[issue22374] Replace contextmanager example and improve explanation

[Berker Peksag]

Berker Peksag  added the comment:

The old tag() example has been replaced with a different example in 
https://github.com/python/cpython/commit/bde782bb594edffeabe978abeee2b7082ab9bc2a
 (bpo-33468)

--
resolution:  -> duplicate
stage: patch review -> resolved
status: open -> closed
superseder:  -> Add try-finally contextlib.contextmanager example

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[issue22374] Replace contextmanager example and improve explanation

[cowlinator]

cowlinator  added the comment:

I would like to second the improved explanation of contextlib.contextmanager, 
and additionally point out another problem:  
A very important piece of information is missing from the documentation:  how 
to return data from the contextmanager-wrapped function.  

I had to go look up the source code, which had a wonderful explanation in the 
comments: https://gist.github.com/enuomi/1385336#file-contextlib-py-L56 

In particular, note that 

@contextmanager
def some_generator():
  yield 

can be used to return  to the caller, via

with some_generator() as :
  print(return_data)

This information is wholly and completely missing from the 
contextlib.contextmanager documentation.

--
nosy: +cowlinator
versions: +Python 2.7, Python 3.4, Python 3.7

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[issue22374] Replace contextmanager example and improve explanation

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

swap_attr() looks too general. I think something more concrete would be better. 
But the documentation already contain other examples for contextmanager. 
closing() is good example, and redirect_stdout() would be good example.

--
nosy: +serhiy.storchaka

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[issue22374] Replace contextmanager example and improve explanation

[Berker Peksag]

Berker Peksag added the comment:

We also have swap_attr() and swap_item() helpers in 
Lib/test/support/__init__.py. I've used a simplified version of swap_attr() in 
my patch.

Since this is basically a document improvement I removed Python 2.7 from the 
versions field.

--
keywords: +patch
nosy: +berker.peksag
stage:  -> patch review
type:  -> enhancement
versions: +Python 3.6 -Python 2.7, Python 3.4
Added file: http://bugs.python.org/file43095/issue22374.diff

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[issue22374] Replace contextmanager example and improve explanation

[Martin Panter]

Martin Panter added the comment:

You should probably use try / finally in your context manager to always restore 
the attribute. Having said that, I recently wrote a similar context manager, 
and then later discovered there is already “unittest.mock.patch” and/or 
“unittest.mock.patch.object” via Issue 11664, which apparently can do this job.

--
nosy: +vadmium

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[issue22374] Replace contextmanager example and improve explanation

[Terry J. Reedy]

New submission from Terry J. Reedy:

https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager
The current html contextmanager example is 'not recommended' for actual use, 
because there are better ways to accomplish the same goal.  To me, is also 
unsatifactory in that the context management is only metaphorical (conceptual) 
and not actual.

I propose the following as a replacement. It actually manages context and is, I 
believe, both useful and currently* the best way to accomplish the goal of 
temporarily monkeypatching a module,  It was directly inspired by #20752, see 
msg226657, though there have been other issues where monkeypatching as a 
solution has been discussed.

---
from contextlib import contextmanager
import itertools as its

@contextmanager
def mp(ob, attr, new):
old = getattr(ob, attr)
setattr(ob, attr, new)
yield
setattr(ob, attr, old)

def f(): pass

print(its.count, its.cycle)
with mp(its, 'count', f), mp(its, 'cycle', f):
print(its.count, its.cycle)
print(its.count, its.cycle)

# class 'itertools.count'
# function f at 0x035A91E0
# class 'itertools.count'
---

I am aware that the above does not follow the current style, which I dislike, 
of confusingly mixing together batch file code and interactive input code.  I 
think the above is how the example should be written.  It would work even 
better if Sphinx gave comment lines a different background color. (A '##' 
prefix could be used to differentiate code comments from commented-out output 
lines.)

In the same section, I find the following paragraph a bit confusing (perhaps is 
tries to say too much):

contextmanager() uses ContextDecorator so the context managers it creates can 
be used as decorators as well as in with statements. When used as a decorator, 
a new generator instance is implicitly created on each function call (this 
allows the otherwise “one-shot” context managers created by contextmanager() to 
meet the requirement that context managers support multiple invocations in 
order to be used as decorators).

I am guessing that this means, among other things, that ContextDecorator is 
necessary and sufficient to use mp twice in one with statement.  I 
intentionally added the double use to the example to make this possibility 
clear.

* The only better way I know of would be if mp (spelled out as 'monkeypatch' 
were considered useful enough to be added to contextlib.

--
assignee: docs@python
components: Documentation
messages: 226661
nosy: docs@python, ncoghlan, terry.reedy
priority: normal
severity: normal
status: open
title: Replace contextmanager example and improve explanation
versions: Python 2.7, Python 3.4, Python 3.5

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