.. currentmodule:: asyncio ========== Event Loop ========== .. rubric:: Preface The event loop is a central component of every asyncio application. Event loops run asynchronous tasks and callbacks, perform network IO operations, and run subprocesses. Application developers will typically use high-level asyncio functions to interact with the event loop. In general, high-level asyncio applications should not need to work directly with event loops and, instead, should use the :func:`asyncio.run` function to initialize, manage the event loop, and run asynchronous code. Alternatively, developers of low-level code, such as libraries and framework, may need access to the event loop. .. rubric:: Accessing Event Loop The following low-level functions can be used to get, set, or create an event loop: .. function:: get_running_loop() Return the running event loop in the current OS thread. If there is no running event loop a :exc:`RuntimeError` is raised. This function can only be called from a coroutine or a callback. .. versionadded:: 3.7 .. function:: get_event_loop() Get the current event loop. If there is no current event loop set in the current OS thread and :func:`set_event_loop` has not yet been called, asyncio will create a new event loop and set it as the current one. Because this function has rather complex behavior (especially when custom event loop policies are in use), using the :func:`get_running_loop` function is preferred to :func:`get_event_loop` in coroutines and callbacks. Consider also using the :func:`asyncio.run` function instead of using lower level functions to manually create and close an event loop. .. function:: set_event_loop(loop) Set *loop* as a current event loop for the current OS thread. .. function:: new_event_loop() Create a new event loop object. Note that the behaviour of :func:`get_event_loop`, :func:`set_event_loop`, and :func:`new_event_loop` functions can be altered by :ref:`setting a custom event loop policy `. .. rubric:: Contents This documentation page contains the following sections: * The `Event Loop Methods`_ section is the reference documentation of event loop APIs; * The `Callback Handles`_ section documents the :class:`Handle` and :class:`TimerHandle`, instances which are returned from functions, such as :meth:`loop.call_soon` and :meth:`loop.call_later`; * The `Server Objects`_ sections document types returned from event loop methods like :meth:`loop.create_server`; * The `Event Loops Implementations`_ section documents the :class:`SelectorEventLoop` and :class:`ProactorEventLoop` classes; * The `Examples`_ section showcases how to work with some event loop APIs. .. _asyncio-event-loop: Event Loop Methods ================== Event loops have **low-level** APIs for the following: .. contents:: :depth: 1 :local: Running and stopping the loop ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ .. method:: loop.run_until_complete(future) Run until the *future* (an instance of :class:`Future`) is completed. If the argument is a :ref:`coroutine object ` it is implicitly wrapped into an :class:`asyncio.Task`. Return the Future's result or raise its exception. .. method:: loop.run_forever() Run the event loop until :meth:`stop` is called. If :meth:`stop` is called before :meth:`run_forever()` is called, the loop will poll the I/O selector once with a timeout of zero, run all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exit. If :meth:`stop` is called while :meth:`run_forever` is running, the loop will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in this case; instead, they will run the next time :meth:`run_forever` or :meth:`run_until_complete` is called. .. method:: loop.stop() Stop the event loop. .. method:: loop.is_running() Return ``True`` if the event loop is currently running. .. method:: loop.is_closed() Return ``True`` if the event loop was closed. .. method:: loop.close() Close the event loop. The loop must be running when this function is called. Any pending callbacks will be discarded. This method clears all queues and shuts down the executor, but does not wait for the executor to finish. This method is idempotent and irreversible. No other methods should be called after the event loop is closed. .. coroutinemethod:: loop.shutdown_asyncgens() Schedule all currently open :term:`asynchronous generator` objects to close with an :meth:`~agen.aclose()` call. After calling this method, the event loop will issue a warning if a new asynchronous generator is iterated. This should be used to reliably finalize all scheduled asynchronous generators, e.g.:: try: loop.run_forever() finally: loop.run_until_complete(loop.shutdown_asyncgens()) loop.close() .. versionadded:: 3.6 .. _asyncio-pass-keywords: Scheduling callbacks ^^^^^^^^^^^^^^^^^^^^ .. method:: loop.call_soon(callback, *args, context=None) Schedule a *callback* to be called with *args* arguments at the next iteration of the event loop. Callbacks are called in the order in which they are registered. Each callback will be called exactly once. An optional keyword-only *context* argument allows