The event loop is the central execution device provided by asyncio. It provides multiple facilities, amongst which:
The easiest way to get an event loop is to call the get_event_loop() function.
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.
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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 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:
A coroutine 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.
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Run subprocesses asynchronously using the subprocess module.
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This method returns a coroutine.
See the constructor of the subprocess.Popen class for parameters.
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This method returns a coroutine.
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.
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.
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().
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