Event Loop
**********

-[ Preface ]-

The event loop is the core of every asyncio application. Event loops
run asynchronous tasks and callbacks, perform network IO operations,
and run subprocesses.

Application developers should typically use the high-level asyncio
functions, such as "asyncio.run()", and should rarely need to
reference the loop object or call its methods.  This section is
intended mostly for authors of lower-level code, libraries, and
frameworks, who need finer control over the event loop behavior.

-[ Obtaining the Event Loop ]-

The following low-level functions can be used to get, set, or create
an event loop:

asyncio.get_running_loop()

   Return the running event loop in the current OS thread.

   If there is no running event loop a "RuntimeError" is raised. This
   function can only be called from a coroutine or a callback.

   3.7 版新加入.

asyncio.get_event_loop()

   Get the current event loop.  If there is no current event loop set
   in the current OS thread and "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
   "get_running_loop()" function is preferred to "get_event_loop()" in
   coroutines and callbacks.

   Consider also using the "asyncio.run()" function instead of using
   lower level functions to manually create and close an event loop.

asyncio.set_event_loop(loop)

   Set *loop* as a current event loop for the current OS thread.

asyncio.new_event_loop()

   Create a new event loop object.

Note that the behaviour of "get_event_loop()", "set_event_loop()", and
"new_event_loop()" functions can be altered by setting a custom event
loop policy.

-[ Contents ]-

This documentation page contains the following sections:

* The Event Loop Methods section is the reference documentation of
  the event loop APIs;

* The Callback Handles section documents the "Handle" and
  "TimerHandle" instances which are returned from scheduling methods
  such as "loop.call_soon()" and "loop.call_later()";

* The Server Objects section documents types returned from event
  loop methods like "loop.create_server()";

* The Event Loop Implementations section documents the
  "SelectorEventLoop" and "ProactorEventLoop" classes;

* The Examples section showcases how to work with some event loop
  APIs.


Event Loop Methods
==================

Event loops have **low-level** APIs for the following:

* Running and stopping the loop

* Scheduling callbacks

* Scheduling delayed callbacks

* Creating Futures and Tasks

* Opening network connections

* Creating network servers

* Transferring files

* TLS Upgrade

* Watching file descriptors

* Working with socket objects directly

* DNS

* Working with pipes

* Unix signals

* Executing code in thread or process pools

* Error Handling API

* Enabling debug mode

* Running Subprocesses


Running and stopping the loop
-----------------------------

loop.run_until_complete(future)

   Run until the *future* (an instance of "Future") has completed.

   If the argument is a coroutine object it is implicitly scheduled to
   run as a "asyncio.Task".

   Return the Future's result or raise its exception.

loop.run_forever()

   Run the event loop until "stop()" is called.

   If "stop()" is called before "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 "stop()" is called while "run_forever()" is running, the loop
   will run the current batch of callbacks and then exit. Note that
   new callbacks scheduled by callbacks will not run in this case;
   instead, they will run the next time "run_forever()" or
   "run_until_complete()" is called.

loop.stop()

   Stop the event loop.

loop.is_running()

   Return "True" if the event loop is currently running.

loop.is_closed()

   Return "True" if the event loop was closed.

loop.close()

   Close the event loop.

   The loop must not 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.

coroutine loop.shutdown_asyncgens()

   Schedule all currently open *asynchronous generator* objects to
   close with an "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.

   Note that there is no need to call this function when
   "asyncio.run()" is used.

   Example:

      try:
          loop.run_forever()
      finally:
          loop.run_until_complete(loop.shutdown_asyncgens())
          loop.close()

   3.6 版新加入.


Scheduling callbacks
--------------------

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 "contextvars.Context" for the *callback* to run in. The
   current context is used when no *context* is provided.

   An instance of "asyncio.Handle" is returned, which can be used
   later to cancel the callback.

   This method is not thread-safe.

loop.call_soon_threadsafe(callback, *args, context=None)

   A thread-safe variant of "call_soon()".  Must be used to schedule
   callbacks *from another thread*.

   See the concurrency and multithreading section of the
   documentation.