specifying a custom :class:`contextvars.Context` for the *callback* to run in. The current context is used when no *context* is provided. An instance of :class:`asyncio.Handle` is returned, which can be used later to cancel the callback. This method is not thread-safe. .. method:: loop.call_soon_threadsafe(callback, *args, context=None) A thread-safe variant of :meth:`call_soon`. Must be used to schedule callbacks *from another thread*. See the :ref:`concurrency and multithreading ` section of the documentation. .. versionchanged:: 3.7 The *context* keyword-only parameter was added. See :pep:`567` for more details. .. note:: Most :mod:`asyncio` scheduling functions don't allow passing keyword arguments. To do that, use :func:`functools.partial`, e.g.:: # will schedule "print("Hello", flush=True)" loop.call_soon( functools.partial(print, "Hello", flush=True)) Using partial objects is usually more convenient than using lambdas, as asyncio can better render partial objects in debug and error messages. .. _asyncio-delayed-calls: Scheduling delayed callbacks ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Event loop provides mechanisms to schedule callback functions to be called at some point in the future. Event loop uses monotonic clocks to track time. .. method:: loop.call_later(delay, callback, *args, context=None) Schedule *callback* to be called after the given *delay* number of seconds (can be either an int or a float). An instance of :class:`asyncio.TimerHandle` is returned which can be used to cancel the callback. *callback* will be called exactly once. If two callbacks are scheduled for exactly the same time, it is undefined which one 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 keyword arguments use :func:`functools.partial`. An optional keyword-only *context* argument allows specifying a custom :class:`contextvars.Context` for the *callback* to run in. The current context is used when no *context* is provided. .. versionchanged:: 3.7 The *context* keyword-only parameter was added. See :pep:`567` for more details. .. method:: loop.call_at(when, callback, *args, context=None) Schedule *callback* to be called at the given absolute timestamp *when* (an int or a float), using the same time reference as :meth:`loop.time`. This method's behavior is the same as :meth:`call_later`. An instance of :class:`asyncio.TimerHandle` is returned which can be used to cancel the callback. .. versionchanged:: 3.7 The *context* keyword-only parameter was added. See :pep:`567` for more details. .. method:: loop.time() Return the current time, as a :class:`float` value, according to the event loop's internal monotonic clock. .. note:: Timeouts (relative *delay* or absolute *when*) should not exceed one day. .. seealso:: The :func:`asyncio.sleep` function. Creating Futures and Tasks ^^^^^^^^^^^^^^^^^^^^^^^^^^ .. method:: loop.create_future() Create an :class:`asyncio.Future` object attached to the event loop. This is the preferred way to create Futures in asyncio. This lets third-party event loops provide alternative implementations of the Future object (with better performance or instrumentation). .. versionadded:: 3.5.2 .. method:: loop.create_task(coro, \*, name=None) Schedule the execution of a :ref:`coroutine`. Return a :class:`Task` object. Third-party event loops can use their own subclass of :class:`Task` for interoperability. In this case, the result type is a subclass of :class:`Task`. If the *name* argument is provided and not ``None``, it is set as the name of the task using :meth:`Task.set_name`. .. versionchanged:: 3.8 Added the ``name`` parameter. .. method:: loop.set_task_factory(factory) Set a task factory that will be used by :meth:`loop.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 :class:`asyncio.Future` compatible object. .. method:: loop.get_task_factory() Return a task factory or ``None`` if the default one is in use. Opening network connections ^^^^^^^^^^^^^^^^^^^^^^^^^^^ .. coroutinemethod:: loop.create_connection(protocol_factory, \ host=None, port=None, \*, ssl=None, \ family=0, proto=0, flags=0, sock=None, \ local_addr=None, server_hostname=None, \ ssl_handshake_timeout=None) Open a streaming transport connection to a given address specified by *host* and *port*. The socket family can be either :py:data:`~socket.AF_INET` or :py:data:`~socket.AF_INET6` depending on *host* (or the *family* argument, if provided). The socket type will be :py:data:`~socket.SOCK_STREAM`. *protocol_factory* must be a callable returning an :ref:`asyncio protocol ` implementation. This method will try to establish the connection in the background. When successful, it returns a ``(transport, protocol)`` pair. The chronological synopsis of the underlying operation is as follows: #. The connection is established and a :ref:`transport ` is created for it. #. *protocol_factory* is called without arguments and is expected to return a :ref:`protocol ` instance. #. The protocol instance is coupled with the transport by calling its :meth:`~BaseProtocol.connection_made` method. #. A ``(transport, protocol)`` tuple is returned on success. 