asyncio
An event loop policy is a global object used to get and set the current 事件循环 ，及创建新的事件循环。默认策略可以被 替换 with 内置替代 以使用不同事件循环实现，或被代入通过 自定义策略 可以覆盖这些行为。
The 策略对象 gets and sets a separate event loop per context . This is per-thread by default, though custom policies could define context differently.
Custom event loop policies can control the behavior of get_event_loop() , set_event_loop() ，和 new_event_loop() .
get_event_loop()
set_event_loop()
new_event_loop()
Policy objects should implement the APIs defined in the AbstractEventLoopPolicy abstract base class.
AbstractEventLoopPolicy
The following functions can be used to get and set the policy for the current process:
返回当前进程范围的策略。
Set the current process-wide policy to policy .
若 policy 被设为 None , the default policy is restored.
None
The abstract event loop policy base class is defined as follows:
An abstract base class for asyncio policies.
Get the event loop for the current context.
Return an event loop object implementing the AbstractEventLoop 接口。
AbstractEventLoop
This method should never return None .
3.6 版改变。
Set the event loop for the current context to loop .
Create and return a new event loop object.
Get a child process watcher object.
Return a watcher object implementing the AbstractChildWatcher 接口。
AbstractChildWatcher
This function is Unix specific.
Deprecated since version 3.12.
Set the current child process watcher to watcher .
asyncio ships with the following built-in policies:
The default asyncio policy. Uses SelectorEventLoop on Unix and ProactorEventLoop 在 Windows。
SelectorEventLoop
ProactorEventLoop
There is no need to install the default policy manually. asyncio is configured to use the default policy automatically.
3.8 版改变： 在 Windows， ProactorEventLoop is now used by default.
Deprecated since version 3.12: The get_event_loop() method of the default asyncio policy now emits a DeprecationWarning if there is no current event loop set and it decides to create one. In some future Python release this will become an error.
DeprecationWarning
An alternative event loop policy that uses the SelectorEventLoop event loop implementation.
可用性 ：Windows。
An alternative event loop policy that uses the ProactorEventLoop event loop implementation.
A process watcher allows customization of how an event loop monitors child processes on Unix. Specifically, the event loop needs to know when a child process has exited.
In asyncio, child processes are created with create_subprocess_exec() and loop.subprocess_exec() 函数。
create_subprocess_exec()
loop.subprocess_exec()
asyncio defines the AbstractChildWatcher abstract base class, which child watchers should implement, and has four different implementations: ThreadedChildWatcher (configured to be used by default), MultiLoopChildWatcher , SafeChildWatcher ，和 FastChildWatcher .
ThreadedChildWatcher
MultiLoopChildWatcher
SafeChildWatcher
FastChildWatcher
另请参阅 子进程和线程 章节。
The following two functions can be used to customize the child process watcher implementation used by the asyncio event loop:
Return the current child watcher for the current policy.
Set the current child watcher to watcher for the current policy. watcher must implement methods defined in the AbstractChildWatcher 基类。
注意
Third-party event loops implementations might not support custom child watchers. For such event loops, using set_child_watcher() might be prohibited or have no effect.
set_child_watcher()
Register a new child handler.
Arrange for callback(pid, returncode, *args) to be called when a process with PID equal to pid terminates. Specifying another callback for the same process replaces the previous handler.
callback(pid, returncode, *args)
The callback callable must be thread-safe.
Removes the handler for process with PID equal to pid .
函数返回 True if the handler was successfully removed, False if there was nothing to remove.
True
False
Attach the watcher to an event loop.
If the watcher was previously attached to an event loop, then it is first detached before attaching to the new loop.
Note: loop may be None .
返回 True if the watcher is ready to use.
Spawning a subprocess with inactive current child watcher raises RuntimeError .
RuntimeError
Added in version 3.8.
Close the watcher.
This method has to be called to ensure that underlying resources are cleaned-up.
This implementation starts a new waiting thread for every subprocess spawn.
It works reliably even when the asyncio event loop is run in a non-main OS thread.
There is no noticeable overhead when handling a big number of children ( O (1) each time a child terminates), but starting a thread per process requires extra memory.
This watcher is used by default.
This implementation registers a SIGCHLD signal handler on instantiation. That can break third-party code that installs a custom handler for SIGCHLD 信号。
SIGCHLD
The watcher avoids disrupting other code spawning processes by polling every process explicitly on a SIGCHLD 信号。
There is no limitation for running subprocesses from different threads once the watcher is installed.
The solution is safe but it has a significant overhead when handling a big number of processes ( O ( n ) each time a SIGCHLD is received).
This implementation uses active event loop from the main thread to handle SIGCHLD signal. If the main thread has no running event loop another thread cannot spawn a subprocess ( RuntimeError 被引发)。
This solution is as safe as MultiLoopChildWatcher and has the same O ( n ) complexity but requires a running event loop in the main thread to work.
This implementation reaps every terminated processes by calling os.waitpid(-1) directly, possibly breaking other code spawning processes and waiting for their termination.
os.waitpid(-1)
There is no noticeable overhead when handling a big number of children ( O (1) each time a child terminates).
This solution requires a running event loop in the main thread to work, as SafeChildWatcher .
This implementation polls process file descriptors (pidfds) to await child process termination. In some respects, PidfdChildWatcher is a “Goldilocks” child watcher implementation. It doesn’t require signals or threads, doesn’t interfere with any processes launched outside the event loop, and scales linearly with the number of subprocesses launched by the event loop. The main disadvantage is that pidfds are specific to Linux, and only work on recent (5.3+) kernels.
PidfdChildWatcher
Added in version 3.9.
To implement a new event loop policy, it is recommended to subclass DefaultEventLoopPolicy and override the methods for which custom behavior is wanted, e.g.:
DefaultEventLoopPolicy
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())
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