3.7 版更變: The *context* keyword-only parameter was added. See **PEP
567** for more details.

備註: Most "asyncio" scheduling functions don't allow passing
  keyword arguments.  To do that, use "functools.partial()":

     # 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 render partial objects better in debug and error
  messages.


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.

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 "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, the order in which they are
   called is undefined.

   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 "functools.partial()".

   An optional keyword-only *context* argument allows specifying a
   custom "contextvars.Context" for the *callback* to run in. The
   current context is used when no *context* is provided.

   3.7 版更變: The *context* keyword-only parameter was added. See
   **PEP 567** for more details.

   3.8 版更變: In Python 3.7 and earlier with the default event loop
   implementation, the *delay* could not exceed one day. This has been
   fixed in Python 3.8.

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
   "loop.time()".

   This method's behavior is the same as "call_later()".

   An instance of "asyncio.TimerHandle" is returned which can be used
   to cancel the callback.

   3.7 版更變: The *context* keyword-only parameter was added. See
   **PEP 567** for more details.

   3.8 版更變: In Python 3.7 and earlier with the default event loop
   implementation, the difference between *when* and the current time
   could not exceed one day.  This has been fixed in Python 3.8.

loop.time()

   Return the current time, as a "float" value, according to the event
   loop's internal monotonic clock.

備註: 3.8 版更變: In Python 3.7 and earlier timeouts (relative
  *delay* or absolute *when*) should not exceed one day.  This has
  been fixed in Python 3.8.

也參考: The "asyncio.sleep()" function.


Creating Futures and Tasks
--------------------------

loop.create_future()

   Create an "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).

   3.5.2 版新加入.

loop.create_task(coro, *, name=None)

   Schedule the execution of a Coroutines. 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".

   If the *name* argument is provided and not "None", it is set as the
   name of the task using "Task.set_name()".

   3.8 版更變: Added the "name" parameter.

loop.set_task_factory(factory)

   Set a task factory that will be used by "loop.create_task()".

   If *factory* is "None" the default task factory will be set.
   Otherwise, *factory* must be a *callable* with the signature
   matching "(loop, coro)", where *loop* is a reference to the active
   event loop, and *coro* is a coroutine object.  The callable must
   return a "asyncio.Future"-compatible object.

loop.get_task_factory()

   Return a task factory or "None" if the default one is in use.


Opening network connections
---------------------------

coroutine 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 "AF_INET" or "AF_INET6" depending
   on *host* (or the *family* argument, if provided).

   The socket type will be "SOCK_STREAM".

   *protocol_factory* must be a callable returning an 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:

   1. The connection is established and a transport is created for
      it.

   2. *protocol_factory* is called without arguments and is
      expected to return a protocol instance.

   3. The protocol instance is coupled with the transport by
      calling its "connection_made()" method.

   4. A "(transport, protocol)" tuple is returned on success.

   The created transport is an implementation-dependent bidirectional
   stream.

   Other arguments:

   * *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 default context returned from
     "ssl.create_default_context()" is used.

     也參考: SSL/TLS security considerations

   * *server_hostname* sets or overrides the hostname that the
     target server's certificate will be matched against.  Should only
     be passed if *ssl* is not "None".  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
     potential 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.

   * *happy_eyeballs_delay*, if given, enables Happy Eyeballs for
     this connection. It should be a floating-point number
     representing the amount of time in seconds to wait for a
     connection attempt to complete, before starting the next attempt
     in parallel. This is the "Connection Attempt Delay" as defined in
     **RFC 8305**. A sensible default value recommended by the RFC is
     "0.25" (250 milliseconds).

   * *interleave* controls address reordering when a host name
     resolves to multiple IP addresses. If "0" or unspecified, no
     reordering is done, and addresses are tried in the order returned
     by "getaddrinfo()". If a positive integer is specified, the
     addresses are interleaved by address family, and the given
     integer is interpreted as "First Address Family Count" as defined
     in **RFC 8305**. The default is "0" if *happy_eyeballs_delay* is
     not specified, and "1" if it is.

