18.5.2. Event loops¶
Source code: Lib/asyncio/events.py
18.5.2.1. Event loop functions¶
The following functions are convenient shortcuts to accessing the methods of the
global policy. Note that this provides access to the default policy, unless an
alternative policy was set by calling set_event_loop_policy()
earlier in
the execution of the process.
-
asyncio.
get_event_loop
()¶ Equivalent to calling
get_event_loop_policy().get_event_loop()
.
-
asyncio.
set_event_loop
(loop)¶ Equivalent to calling
get_event_loop_policy().set_event_loop(loop)
.
-
asyncio.
new_event_loop
()¶ Equivalent to calling
get_event_loop_policy().new_event_loop()
.
18.5.2.2. Available event loops¶
asyncio currently provides two implementations of event loops:
SelectorEventLoop
and ProactorEventLoop
.
-
class
asyncio.
SelectorEventLoop
¶ Event loop based on the
selectors
module. Subclass ofAbstractEventLoop
.Use the most efficient selector available on the platform.
On Windows, only sockets are supported (ex: pipes are not supported): see the MSDN documentation of select.
-
class
asyncio.
ProactorEventLoop
¶ Proactor event loop for Windows using “I/O Completion Ports” aka IOCP. Subclass of
AbstractEventLoop
.Availability: Windows.
Ver também
Example to use a ProactorEventLoop
on Windows:
import asyncio, sys
if sys.platform == 'win32':
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
18.5.2.3. Platform support¶
The asyncio
module has been designed to be portable, but each platform
still has subtle differences and may not support all asyncio
features.
18.5.2.3.1. Windows¶
Common limits of Windows event loops:
create_unix_connection()
andcreate_unix_server()
are not supported: the socket familysocket.AF_UNIX
is specific to UNIXadd_signal_handler()
andremove_signal_handler()
are not supportedEventLoopPolicy.set_child_watcher()
is not supported.ProactorEventLoop
supports subprocesses. It has only one implementation to watch child processes, there is no need to configure it.
SelectorEventLoop
specific limits:
SelectSelector
is used which only supports sockets and is limited to 512 sockets.add_reader()
andadd_writer()
only accept file descriptors of socketsPipes are not supported (ex:
connect_read_pipe()
,connect_write_pipe()
)Subprocesses are not supported (ex:
subprocess_exec()
,subprocess_shell()
)
ProactorEventLoop
specific limits:
create_datagram_endpoint()
(UDP) is not supportedadd_reader()
andadd_writer()
are not supported
The resolution of the monotonic clock on Windows is usually around 15.6 msec. The best resolution is 0.5 msec. The resolution depends on the hardware (availability of HPET) and on the Windows configuration. See asyncio delayed calls.
Alterado na versão 3.5: ProactorEventLoop
now supports SSL.
18.5.2.3.2. Mac OS X¶
Character devices like PTY are only well supported since Mavericks (Mac OS 10.9). They are not supported at all on Mac OS 10.5 and older.
On Mac OS 10.6, 10.7 and 10.8, the default event loop is
SelectorEventLoop
which uses selectors.KqueueSelector
.
selectors.KqueueSelector
does not support character devices on these
versions. The SelectorEventLoop
can be used with
SelectSelector
or PollSelector
to
support character devices on these versions of Mac OS X. Example:
import asyncio
import selectors
selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)
18.5.2.4. Event loop policies and the default policy¶
Event loop management is abstracted with a policy pattern, to provide maximal
flexibility for custom platforms and frameworks. Throughout the execution of a
process, a single global policy object manages the event loops available to the
process based on the calling context. A policy is an object implementing the
AbstractEventLoopPolicy
interface.
For most users of asyncio
, policies never have to be dealt with
explicitly, since the default global policy is sufficient (see below).
The module-level functions
get_event_loop()
and set_event_loop()
provide convenient access to
event loops managed by the default policy.
18.5.2.5. Event loop policy interface¶
An event loop policy must implement the following interface:
-
class
asyncio.
AbstractEventLoopPolicy
¶ Event loop policy.
-
get_event_loop
()¶ Pegar o evento de loop para o contexto atual.
Returns an event loop object implementing the
AbstractEventLoop
interface. In case called from coroutine, it returns the currently running event loop.Raises an exception in case no event loop has been set for the current context and the current policy does not specify to create one. It must never return
None
.Alterado na versão 3.6.
-
set_event_loop
(loop)¶ Definir o evento de loop do contexto atual como loop.
-
new_event_loop
()¶ Create and return a new event loop object according to this policy’s rules.
If there’s need to set this loop as the event loop for the current context,
set_event_loop()
must be called explicitly.
-
The default policy defines context as the current thread, and manages an event
loop per thread that interacts with asyncio
. If the current thread
doesn’t already have an event loop associated with it, the default policy’s
get_event_loop()
method creates one when
called from the main thread, but raises RuntimeError
otherwise.
18.5.2.6. Access to the global loop policy¶
-
asyncio.
get_event_loop_policy
()¶ Get the current event loop policy.
-
asyncio.
set_event_loop_policy
(policy)¶ Set the current event loop policy. If policy is
None
, the default policy is restored.
18.5.2.7. Customizing the event loop policy¶
To implement a new event loop policy, it is recommended you subclass the
concrete default event loop policy DefaultEventLoopPolicy
and override the methods for which you want to change behavior, for example:
class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
def get_event_loop(self):
"""Get the event loop.
This may be None or an instance of EventLoop.
"""
loop = super().get_event_loop()
# Do something with loop ...
return loop
asyncio.set_event_loop_policy(MyEventLoopPolicy())