New submission from Luca Mattiello <lucae.mattie...@gmail.com>:
Reading the contextlib documentation, one might assume the following to be
functionally equivalent, when used in a with statement:
@contextlib.contextmanager
def managed_resource():
resource = acquire()
try:
yield resource
finally:
resource.release()
class ManagedResource:
def __init__(self):
self.resource = acquire()
def __enter__(self):
return self.resource
def __exit__(self, *args):
self.resource.release()
However, the first version has a seemingly unexpected behavior when used in
conjunction with an ExitStack, and pop_all().
with contextlib.ExitStack() as es:
r = es.enter_context(managed_resource())
es.pop_all()
# Uh-oh, r gets released anyway
with contextlib.ExitStack() as es:
r = es.enter_context(ManagedResource())
es.pop_all()
# Works as expected
I think the reason is
https://docs.python.org/3/reference/expressions.html#yield-expressions, in
particular
> Yield expressions are allowed anywhere in a try construct.
> If the generator is not resumed before it is finalized (by
> reaching a zero reference count or by being garbage collected),
> the generator-iterator’s close() method will be called,
> allowing any pending finally clauses to execute.
I guess this is working according to the specs, but I found it very
counter-intuitive. Could we improve the documentation to point out this subtle
difference?
Full repro:
import contextlib
@contextlib.contextmanager
def cm():
print("acquire cm")
try:
yield 1
finally:
print("release cm")
class CM:
def __init__(self):
print("acquire CM")
def __enter__(self):
return 1
def __exit__(self, *args):
print("release CM")
def f1():
with contextlib.ExitStack() as es:
es.enter_context(cm())
es.pop_all()
def f2():
with contextlib.ExitStack() as es:
es.enter_context(CM())
es.pop_all()
f1()
f2()
Output:
acquire cm
release cm
acquire CM
----------
assignee: docs@python
components: Documentation, Library (Lib)
messages: 394948
nosy: docs@python, lucae.mattiello
priority: normal
severity: normal
status: open
title: contextmanager + ExitStack.pop_all()
versions: Python 3.6, Python 3.7, Python 3.8, Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44292>
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