   * *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*,
     *happy_eyeballs_delay*, *interleave* 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 a TLS connection) the time in
     seconds to wait for the TLS handshake to complete before aborting
     the connection. "60.0" seconds if "None" (default).

   3.8 版新加入: The *happy_eyeballs_delay* and *interleave*
   parameters.

   3.7 版新加入: The *ssl_handshake_timeout* parameter.

   3.6 版更變: The socket option "TCP_NODELAY" is set by default for
   all TCP connections.

   3.5 版更變: Added support for SSL/TLS in "ProactorEventLoop".

   也參考: The "open_connection()" function is a high-level
     alternative API. It returns a pair of ("StreamReader",
     "StreamWriter") that can be used directly in async/await code.

coroutine 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 "AF_INET", "AF_INET6", or
   "AF_UNIX", depending on *host* (or the *family* argument, if
   provided).

   The socket type will be "SOCK_DGRAM".

   *protocol_factory* must be a callable returning a 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 "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
     "getaddrinfo()".

   * *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.

   * *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 Unixes. If the
     "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, "socket.socket" object to be used
     by the transport. If specified, *local_addr* and *remote_addr*
     should be omitted (must be "None").

   See UDP echo client protocol and UDP echo server protocol examples.

   3.4.4 版更變: The *family*, *proto*, *flags*, *reuse_address*,
   *reuse_port, *allow_broadcast*, and *sock* parameters were added.

   3.8 版更變: Added support for Windows.

coroutine loop.create_unix_connection(protocol_factory, path=None, *, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None)

   Create a Unix connection.

   The socket family will be "AF_UNIX"; socket type will be
   "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, "str",
   "bytes", and "Path" paths are supported.

   See the documentation of the "loop.create_connection()" method for
   information about arguments to this method.

   可用性: Unix。

   3.7 版新加入: The *ssl_handshake_timeout* parameter.

   3.7 版更變: The *path* parameter can now be a *path-like object*.


Creating network servers
------------------------

coroutine 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 "SOCK_STREAM") listening on *port*
   of the *host* address.

   Returns a "Server" object.

   Arguments:

   * *protocol_factory* must be a callable returning a protocol
     implementation.

   * The *host* parameter can be set to several types which
     determine where the server would be listening:

     * If *host* is a string, the TCP server is bound to a single
       network interface specified by *host*.

     * If *host* is a sequence of strings, the TCP server is bound
       to all network interfaces specified by 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 "socket.AF_INET" or "AF_INET6" to
     force the socket to use IPv4 or IPv6. If not set, the *family*
     will be determined from host name (defaults to "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* must
     not be specified.

   * *backlog* is the maximum number of queued connections passed to
     "listen()" (defaults to 100).

   * *ssl* can be set to an "SSLContext" instance to enable TLS 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 a TLS server) the time in
     seconds to wait for the TLS 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 "Server.start_serving()" or
     "Server.serve_forever()" to make the server to start accepting
     connections.

   3.7 版新加入: Added *ssl_handshake_timeout* and *start_serving*
   parameters.

   3.6 版更變: The socket option "TCP_NODELAY" is set by default for
   all TCP connections.

   3.5 版更變: Added support for SSL/TLS in "ProactorEventLoop".

   3.5.1 版更變: The *host* parameter can be a sequence of strings.

   也參考: The "start_server()" function is a higher-level
     alternative API that returns a pair of "StreamReader" and
     "StreamWriter" that can be used in an async/await code.

coroutine loop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, start_serving=True)

   Similar to "loop.create_server()" but works with the "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, "str",
   "bytes", and "Path" paths are supported.

   See the documentation of the "loop.create_server()" method for
   information about arguments to this method.

   可用性: Unix。

   3.7 版新加入: The *ssl_handshake_timeout* and *start_serving*
   parameters.

   3.7 版更變: The *path* parameter can now be a "Path" object.

coroutine 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:

   * *protocol_factory* must be a callable returning a protocol
     implementation.

   * *sock* is a preexisting socket object returned from
     "socket.accept".

   * *ssl* can be set to an "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.

   3.7 版新加入: The *ssl_handshake_timeout* parameter.

