The event loop is the central execution device provided by asyncio. It provides multiple facilities, including:
Base class of event loops.
This class is not thread safe.
Run until stop() is called. If stop() is called before run_forever() is called, this polls the I/O selector once with a timeout of zero, runs all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exits. If stop() is called while run_forever() is running, this will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in that case; they will run the next time run_forever() is called.
Changed in version 3.4.4.
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.
Returns running status of event loop.
Stop running the event loop.
This causes run_forever() to exit at the next suitable opportunity (see there for more details).
Changed in version 3.4.4.
Returns True if the event loop was closed.
New in version 3.4.2.
Close the event loop. The loop must not be running. Pending callbacks will be lost.
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.
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).
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, which can be used to cancel the callback.
Like call_soon(), but thread safe.
See the concurrency and multithreading section of the documentation.
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.
Arrange for the callback to be called after the given delay seconds (either an int or float).
An instance of asyncio.Handle is returned, which can be used to cancel the callback.
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 BaseEventLoop.time().
This method’s behavior is the same as call_later().
An instance of asyncio.Handle is returned, which can be used to cancel the callback.
Return the current time, as a float value, according to the event loop’s internal clock.
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.
Set a task factory that will be used by BaseEventLoop.create_task().
If factory is None the default task factory will be set.
If factory is a callable, it should have a signature matching (loop, coro), where loop will be a reference to the active event loop, coro will be a coroutine object. The callable must return an asyncio.Future compatible object.
New in version 3.4.4.
Return a task factory, or None if the default one is in use.
New in version 3.4.4.
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:
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.
Create datagram connection: socket family AF_INET or AF_INET6 depending on host (or family if specified), socket type SOCK_DGRAM. 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.
Options changing how the connection is created:
On Windows with ProactorEventLoop, this method is not supported.
See UDP echo client protocol and UDP echo server protocol examples.
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.
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:
This method is a coroutine.
On Windows with ProactorEventLoop, SSL/TLS is not supported.
Changed in version 3.4.4: The host parameter can now be a sequence of strings.
Similar to BaseEventLoop.create_server(), but specific to the socket family AF_UNIX.
This method is a coroutine.
Availability: UNIX.
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.
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.
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.
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.
This method is a coroutine, similar to socket.getaddrinfo() function but non-blocking.
This method is a coroutine, similar to socket.getnameinfo() function but non-blocking.
On Windows with SelectorEventLoop, these methods are not supported. Use ProactorEventLoop to support pipes on Windows.
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.
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.
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.
Allows to customize how exceptions are handled in the event loop.
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).
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().
Call the current event loop exception handler.
context is a dict object containing the following keys (new keys may be introduced later):
Note
Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use set_exception_handler() method.
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.
Set the debug mode of the event loop.
New in version 3.4.2.
Server listening on sockets.
Object created by the BaseEventLoop.create_server() method and the start_server() function. Don’t instantiate the class directly.
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.
List of socket.socket objects the server is listening to, or None if the server is closed.
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()
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()
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()
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.
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