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``. Other arguments: * *ssl*: if given and not false, an SSL/TLS transport is created (by default a plain TCP transport is created). If *ssl* is a :class:`ssl.SSLContext` object, this context is used to create the transport; if *ssl* is :const:`True`, a context with some unspecified default settings is used. .. seealso:: :ref:`SSL/TLS security considerations ` * *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 :mod:`socket` module constants. * *sock*, if given, should be an existing, already connected :class:`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*. * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to wait for the SSL handshake to complete before aborting the connection. ``60.0`` seconds if ``None`` (default). .. versionadded:: 3.7 The *ssl_handshake_timeout* parameter. .. versionchanged:: 3.6 The socket option :py:data:`~socket.TCP_NODELAY` is set by default for all TCP connections. .. versionchanged:: 3.5 Added support for SSL/TLS for :class:`ProactorEventLoop`. .. seealso:: The :func:`open_connection` function is a high-level alternative API. It returns a pair of (:class:`StreamReader`, :class:`StreamWriter`) that can be used directly in async/await code. .. coroutinemethod:: loop.create_datagram_endpoint(protocol_factory, \ local_addr=None, remote_addr=None, \*, \ family=0, proto=0, flags=0, \ reuse_address=None, reuse_port=None, \ allow_broadcast=None, sock=None) Create a datagram connection. The socket family can be either :py:data:`~socket.AF_INET`, :py:data:`~socket.AF_INET6`, or :py:data:`~socket.AF_UNIX`, depending on *host* (or the *family* argument, if provided). The socket type will be :py:data:`~socket.SOCK_DGRAM`. *protocol_factory* must be a callable returning a :ref:`protocol ` implementation. A tuple of ``(transport, protocol)`` is returned on success. Other arguments: * *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 :meth:`getaddrinfo`. * *remote_addr*, if given, is a ``(remote_host, remote_port)`` tuple used to connect the socket to a remote address. The *remote_host* and *remote_port* are looked up using :meth:`getaddrinfo`. * *family*, *proto*, *flags* are the optional address family, protocol and flags to be passed through to :meth:`getaddrinfo` for *host* resolution. If given, these should all be integers from the corresponding :mod:`socket` module constants. * *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. * *reuse_port* tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows and some UNIX's. If the :py:data:`~socket.SO_REUSEPORT` constant is not defined then this capability is unsupported. * *allow_broadcast* tells the kernel to allow this endpoint to send messages to the broadcast address. * *sock* can optionally be specified in order to use a preexisting, already connected, :class:`socket.socket` object to be used by the transport. If specified, *local_addr* and *remote_addr* should be omitted (must be :const:`None`). On Windows with :class:`ProactorEventLoop`, this method is not supported. See :ref:`UDP echo client protocol ` and :ref:`UDP echo server protocol ` examples. .. versionchanged:: 3.4.4 The *family*, *proto*, *flags*, *reuse_address*, *reuse_port, *allow_broadcast*, and *sock* parameters were added. .. coroutinemethod:: loop.create_unix_connection(protocol_factory, \ path=None, \*, ssl=None, sock=None, \ server_hostname=None, ssl_handshake_timeout=None) Create UNIX connection. The socket family will be :py:data:`~socket.AF_UNIX`; socket type will be :py:data:`~socket.SOCK_STREAM`. A tuple of ``(transport, protocol)`` is returned on success. *path* is the name of a UNIX domain socket and is required, unless a *sock* parameter is specified. Abstract UNIX sockets, :class:`str`, :class:`bytes`, and :class:`~pathlib.Path` paths are supported. See the documentation of the :meth:`loop.create_connection` method for information about arguments to this method. Availability: UNIX. .. versionadded:: 3.7 The *ssl_handshake_timeout* parameter. .. versionchanged:: 3.7 The *path* parameter can now be a :term:`path-like object`. Creating network servers ^^^^^^^^^^^^^^^^^^^^^^^^ .. coroutinemethod:: loop.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, reuse_port=None, \ ssl_handshake_timeout=None, start_serving=True) Create a TCP server (socket type :data:`~socket.SOCK_STREAM`) listening on the *host* and *port* address. Returns a :class:`Server` object. Arguments: * The *host* parameter can be set to several types which determine behavior: - If *host* is a string, the TCP server is bound to *host* and *port*. - if *host* is a sequence of strings, the TCP server is bound to all hosts of the sequence. - 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 :data:`socket.AF_INET` or :data:`~socket.AF_INET6` to force the socket to use IPv4 or IPv6. If not set, the *family* will be determined from host name (defaults to :data:`~socket.AF_UNSPEC`). * *flags* is a bitmask for :meth:`getaddrinfo`. * *sock* can optionally be specified in order to use a preexisting socket object. If specified, *host* and *port* should be omitted (must be :const:`None`). * *backlog* is the maximum number of queued connections passed to :meth:`~socket.socket.listen` (defaults to 100). * *ssl* can be set to an :class:`~ssl.