   3.5.3 版新加入.


Transferring files
------------------

coroutine 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 "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 always
   updated, even when this method raises an error, and "file.tell()"
   can be used to obtain the actual number of bytes 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 "SendfileNotAvailableError" if the system does not support
   the *sendfile* syscall and *fallback* is "False".

   3.7 版新加入.


TLS Upgrade
-----------

coroutine 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
     "create_server()" and "create_connection()" return.

   * *sslcontext*: a configured instance of "SSLContext".

   * *server_side* pass "True" when a server-side connection is
     being upgraded (like the one created by "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 a TLS connection) the time in
     seconds to wait for the TLS handshake to complete before aborting
     the connection. "60.0" seconds if "None" (default).

   3.7 版新加入.


Watching file descriptors
-------------------------

loop.add_reader(fd, callback, *args)

   Start monitoring the *fd* file descriptor for read availability and
   invoke *callback* with the specified arguments once *fd* is
   available for reading.

loop.remove_reader(fd)

   Stop monitoring the *fd* file descriptor for read availability.

loop.add_writer(fd, callback, *args)

   Start monitoring the *fd* file descriptor for write availability
   and invoke *callback* with the specified arguments once *fd* is
   available for writing.

   使用 "functools.partial()" 传递关键字参数  给 *callback*.

loop.remove_writer(fd)

   Stop monitoring the *fd* file descriptor for write availability.

See also 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 "loop.create_connection()" and "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 "socket" objects directly is more convenient.

coroutine loop.sock_recv(sock, nbytes)

   Receive up to *nbytes* from *sock*.  Asynchronous version of
   "socket.recv()".

   Return the received data as a bytes object.

   *sock* must be a non-blocking socket.

   3.7 版更變: Even though this method was always documented as a
   coroutine method, releases before Python 3.7 returned a "Future".
   Since Python 3.7 this is an "async def" method.

coroutine loop.sock_recv_into(sock, buf)

   Receive data from *sock* into the *buf* buffer.  Modeled after the
   blocking "socket.recv_into()" method.

   Return the number of bytes written to the buffer.

   *sock* must be a non-blocking socket.

   3.7 版新加入.

coroutine loop.sock_sendall(sock, data)

   Send *data* to the *sock* socket. Asynchronous version of
   "socket.sendall()".

   This method continues to send 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.

   *sock* must be a non-blocking socket.

   3.7 版更變: Even though the method was always documented as a
   coroutine method, before Python 3.7 it returned an "Future". Since
   Python 3.7, this is an "async def" method.

coroutine loop.sock_connect(sock, address)

   Connect *sock* to a remote socket at *address*.

   Asynchronous version of "socket.connect()".

   *sock* must be a non-blocking socket.

   3.5.2 版更變: "address" no longer needs to be resolved.
   "sock_connect" will try to check if the *address* is already
   resolved by calling "socket.inet_pton()".  If not,
   "loop.getaddrinfo()" will be used to resolve the *address*.

   也參考: "loop.create_connection()" and
     "asyncio.open_connection()".

coroutine loop.sock_accept(sock)

   Accept a connection.  Modeled after the blocking "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.

   *sock* must be a non-blocking socket.

   3.7 版更變: Even though the method was always documented as a
   coroutine method, before Python 3.7 it returned a "Future". Since
   Python 3.7, this is an "async def" method.

   也參考: "loop.create_server()" and "start_server()".

coroutine loop.sock_sendfile(sock, file, offset=0, count=None, *, fallback=True)

   Send a file using high-performance "os.sendfile" if possible.
   Return the total number of bytes sent.

   Asynchronous version of "socket.sendfile()".

   *sock* must be a non-blocking "socket.SOCK_STREAM" "socket".

   *file* must be a regular file object open 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 always
   updated, even when this method raises an error, and "file.tell()"
   can be used to obtain the actual number of bytes 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 "SendfileNotAvailableError" if the system does not support
   *sendfile* syscall and *fallback* is "False".

   *sock* must be a non-blocking socket.

   3.7 版新加入.


