[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:

Thanks for the PR, Matthias Bussonnier!

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nosy: +mbussonn
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions: +Python 2.7

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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:


New changeset 4e166ffd29b675238ccd94964743f6cb206d2235 by Tal Einat (Miss 
Islington (bot)) in branch '3.7':
bpo-33468: Add try-finally contextlib.contextmanager example (GH-7816) (GH-8425)
https://github.com/python/cpython/commit/4e166ffd29b675238ccd94964743f6cb206d2235


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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:


New changeset 5b3643d1a5114551350a9d17fb0aaab2d503c290 by Tal Einat (Miss 
Islington (bot)) in branch '3.6':
bpo-33468: Add try-finally contextlib.contextmanager example (GH-7816) (GH-8426)
https://github.com/python/cpython/commit/5b3643d1a5114551350a9d17fb0aaab2d503c290


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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:


New changeset f7e60a69485097dc28f000c55615038278f84333 by Tal Einat in branch 
'2.7':
[2.7] bpo-33468: Add try-finally contextlib.contextmanager example (GH-7816) 
(GH-8427)
https://github.com/python/cpython/commit/f7e60a69485097dc28f000c55615038278f84333


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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Change by Tal Einat :


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[issue33468] Add try-finally contextlib.contextmanager example

[miss-islington]

Change by miss-islington :


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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:


New changeset bde782bb594edffeabe978abeee2b7082ab9bc2a by Tal Einat (Matthias 
Bussonnier) in branch 'master':
bpo-33468: Add try-finally contextlib.contextmanager example (GH-7816)
https://github.com/python/cpython/commit/bde782bb594edffeabe978abeee2b7082ab9bc2a


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[issue33468] Add try-finally contextlib.contextmanager example

[miss-islington]

Change by miss-islington :


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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:

Nick's abstract example LGTM as well.

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[issue33468] Add try-finally contextlib.contextmanager example

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The entirely abstract example LGTM.

As for concrete real word examples, the possible one is changing the current 
working directory:

cwd = os.getcwd()
try:
os.chdir(target_dir)
yield
finally:
os.chdir(cwd)

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[issue33468] Add try-finally contextlib.contextmanager example

[Nick Coghlan]

Nick Coghlan  added the comment:

Similar to closing, we have dedicated context managers for stdout and stderr 
redirection now: 
https://docs.python.org/3/library/contextlib.html#contextlib.redirect_stdout

Redirecting stdin could be a good example though, since we've so far chosen not 
to provide a standard context manager for that (it's a sufficiently uncommon 
use case that we haven't seen the need to add it directly).

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[issue33468] Add try-finally contextlib.contextmanager example

[Tal Einat]

Tal Einat  added the comment:

A possible concrete example: Capturing sys.stdout and/or sys.stderr for 
testing, as in test.support.captured_output.

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[issue33468] Add try-finally contextlib.contextmanager example

[Nick Coghlan]

Nick Coghlan  added the comment:

Right, the HTML example was always a bit cutesy (hence the disclaimer in the 
header), but it's hard to come up with a good illustrative example that isn't 
already a native context manager in the standard library.

Perhaps it would make sense to change the example text completely, and write 
something entirely abstract like:

from contextlib import contextmanager

@contextmanager
def managed_resource(*args, **kwds):
# Code to acquire resource, e.g.:
resource = acquire_resource(*args, **kwds)
try:
yield resource
finally:
 # Code to release resource, e.g.:
 release_resource(resource)

>>> with managed_resource(*args, **kwds) as resource:
... # Resource is released at the end of this block,
... # even if code in the block raises an exception

Then the introductory text could be updated to say something like "While many 
objects natively support use in with statements, sometimes a resource needs to 
be managed that isn't a context manager in its own right, and doesn't implement 
a ``close()`` method for use with ``contextlib.closing``.".

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[issue33468] Add try-finally contextlib.contextmanager example

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It doesn't look as a good example. It is not always good to write a closing tag 
if an error was occurred while write an inner content. In many cases it is 
better to stop writing after error.

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[issue33468] Add try-finally contextlib.contextmanager example

[Matthias Bussonnier]

Change by Matthias Bussonnier :


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keywords: +patch
pull_requests: +7424
stage: needs patch -> patch review

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[issue33468] Add try-finally contextlib.contextmanager example

[Petr Viktorin]

Change by Petr Viktorin :


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[issue33468] Add try-finally contextlib.contextmanager example

[Nick Coghlan]

New submission from Nick Coghlan :

The current example for contextlib.contextmanager doesn't use try/finally, 
which sets folks up for writing resource management context managers that don't 
clean up after exceptions properly.

There's an example with try/finally down in the contextlib.closing docs, but 
I'm thinking it would be worthwhile to change the presentation of 
contextmanager itself.

--
assignee: ncoghlan
components: Documentation
messages: 316416
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Add try-finally contextlib.contextmanager example
type: enhancement
versions: Python 3.6, Python 3.7, Python 3.8

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