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. * *reuse_port* tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows. * *ssl_handshake_timeout* is (for an SSL server) the time in seconds to wait for the SSL handshake to complete before aborting the connection. ``60.0`` seconds if ``None`` (default). * *start_serving* set to ``True`` (the default) causes the created server to start accepting connections immediately. When set to ``False``, the user should await on :meth:`Server.start_serving` or :meth:`Server.serve_forever` to make the server to start accepting connections. .. versionadded:: 3.7 Added *ssl_handshake_timeout* and *start_serving* parameters. .. versionchanged:: 3.6 The socket option :py:data:`~socket.TCP_NODELAY` is set by default for all TCP connections. .. versionchanged:: 3.5 Added support for SSL/TLS on Windows with :class:`ProactorEventLoop`. .. versionchanged:: 3.5.1 The *host* parameter can be a sequence of strings. .. seealso:: The :func:`start_server` function is a higher-level alternative API that returns a pair of :class:`StreamReader` and :class:`StreamWriter` that can be used in an async/await code. .. coroutinemethod:: loop.create_unix_server(protocol_factory, path=None, \ \*, sock=None, backlog=100, ssl=None, \ ssl_handshake_timeout=None, start_serving=True) Similar to :meth:`loop.create_server` but works with the :py:data:`~socket.AF_UNIX` socket family. *path* is the name of a UNIX domain socket, and is required, unless a *sock* argument is provided. Abstract UNIX sockets, :class:`str`, :class:`bytes`, and :class:`~pathlib.Path` paths are supported. Availability: UNIX. .. versionadded:: 3.7 The *ssl_handshake_timeout* and *start_serving* parameters. .. versionchanged:: 3.7 The *path* parameter can now be a :class:`~pathlib.Path` object. .. coroutinemethod:: loop.connect_accepted_socket(protocol_factory, \ sock, \*, ssl=None, ssl_handshake_timeout=None) Wrap an already accepted connection into a transport/protocol pair. This method can be used by servers that accept connections outside of asyncio but that use asyncio to handle them. Parameters: * *sock* is a preexisting socket object returned from :meth:`socket.accept `. * *ssl* can be set to an :class:`~ssl.SSLContext` to enable SSL over the accepted connections. * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to wait for the SSL handshake to complete before aborting the connection. ``60.0`` seconds if ``None`` (default). Returns a ``(transport, protocol)`` pair. .. versionadded:: 3.7 The *ssl_handshake_timeout* parameter. .. versionadded:: 3.5.3 Transferring files ^^^^^^^^^^^^^^^^^^ .. coroutinemethod:: loop.sendfile(transport, file, \ offset=0, count=None, *, fallback=True) Send a *file* over a *transport*. Return the total number of bytes sent. The method uses high-performance :meth:`os.sendfile` if available. *file* must be a regular file object opened in binary mode. *offset* tells from where to start reading the file. If specified, *count* is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is updated on return or also in case of error in which case :meth:`file.tell() ` can be used to figure out the number of bytes which were sent. *fallback* set to ``True`` makes asyncio to manually read and send the file when the platform does not support the sendfile system call (e.g. Windows or SSL socket on Unix). Raise :exc:`SendfileNotAvailableError` if the system does not support *sendfile* syscall and *fallback* is ``False``. .. versionadded:: 3.7 TLS Upgrade ^^^^^^^^^^^ .. coroutinemethod:: loop.start_tls(transport, protocol, \ sslcontext, \*, server_side=False, \ server_hostname=None, ssl_handshake_timeout=None) Upgrade an existing transport-based connection to TLS. Return a new transport instance, that the *protocol* must start using immediately after the *await*. The *transport* instance passed to the *start_tls* method should never be used again. Parameters: * *transport* and *protocol* instances that methods like :meth:`~loop.create_server` and :meth:`~loop.create_connection` return. * *sslcontext*: a configured instance of :class:`~ssl.SSLContext`. * *server_side* pass ``True`` when a server-side connection is being upgraded (like the one created by :meth:`~loop.create_server`). * *server_hostname*: sets or overrides the host name that the target server's certificate will be matched against. * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to wait for the SSL handshake to complete before aborting the connection. ``60.0`` seconds if ``None`` (default). .. versionadded:: 3.7 Watching file descriptors ^^^^^^^^^^^^^^^^^^^^^^^^^ .. method:: loop.add_reader(fd, callback, \*args) Start watching the file descriptor *fd* for read availability and call the *callback* with specified arguments. .. method:: loop.remove_reader(fd) Stop watching