DNS
---

coroutine loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

   Asynchronous version of "socket.getaddrinfo()".

coroutine loop.getnameinfo(sockaddr, flags=0)

   Asynchronous version of "socket.getnameinfo()".

3.7 版更變: Both *getaddrinfo* and *getnameinfo* methods were always
documented to return a coroutine, but prior to Python 3.7 they were,
in fact, returning "asyncio.Future" objects.  Starting with Python 3.7
both methods are coroutines.


Working with pipes
------------------

coroutine loop.connect_read_pipe(protocol_factory, pipe)

   Register the read end of *pipe* in the event loop.

   *protocol_factory* must be a callable returning an asyncio protocol
   implementation.

   *pipe* is a *file-like object*.

   Return pair "(transport, protocol)", where *transport* supports the
   "ReadTransport" interface and *protocol* is an object instantiated
   by the *protocol_factory*.

   With "SelectorEventLoop" event loop, the *pipe* is set to non-
   blocking mode.

coroutine loop.connect_write_pipe(protocol_factory, pipe)

   Register the write end of *pipe* in the event loop.

   *protocol_factory* must be a callable returning an asyncio protocol
   implementation.

   *pipe* is *file-like object*.

   Return pair "(transport, protocol)", where *transport* supports
   "WriteTransport" interface and *protocol* is an object instantiated
   by the *protocol_factory*.

   With "SelectorEventLoop" event loop, the *pipe* is set to non-
   blocking mode.

備註: "SelectorEventLoop" does not support the above methods on
  Windows. Use "ProactorEventLoop" instead for Windows.

也參考: The "loop.subprocess_exec()" and "loop.subprocess_shell()"
  methods.


Unix signals
------------

loop.add_signal_handler(signum, callback, *args)

   Set *callback* as the handler for the *signum* signal.

   The callback will be invoked by *loop*, along with other queued
   callbacks and runnable coroutines of that event loop. Unlike signal
   handlers registered using "signal.signal()", a callback registered
   with this function is allowed to interact with the event loop.

   Raise "ValueError" if the signal number is invalid or uncatchable.
   Raise "RuntimeError" if there is a problem setting up the handler.

   使用 "functools.partial()" 传递关键字参数  给 *callback*.

   和 "signal.signal()" 一样，这个函数只能在主线程中调用。

loop.remove_signal_handler(sig)

   Remove the handler for the *sig* signal.

   Return "True" if the signal handler was removed, or "False" if no
   handler was set for the given signal.

   可用性: Unix。

也參考: The "signal" module.


Executing code in thread or process pools
-----------------------------------------

awaitable loop.run_in_executor(executor, func, *args)

   Arrange for *func* to be called in the specified executor.

   The *executor* argument should be an "concurrent.futures.Executor"
   instance. The default executor is used if *executor* is "None".

   Example:

      import asyncio
      import concurrent.futures

      def blocking_io():
          # File operations (such as logging) can block the
          # event loop: run them in a thread pool.
          with open('/dev/urandom', 'rb') as f:
              return f.read(100)

      def cpu_bound():
          # CPU-bound operations will block the event loop:
          # in general it is preferable to run them in a
          # process pool.
          return sum(i * i for i in range(10 ** 7))

      async def main():
          loop = asyncio.get_running_loop()

          ## Options:

          # 1. Run in the default loop's executor:
          result = await loop.run_in_executor(
              None, blocking_io)
          print('default thread pool', result)

          # 2. Run in a custom thread pool:
          with concurrent.futures.ThreadPoolExecutor() as pool:
              result = await loop.run_in_executor(
                  pool, blocking_io)
              print('custom thread pool', result)

          # 3. Run in a custom process pool:
          with concurrent.futures.ProcessPoolExecutor() as pool:
              result = await loop.run_in_executor(
                  pool, cpu_bound)
              print('custom process pool', result)

      asyncio.run(main())

   This method returns a "asyncio.Future" object.

   Use "functools.partial()" to pass keyword arguments to *func*.

