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
ensure_future()
.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 afterstop()
is called will not run. However, those callbacks will run ifrun_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, which can be used to cancel 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, 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()
.
-
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()
.An instance of
asyncio.Handle
is returned, which can be used to cancel 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. Tasks
-
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 ofTask
.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.
-
BaseEventLoop.
set_task_factory
(factory) 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 anasyncio.Future
compatible object.New in version 3.4.4.
-
BaseEventLoop.
get_task_factory
() Return a task factory, or
None
if the default one is in use.New in version 3.4.4.
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
orAF_INET6
depending on host (or family if specified), socket typeSOCK_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 connection is established, and a transport is created to represent it.
- protocol_factory is called without arguments and must return a protocol instance.
- The protocol instance is tied to the transport, and its
connection_made()
method is called. - 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 isTrue
, a context with some unspecified default settings is used.See also
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
orAF_INET6
depending on host (or family if specified), socket typeSOCK_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 typeSOCK_STREAM
. TheAF_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, itssockets
attribute contains created sockets. Use theServer.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
orAF_INET6
to force the socket to use IPv4 or IPv6. If not set it will be determined from host (defaults tosocket.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.- If host is an empty string or
-
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 familyAF_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.
-
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.
-
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
andAF_INET6
address families. Usegetaddrinfo()
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, theopen_connection()
function and thesocket.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, thestart_server()
function and thesocket.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 theReadTransport
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 supportsWriteTransport
interface.With
SelectorEventLoop
event loop, the pipe is set to non-blocking mode.This method is a coroutine.
See also
The BaseEventLoop.subprocess_exec()
and
BaseEventLoop.subprocess_shell()
methods.
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. RaiseRuntimeError
if there is a problem setting up the handler.
-
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 isNone
.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)
, whereloop
will be a reference to the active event loop,context
will be adict
object (seecall_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 variablePYTHONASYNCIODEBUG
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
18.5.1.15. Server
-
class
asyncio.
Server
Server listening on sockets.
Object created by the
BaseEventLoop.create_server()
method and thestart_server()
function. Don’t instantiate the class directly.-
close
() Stop serving: close listening sockets and set the
sockets
attribute toNone
.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.
-
sockets
List of
socket.socket
objects the server is listening to, orNone
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()
, andBaseEventLoop.call_at()
.-
cancel
() Cancel the call. If the callback is already canceled or executed, this method has no effect.
-
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.