the file descriptor *fd* for read availability. .. method:: loop.add_writer(fd, callback, \*args) Start watching the file descriptor *fd* for write availability and then call the *callback* with specified arguments. Use :func:`functools.partial` :ref:`to pass keywords ` to *func*. .. method:: loop.remove_writer(fd) Stop watching the file descriptor *fd* for write availability. See also :ref:`Platform Support ` section for some limitations of these methods. Working with socket objects directly ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In general, protocol implementations that use transport-based APIs such as :meth:`loop.create_connection` and :meth:`loop.create_server` are faster than implementations that work with sockets directly. However, there are some use cases when performance is not critical, and working with :class:`~socket.socket` objects directly is more convenient. .. coroutinemethod:: loop.sock_recv(sock, nbytes) Receive data. Asynchronous version of :meth:`socket.recv() `. The received data is returned as a bytes object. The maximum amount of data to be received is specified by the *nbytes* argument. The socket *sock* must be non-blocking. .. versionchanged:: 3.7 Even though this method was always documented as a coroutine method, releases before Python 3.7 returned a :class:`Future`. Since Python 3.7 this is an ``async def`` method. .. coroutinemethod:: loop.sock_recv_into(sock, buf) Receive data into a buffer. Modeled after the blocking :meth:`socket.recv_into() ` method. Return the number of bytes written to the buffer. The socket *sock* must be non-blocking. .. versionadded:: 3.7 .. coroutinemethod:: loop.sock_sendall(sock, data) Send data to the socket. Asynchronous version of :meth:`socket.sendall() `. This method continues to send data from *data* to the socket until either all data in *data* has been sent or an error occurs. ``None`` is returned on success. On error, an exception is raised. Additionally, there is no way to determine how much data, if any, was successfully processed by the receiving end of the connection. The socket *sock* must be non-blocking. .. versionchanged:: 3.7 Even though the method was always documented as a coroutine method, before Python 3.7 it returned an :class:`Future`. Since Python 3.7, this is an ``async def`` method. .. coroutinemethod:: loop.sock_connect(sock, address) Connect to a remote socket at *address*. Asynchronous version of :meth:`socket.connect() `. The socket *sock* must be non-blocking. .. versionchanged:: 3.5.2 ``address`` no longer needs to be resolved. ``sock_connect`` will try to check if the *address* is already resolved by calling :func:`socket.inet_pton`. If not, :meth:`loop.getaddrinfo` will be used to resolve the *address*. .. seealso:: :meth:`loop.create_connection` and :func:`asyncio.open_connection() `. .. coroutinemethod:: loop.sock_accept(sock) Accept a connection. Modeled after the blocking :meth:`socket.accept() ` method. 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. .. versionchanged:: 3.7 Even though the method was always documented as a coroutine method, before Python 3.7 it returned a :class:`Future`. Since Python 3.7, this is an ``async def`` method. .. seealso:: :meth:`loop.create_server` and :func:`start_server`. .. coroutinemethod:: loop.sock_sendfile(sock, file, offset=0, count=None, \ \*, fallback=True) Send a file using high-performance :mod:`os.sendfile` if possible. Return the total number of bytes which were sent. Asynchronous version of :meth:`socket.sendfile() `. *sock* must be non-blocking :class:`~socket.socket` of :const:`socket.SOCK_STREAM` type. *file* must be a regular file object opened in binary mode. *offset* tells from where to start reading the file. If specified, *count* is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is updated on return or also in case of error in which case :meth:`file.tell() ` can be used to figure out the number of bytes which were sent. *fallback*, when set to ``True``, makes asyncio manually read and send the file when the platform does not support the sendfile syscall (e.g. Windows or SSL socket on Unix). Raise :exc:`SendfileNotAvailableError` if the system does not support *sendfile* syscall and *fallback* is ``False``. The socket *sock* must be non-blocking. .. versionadded:: 3.7 DNS ^^^ .. coroutinemethod:: loop.getaddrinfo(host, port, \*, family=0, \ type=0, proto=0, flags=0) Asynchronous version of :meth:`socket.getaddrinfo`. .. coroutinemethod:: loop.getnameinfo(sockaddr, flags=0) Asynchronous version of :meth:`socket.getnameinfo`. .. versionchanged:: 3.7 Both *getaddrinfo* and *getnameinfo* methods were always documented to return a coroutine, but prior to Python 3.7 they were, in fact, returning :class:`asyncio.Future` objects. Starting with Python 3.7 both methods are coroutines. Working with pipes ^^^^^^^^^^^^^^^^^^ .. coroutinemethod:: loop.connect_read_pipe(protocol_factory, pipe) Register a read-pipe in the event loop. *protocol_factory* must be a callable returning an :ref:`asyncio protocol ` implementation. *pipe* is a :term:`file-like object `. Return pair ``(transport, protocol)``, where *transport* supports the :class:`ReadTransport` interface. With :class:`SelectorEventLoop` event loop, the *pipe* is set to non-blocking mode. .. coroutinemethod:: loop.connect_write_pipe(protocol_factory, pipe) Register a write-pipe in the event loop. *protocol_factory* must be a callable returning an :ref:`asyncio protocol ` implementation. *pipe* is :term:`file-like object `. Return pair ``(transport, protocol)``, where *transport* supports :class:`WriteTransport` interface. With :class:`SelectorEventLoop` event loop, the *pipe* is set to non-blocking mode. .. note:: :class:`SelectorEventLoop` does not support the above methods on Windows. Use :class:`ProactorEventLoop` instead for Windows. .. seealso:: The :meth:`loop.subprocess_exec` and :meth:`loop.subprocess_shell` methods. UNIX signals ^^^^^^^^^^^^ .. method:: loop.add_signal_handler(signum, callback, \*args) Add a handler for a signal. Raise :exc:`ValueError` if the signal number is invalid or uncatchable. Raise :exc:`RuntimeError` if there is a problem setting up the handler. Use :func:`functools.partial` :ref:`to pass keywords ` to *func*. .. method:: loop.remove_signal_handler(sig) Remove a handler for a signal. Return ``True`` if a signal handler was removed, ``False`` if not. Availability: UNIX. .. seealso:: The :mod:`signal` module. Executing code in thread or process pools ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ .. method:: loop.run_in_executor(executor, func, \*args) Arrange for a *func* to be called in the specified executor. The *executor* argument should be an :class:`concurrent.futures.Executor` instance. The default executor is used if *executor* is ``None``. Use :func:`functools.partial` :ref:`to pass keywords ` to *func*. This method returns a :class:`asyncio.Future` object. .. versionchanged:: 3.5.3 :meth:`loop.run_in_executor` no longer configures the ``max_workers`` of the thread pool executor it creates, instead leaving it up to the thread pool executor (:class:`~concurrent.futures.ThreadPoolExecutor`) to set the default. .. method:: loop.set_default_executor(executor) Set *executor* as the default executor used by :meth:`run_in_executor`. *executor* should be an instance of :class:`~concurrent.futures.ThreadPoolExecutor`. .. deprecated:: 3.8 Using an executor that is not an instance of :class:`~concurrent.futures.ThreadPoolExecutor` is deprecated and will trigger an error in Python 3.9. *executor* must be an instance of :class:`concurrent.futures.ThreadPoolExecutor`. Error Handling API ^^^^^^^^^^^^^^^^^^ Allows customizing how exceptions are handled in the event loop. .. method:: loop.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 :meth:`call_exception_handler` documentation for details about context). .. method:: loop.get_exception_handler() Return the exception handler, or ``None`` if the default one is in use. .. versionadded:: 3.5.2 .. method:: loop.default_exception_handler(context) Default exception handler. This is called when an exception occurs and no exception handler is set. This can be called by a custom exception handler that wants to defer to the default handler behavior. *context* parameter has the same meaning as in :meth:`call_exception_handler`. .. method:: loop.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): :class:`asyncio.Future` instance; * 'handle' (optional): :class:`asyncio.Handle` instance; * 'protocol' (optional): :ref:`Protocol ` instance; * 'transport' (optional): :ref:`Transport ` instance; * 'socket' (optional): :class:`socket.socket` instance. .. note:: This method should not be overloaded in subclassed event loops. For any custom exception handling, use :meth:`set_exception_handler()` method. Enabling debug mode ^^^^^^^^^^^^^^^^^^^ .. method:: loop.get_debug() Get the debug mode (:class:`bool`) of the event loop. The default value is ``True`` if the environment variable :envvar:`PYTHONASYNCIODEBUG` is set to a non-empty string, ``False`` otherwise. .. method:: loop.set_debug(enabled: bool) Set the debug mode of the event loop. .. seealso:: The :ref:`debug mode of asyncio `. Running Subprocesses ^^^^^^^^^^^^^^^^^^^^ Methods described in this subsections are low-level. In an async/await code consider using high-level convenient :func:`asyncio.create_subprocess_shell` and :func:`asyncio.create_subprocess_exec` functions instead. .. note:: The default event loop that asyncio is pre-configured to use on **Windows** does not support subprocesses. See :ref:`Subprocess Support on Windows ` for details. .. coroutinemethod:: loop.subprocess_exec(protocol_factory, \*args, \ stdin=subprocess.PIPE, stdout=subprocess.PIPE, \ stderr=subprocess.PIPE, \*\*kwargs) Create a subprocess from one or more string arguments specified by *args*. *args* must be a list of strings represented by: * :class:`str`; * or :class:`bytes`, encoded to the :ref:`filesystem encoding `. The first string specifies the program to execute, and the remaining strings specify the arguments. Together, string arguments form the ``argv`` of