   3.5.3 版更變: "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 ("ThreadPoolExecutor") to
   set the default.

loop.set_default_executor(executor)

   Set *executor* as the default executor used by "run_in_executor()".
   *executor* should be an instance of "ThreadPoolExecutor".

   3.8 版後已棄用: Using an executor that is not an instance of
   "ThreadPoolExecutor" is deprecated and will trigger an error in
   Python 3.9.

   *executor* must be an instance of
   "concurrent.futures.ThreadPoolExecutor".


Error Handling API
------------------

Allows customizing how exceptions are handled in the event loop.

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.
   Otherwise, *handler* must be a callable with the signature matching
   "(loop, context)", where "loop" is a reference to the active event
   loop, and "context" is a "dict" object containing the details of
   the exception (see "call_exception_handler()" documentation for
   details about context).

loop.get_exception_handler()

   Return the current exception handler, or "None" if no custom
   exception handler was set.

   3.5.2 版新加入.

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
   "call_exception_handler()".

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 in future Python versions):

   * '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.

   備註: This method should not be overloaded in subclassed event
     loops. For custom exception handling, use the
     "set_exception_handler()" method.


Enabling debug mode
-------------------

loop.get_debug()

   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.

loop.set_debug(enabled: bool)

   Set the debug mode of the event loop.

   3.7 版更變: The new "-X dev" command line option can now also be
   used to enable the debug mode.

也參考: The debug mode of asyncio.


Running Subprocesses
--------------------

Methods described in this subsections are low-level.  In regular
async/await code consider using the high-level
"asyncio.create_subprocess_shell()" and
"asyncio.create_subprocess_exec()" convenience functions instead.

備註: The default asyncio event loop on **Windows** does not support
  subprocesses. See Subprocess Support on Windows for details.

coroutine 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:

   * "str";

   * or "bytes", encoded to the filesystem encoding.

   The first string specifies the program executable, and the
   remaining strings specify the arguments.  Together, string
   arguments form the "argv" of the program.

   This is similar to the standard library "subprocess.Popen" class
   called with "shell=False" and the list of strings passed as the
   first argument; however, where "Popen" takes a single argument
   which is list of strings, *subprocess_exec* takes multiple string
   arguments.

   The *protocol_factory* must be a callable returning a subclass of
   the "asyncio.SubprocessProtocol" class.

   Other parameters:

   * *stdin* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard input stream using "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

   * *stdout* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard output stream using
       "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

   * *stderr* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard error stream using "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

     * the "subprocess.STDOUT" constant which will connect the
       standard error stream to the process' standard output stream

   * All other keyword arguments are passed to "subprocess.Popen"
     without interpretation, except for *bufsize*,
     *universal_newlines*, *shell*, *text*, *encoding* and *errors*,
     which should not be specified at all.

     The "asyncio" subprocess API does not support decoding the
     streams as text. "bytes.decode()" can be used to convert the
     bytes returned from the stream to text.

   See the constructor of the "subprocess.Popen" class for
   documentation on other arguments.

   Returns a pair of "(transport, protocol)", where *transport*
   conforms to the "asyncio.SubprocessTransport" base class and
   *protocol* is an object instantiated by the *protocol_factory*.

coroutine 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 "str" or a "bytes"
   string encoded to the filesystem encoding, using the platform's
   "shell" syntax.

   This is similar to the standard library "subprocess.Popen" class
   called with "shell=True".

   The *protocol_factory* must be a callable returning a subclass of
   the "SubprocessProtocol" class.

   See "subprocess_exec()" for more details about the remaining
   arguments.

   Returns a pair of "(transport, protocol)", where *transport*
   conforms to the "SubprocessTransport" base class and *protocol* is
   an object instantiated by the *protocol_factory*.

備註: It is the application's responsibility to ensure that all
  whitespace and special characters are quoted appropriately to avoid
  shell injection vulnerabilities. The "shlex.quote()" function can be
  used to properly escape whitespace and special characters in strings
  that are going to be used to construct shell commands.


Callback Handles
================

class asyncio.Handle

   A callback wrapper object returned by "loop.call_soon()",
   "loop.call_soon_threadsafe()".

   cancel()

      Cancel the callback.  If the callback has already been canceled
      or executed, this method has no effect.

   cancelled()

      Return "True" if the callback was cancelled.

