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Diffstat (limited to 'Doc/reference/expressions.rst')
| -rw-r--r-- | Doc/reference/expressions.rst | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst index 512aa5af95..8ac6264448 100644 --- a/Doc/reference/expressions.rst +++ b/Doc/reference/expressions.rst @@ -643,6 +643,16 @@ after resuming depends on the method which resumed the execution. If :meth:`~agen.asend` is used, then the result will be the value passed in to that method. +If an asynchronous generator happens to exit early by :keyword:`break`, the caller +task being cancelled, or other exceptions, the generator's async cleanup code +will run and possibly raise exceptions or access context variables in an +unexpected context--perhaps after the lifetime of tasks it depends, or +during the event loop shutdown when the async-generator garbage collection hook +is called. +To prevent this, the caller must explicitly close the async generator by calling +:meth:`~agen.aclose` method to finalize the generator and ultimately detach it +from the event loop. + In an asynchronous generator function, yield expressions are allowed anywhere in a :keyword:`try` construct. However, if an asynchronous generator is not resumed before it is finalized (by reaching a zero reference count or by @@ -654,9 +664,9 @@ generator-iterator's :meth:`~agen.aclose` method and run the resulting coroutine object, thus allowing any pending :keyword:`!finally` clauses to execute. -To take care of finalization, an event loop should define -a *finalizer* function which takes an asynchronous generator-iterator -and presumably calls :meth:`~agen.aclose` and executes the coroutine. +To take care of finalization upon event loop termination, an event loop should +define a *finalizer* function which takes an asynchronous generator-iterator and +presumably calls :meth:`~agen.aclose` and executes the coroutine. This *finalizer* may be registered by calling :func:`sys.set_asyncgen_hooks`. When first iterated over, an asynchronous generator-iterator will store the registered *finalizer* to be called upon finalization. For a reference example |