[issue37743] How should contextmanager/ContextDecorator work with generators?

[Antony Lee]

Change by Antony Lee :


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nosy:  -Antony.Lee

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[issue37743] How should contextmanager/ContextDecorator work with generators?

[Nick Coghlan]

Nick Coghlan  added the comment:

This is a documentation bug, as the current behaviour is as intended, but the 
documented equivalence doesn't hold for generator functions: for a generator 
function, the CM will only be applied when the generator is instantiated, 
whereas the inline context manager version will be held open until the 
generator is closed or destroyed.

That said, an approach similar to the one discussed in #37398 could also be 
applied, here, with a separate "ContextDecorator.generator()" class method 
added to give the "wrapped yield from" behaviour. If anyone is interested in 
pursuing that, it can be filed as a separate enhancement issue (leaving this 
bug to cover the fact that the existing documentation is only accurate for 
regular synchronous functions)

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assignee: ncoghlan -> 
components: +Documentation -Library (Lib)

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[issue37743] How should contextmanager/ContextDecorator work with generators?

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee:  -> ncoghlan

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[issue37743] How should contextmanager/ContextDecorator work with generators?

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +ncoghlan

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[issue37743] How should contextmanager/ContextDecorator work with generators?

[Antony Lee]

New submission from Antony Lee :

The docs for ContextDecorator (of which contextmanager is a case) describe its 
semantics as:

... for any construct of the following form:

def f():
with cm():
# Do stuff

ContextDecorator lets you instead write:

@cm()
def f():
# Do stuff

However, when decorating a generator, the equivalence is broken:

from contextlib import contextmanager

@contextmanager
def cm():
print("start")
yield
print("stop")

def gen_using_with():
with cm():
yield from map(print, range(2))

@cm()
def gen_using_decorator():
yield from map(print, range(2))

print("using with")
list(gen_using_with())
print("==")
print("using decorator")
list(gen_using_decorator())

results in

using with
start
0
1
stop
==
using decorator
start
stop
0
1

i.e., when used as a decorator, the entire contextmanager is executed first 
before iterating over the generator (which is unsurprising given the 
implementation of ContextDecorator: ContextDecorator returns a function that 
executes the context manager and returns the generator, which is only iterated 
over later).

Should this be considered as a bug in ContextDecorator, and should 
ContextDecorator instead detect when it is used to decorate a generator (e.g. 
with inspect.isgeneratorfunction), and switch its implementation accordingly in 
that case?

--
components: Library (Lib)
messages: 348878
nosy: Antony.Lee
priority: normal
severity: normal
status: open
title: How should contextmanager/ContextDecorator work with generators?
versions: Python 3.8

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