      3.7 版新加入.

class asyncio.TimerHandle

   A callback wrapper object returned by "loop.call_later()", and
   "loop.call_at()".

   This class is a subclass of "Handle".

   when()

      Return a scheduled callback time as "float" seconds.

      The time is an absolute timestamp, using the same time reference
      as "loop.time()".

      3.7 版新加入.


Server Objects
==============

Server objects are created by "loop.create_server()",
"loop.create_unix_server()", "start_server()", and
"start_unix_server()" functions.

Do not instantiate the class directly.

class asyncio.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.

   3.7 版更變: Server object is an asynchronous context manager since
   Python 3.7.

   close()

      Stop serving: close listening sockets and set the "sockets"
      attribute to "None".

      The sockets that represent existing incoming client connections
      are left open.

      The server is closed asynchronously, use the "wait_closed()"
      coroutine to wait until the server is closed.

   get_loop()

      Return the event loop associated with the server object.

      3.7 版新加入.

   coroutine start_serving()

      Start accepting connections.

      This method is idempotent, so it can be called when the server
      is already being serving.

      The *start_serving* keyword-only parameter to
      "loop.create_server()" and "asyncio.start_server()" allows
      creating a Server object that is not accepting connections
      initially.  In this case "Server.start_serving()", or
      "Server.serve_forever()" can be used to make the Server start
      accepting connections.

      3.7 版新加入.

   coroutine 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))

      3.7 版新加入.

   is_serving()

      Return "True" if the server is accepting new connections.

      3.7 版新加入.

   coroutine wait_closed()

      Wait until the "close()" method completes.

   sockets

      List of "socket.socket" objects the server is listening on.

      3.7 版更變: Prior to Python 3.7 "Server.sockets" used to return
      an internal list of server sockets directly.  In 3.7 a copy of
      that list is returned.


Event Loop Implementations
==========================

asyncio ships with two different event loop implementations:
"SelectorEventLoop" and "ProactorEventLoop".

By default asyncio is configured to use "SelectorEventLoop" on Unix
and "ProactorEventLoop" on Windows.

class asyncio.SelectorEventLoop

   An event loop based on the "selectors" module.

   Uses the most efficient *selector* available for the given
   platform.  It is also possible to manually configure the exact
   selector implementation to be used:

      import asyncio
      import selectors

      selector = selectors.SelectSelector()
      loop = asyncio.SelectorEventLoop(selector)
      asyncio.set_event_loop(loop)

   可用性: Unix, Windows。

class asyncio.ProactorEventLoop

   An event loop for Windows that uses "I/O Completion Ports" (IOCP).

   可用性: Windows。

   也參考: MSDN documentation on I/O Completion Ports.

class asyncio.AbstractEventLoop

   Abstract base class for asyncio-compliant event loops.

   The 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 the low-level event loop APIs, such as "loop.run_forever()" and
"loop.call_soon()".  Modern asyncio applications rarely need to be
written this way; consider using the high-level functions like
"asyncio.run()".


Hello World with call_soon()
----------------------------

An example using the "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()

也參考: A similar Hello World example created with a coroutine and
  the "run()" function.


Display the current date with call_later()
------------------------------------------

An example of a callback displaying the current date every second. The
callback uses the "loop.call_later()" method to reschedule itself
after 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()

也參考: A similar current date example created with a coroutine and
  the "run()" function.


Watch a file descriptor for read events
---------------------------------------

Wait until a file descriptor received some data using the
"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()

也參考:

  * A similar example using transports, protocols, and the
    "loop.create_connection()" method.

  * Another similar example using the high-level "asyncio.connect()"
    function and streams.


Set signal handlers for SIGINT and SIGTERM
------------------------------------------

(This "signals" example only works on Unix.)

Register handlers for signals "SIGINT" and "SIGTERM" using the
"loop.add_signal_handler()" method:

   import asyncio
   import functools
   import os
   import signal

   def ask_exit(signame, loop):
       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, loop))

       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())