the program. This is similar to the standard library :class:`subprocess.Popen` class called with ``shell=False`` and the list of strings passed as the first argument; however, where :class:`~subprocess.Popen` takes a single argument which is list of strings, *subprocess_exec* takes multiple string arguments. The *protocol_factory* must instantiate a subclass of the :class:`asyncio.SubprocessProtocol` class. Other parameters: * *stdin*: either a file-like object representing a pipe to be connected to the subprocess's standard input stream using :meth:`~loop.connect_write_pipe`, or the :const:`subprocess.PIPE` constant (default). By default a new pipe will be created and connected. * *stdout*: either a file-like object representing the pipe to be connected to the subprocess's standard output stream using :meth:`~loop.connect_read_pipe`, or the :const:`subprocess.PIPE` constant (default). By default a new pipe will be created and connected. * *stderr*: either a file-like object representing the pipe to be connected to the subprocess's standard error stream using :meth:`~loop.connect_read_pipe`, or one of :const:`subprocess.PIPE` (default) or :const:`subprocess.STDOUT` constants. By default a new pipe will be created and connected. When :const:`subprocess.STDOUT` is specified, the subprocess' standard error stream will be connected to the same pipe as the standard output stream. * All other keyword arguments are passed to :class:`subprocess.Popen` without interpretation, except for *bufsize*, *universal_newlines* and *shell*, which should not be specified at all. See the constructor of the :class:`subprocess.Popen` class for documentation on other arguments. Returns a pair of ``(transport, protocol)``, where *transport* conforms to the :class:`asyncio.SubprocessTransport` base class. .. coroutinemethod:: loop.subprocess_shell(protocol_factory, cmd, \*, \ stdin=subprocess.PIPE, stdout=subprocess.PIPE, \ stderr=subprocess.PIPE, \*\*kwargs) Create a subprocess from *cmd*, which can be a :class:`str` or a :class:`bytes` string encoded to the :ref:`filesystem encoding `, using the platform's "shell" syntax. This is similar to the standard library :class:`subprocess.Popen` class called with ``shell=True``. The *protocol_factory* must instantiate a subclass of the :class:`SubprocessProtocol` class. See :meth:`~loop.subprocess_exec` for more details about the remaining arguments. Returns a pair of ``(transport, protocol)``, where *transport* conforms to the :class:`SubprocessTransport` base class. .. note:: It is the application's responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid `shell injection `_ vulnerabilities. The :func:`shlex.quote` function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands. Callback Handles ================ .. class:: Handle A callback wrapper object returned by :meth:`loop.call_soon`, :meth:`loop.call_soon_threadsafe`. .. method:: cancel() Cancel the call. If the callback is already canceled or executed, this method has no effect. .. method:: cancelled() Return ``True`` if the call was cancelled. .. versionadded:: 3.7 .. class:: TimerHandle A callback wrapper object returned by :meth:`loop.call_later`, and :meth:`loop.call_at`. The class is inherited from :class:`Handle`. .. method:: when() Return a scheduled callback time as :class:`float` seconds. The time is an absolute timestamp, using the same time reference as :meth:`loop.time`. .. versionadded:: 3.7 Server Objects ============== Server objects are created by :meth:`loop.create_server`, :meth:`loop.create_unix_server`, :func:`start_server`, and :func:`start_unix_server` functions. Do not instantiate the class directly. .. class:: Server *Server* objects are asynchronous context managers. When used in an ``async with`` statement, it's guaranteed that the Server object is closed and not accepting new connections when the ``async with`` statement is completed:: srv = await loop.create_server(...) async with srv: # some code # At this point, srv is closed and no longer accepts new connections. .. versionchanged:: 3.7 Server object is an asynchronous context manager since Python 3.7. .. method:: close() Stop serving: close listening sockets and set the :attr:`sockets` attribute to ``None``. The sockets that represent existing incoming client connections are left open. The server is closed asynchronously, use the :meth:`wait_closed` coroutine to wait until the server is closed. .. method:: get_loop() Gives the event loop associated with the server object. .. versionadded:: 3.7 .. coroutinemethod:: start_serving() Start accepting connections. This method is idempotent, so it can be called when the server is already being serving. The new *start_serving* keyword-only parameter to :meth:`loop.create_server` and :meth:`asyncio.start_server` allows to create a Server object that is not accepting connections right away. In which case this method, or :meth:`Server.serve_forever` can be used to make the Server object to start accepting connections. .. versionadded:: 3.7 .. coroutinemethod:: serve_forever() Start accepting