uses a thread pool executor (ThreadPoolExecutor).
Arrange for a callback to be called in the specified executor.
executor is a Executor instance, the default executor is used if executor is None.
Set the default executor used by run_in_executor().
Abstract server returned by BaseEventLoop.create_server().
Stop serving. This leaves existing connections open.
Coroutine to wait until service is closed.
Print Hello World every two seconds, using a callback:
import asyncio
def print_and_repeat(loop):
print('Hello World')
loop.call_later(2, print_and_repeat, loop)
loop = asyncio.get_event_loop()
loop.call_soon(print_and_repeat, loop)
loop.run_forever()
See also
Register handlers for signals SIGINT and SIGTERM:
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())
loop.run_forever()