[issue34067] Problem with contextlib.nullcontext

[Irit Katriel]

Change by Irit Katriel :


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stage: patch review -> needs patch

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[issue34067] Problem with contextlib.nullcontext

[Irit Katriel]

Change by Irit Katriel :


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pull_requests:  -21767

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[issue34067] Problem with contextlib.nullcontext

[Irit Katriel]

Change by Irit Katriel :


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versions: +Python 3.10, Python 3.9 -Python 3.7

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[issue34067] Problem with contextlib.nullcontext

[Irit Katriel]

Change by Irit Katriel :


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pull_requests: +21767
pull_request: https://github.com/python/cpython/pull/22811

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[issue34067] Problem with contextlib.nullcontext

[STINNER Victor]

Change by STINNER Victor :


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nosy:  -vstinner

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[issue34067] Problem with contextlib.nullcontext

[Irit Katriel]

Irit Katriel  added the comment:

For the original problem, how about this?

  def process_file(file_or_path):
  if isinstance(file_or_path, str):
  # If string, open file
  cm = open
  else:
  # Caller is responsible for closing file
  cm = nullcontext

  with cm(file_or_path) as file:
  # Perform processing on the file

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nosy: +iritkatriel

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[issue34067] Problem with contextlib.nullcontext

[Nick Coghlan]

Nick Coghlan  added the comment:

Right, the merged PR adds a simpler example that handles "ignore exceptions or 
not" by selecting between suppress() and nullcontext().

The documentation still needs updating to improve or remove the file-based 
example, and point out that for resources that get acquired on construction, 
you either need to delay construction to the start of the with statement, or 
else use ExitStack instead of nullcontext.

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[issue34067] Problem with contextlib.nullcontext

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

No, the merged PR is not directly related to the original issue.

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[issue34067] Problem with contextlib.nullcontext

[STINNER Victor]

STINNER Victor  added the comment:

Can we close this issue?

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nosy: +vstinner

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[issue34067] Problem with contextlib.nullcontext

[miss-islington]

miss-islington  added the comment:


New changeset 1b6e21730417c56748d500a96cafebbaa1bd0120 by Miss Islington (bot) 
in branch '3.7':
bpo-34067: Include a more easily understood example for nullcontext (GH-8158)
https://github.com/python/cpython/commit/1b6e21730417c56748d500a96cafebbaa1bd0120


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[issue34067] Problem with contextlib.nullcontext

[miss-islington]

Change by miss-islington :


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pull_requests: +7752

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[issue34067] Problem with contextlib.nullcontext

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset c287545d62edf1a1ee65727d3c57befa8c99c13a by Nick Coghlan (Daniel 
Porteous) in branch 'master':
bpo-34067: Include a more easily understood example for nullcontext (GH-8158)
https://github.com/python/cpython/commit/c287545d62edf1a1ee65727d3c57befa8c99c13a


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[issue34067] Problem with contextlib.nullcontext

[Daniel Porteous]

Change by Daniel Porteous :


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keywords: +patch
pull_requests: +7751
stage:  -> patch review

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[issue34067] Problem with contextlib.nullcontext

[Nick Coghlan]

Nick Coghlan  added the comment:

That's certainly similar to the problems with contextlib.nested, but I don't 
think it's as inherently flawed as nested was. What I'd suggest we change the 
existing example to is this:

  from functools import partial
  from os import fspath

  def process_file(file_or_path):
  try:
  as_fspath = os.fspath(file_or_path)
  except TypeError:
  # If it's a file, caller is responsible for closing it
  make_cm = partial(nullcontext, file_or_path)
  else:
  # If it's a path, open file when the context is entered
  make_cm = partial(open, as_fspath)

  with make_cm() as file:
  # Perform processing on the file

Optionally, we could also present a cleaner example where a pre-created context 
manager is passed in and we're just coping with the fact it may be None:

def update_resource(resource, updates, resource_lock=None):
if resource_lock is None:
resource_lock = nullcontext()
with resource_lock:
resource.apply_updates(updates)

(I'll also note that ExitStack is *far* from being immune to the Ctrl-C 
problem, as it's implemented in Python itself, which allows its __exit__ method 
to be interrupted, as well as for interrupt to occur between a resource being 
created or acquired, and it being registered with the stack)

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[issue34067] Problem with contextlib.nullcontext

[Eric V. Smith]

Change by Eric V. Smith :


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nosy: +eric.smith

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[issue34067] Problem with contextlib.nullcontext

[Serhiy Storchaka]

New submission from Serhiy Storchaka :

There is a flaw in the example in the documentation for contextlib.nullcontext.

  def process_file(file_or_path):
  if isinstance(file_or_path, str):
  # If string, open file
  cm = open(file_or_path)
  else:
  # Caller is responsible for closing file
  cm = nullcontext(file_or_path)

  with cm as file:
  # Perform processing on the file

The execution can be interrupted by Ctrl-C between opening a file and entering 
a 'with' block. There is the same problem with the simple "with open(path) as 
file:", but it can be easily solved (see issue34066). It is harder to do for 
this example, because there are more opcodes between calling open() and 
entering 'with'. I afraid that this problem can't be solved in general case. 
Maybe contextlib.nullcontext has a flaw similar to the flaw in 
contextlib.nested().

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messages: 321226
nosy: ncoghlan, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Problem with contextlib.nullcontext
type: resource usage
versions: Python 3.7, Python 3.8

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