connections until the coroutine is cancelled. Cancellation of ``serve_forever`` task causes the server to be closed. This method can be called if the server is already accepting connections. Only one ``serve_forever`` task can exist per one *Server* object. Example:: async def client_connected(reader, writer): # Communicate with the client with # reader/writer streams. For example: await reader.readline() async def main(host, port): srv = await asyncio.start_server( client_connected, host, port) await srv.serve_forever() asyncio.run(main('127.0.0.1', 0)) .. versionadded:: 3.7 .. method:: is_serving() Return ``True`` if the server is accepting new connections. .. versionadded:: 3.7 .. coroutinemethod:: wait_closed() Wait until the :meth:`close` method completes. .. attribute:: sockets List of :class:`socket.socket` objects the server is listening to, or ``None`` if the server is closed. .. versionchanged:: 3.7 Prior to Python 3.7 ``Server.sockets`` used to return the internal list of server's sockets directly. In 3.7 a copy of that list is returned. .. _asyncio-event-loops: Event Loops Implementations =========================== asyncio ships with two different event loop implementations: :class:`SelectorEventLoop` and :class:`ProactorEventLoop`. By default asyncio is configured to use :class:`SelectorEventLoop` on all platforms. .. class:: SelectorEventLoop An event loop based on the :mod:`selectors` module. Uses the most efficient *selector* available for the given platform. It is also possible to manually configure what exact selector implementation should be used:: import asyncio import selectors selector = selectors.SelectSelector() loop = asyncio.SelectorEventLoop(selector) asyncio.set_event_loop(loop) Availability: UNIX, Windows. .. class:: ProactorEventLoop An event loop for Windows that uses "I/O Completion Ports" (IOCP). Availability: Windows. An example how to use :class:`ProactorEventLoop` on Windows:: import asyncio import sys if sys.platform == 'win32': loop = asyncio.ProactorEventLoop() asyncio.set_event_loop(loop) .. seealso:: `MSDN documentation on I/O Completion Ports `_. .. class:: AbstractEventLoop Abstract base class for asyncio-compliant event loops. The :ref:`Event Loop Methods ` section lists all methods that an alternative implementation of ``AbstractEventLoop`` should have defined. Examples ======== Note that all examples in this section **purposefully** show how to use low-level event loop APIs such as :meth:`loop.run_forever` and :meth:`loop.call_soon`. Modern asyncio applications rarely need to be written this way; consider using high-level functions like :func:`asyncio.run`. .. _asyncio-hello-world-callback: Hello World with call_soon() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ An example using the :meth:`loop.call_soon` method to schedule a callback. The callback displays ``"Hello World"`` and then stops the event loop:: import asyncio def hello_world(loop): """A callback to print 'Hello World' and stop the event 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() try: loop.run_forever() finally: loop.close() .. seealso:: A similar :ref:`Hello World ` example created with a coroutine and the :func:`run` function. .. _asyncio-date-callback: Display the current date with call_later() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ An example of callback displaying the current date every second. The callback uses the :meth:`loop.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() try: loop.run_forever() finally: loop.close() .. seealso:: A similar :ref:`current date ` example created with a coroutine and the :func:`run` function. .. _asyncio-watch-read-event: Watch a file descriptor for read events ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Wait until a file descriptor received some data using the :meth:`loop.add_reader` method and then close the event loop:: import asyncio from socket 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()) try: # Run the event loop loop.run_forever() finally: # We are done. Close sockets and the event loop. rsock.close() wsock.close() loop.close() .. seealso:: * A similar :ref:`example ` using transports, protocols, and the :meth:`loop.create_connection` method. * Another similar :ref:`example ` using the high-level :func:`asyncio.open_connection` function and streams. Set signal handlers for SIGINT and SIGTERM ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ (This ``signals`` example only works on UNIX.) Register handlers for signals :py:data:`SIGINT` and :py:data:`SIGTERM` using the :meth:`loop.add_signal_handler` method:: import asyncio import functools import os import signal def ask_exit(signame): print("got signal %s: exit" % signame) loop.stop() async def main(): loop = asyncio.get_running_loop() for signame in {'SIGINT', 'SIGTERM'}: loop.add_signal_handler( getattr(signal, signame), functools.partial(ask_exit, signame)) await asyncio.sleep(3600) print("Event loop running for 1 hour, press Ctrl+C to interrupt.") print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.") asyncio.run(main())