[issue39682] pathlib.Path objects can be used as context managers

[Barney Gale]

Change by Barney Gale :


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pull_request: https://github.com/python/cpython/pull/30971

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[issue39682] pathlib.Path objects can be used as context managers

[Antony Lee]

Change by Antony Lee :


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[issue39682] pathlib.Path objects can be used as context managers

[Antoine Pitrou]

Change by Antoine Pitrou :


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versions: +Python 3.9 -Python 3.8

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[issue39682] pathlib.Path objects can be used as context managers

[Antoine Pitrou]

Antoine Pitrou  added the comment:


New changeset 2e6d8b0ccdb6e0d9e98a9a7f9c9edfdf1311 by Barney Gale in branch 
'master':
bpo-39682: make `pathlib.Path` immutable by removing (undocumented) support for 
"closing" a path by using it as a context manager (GH-18846)
https://github.com/python/cpython/commit/2e6d8b0ccdb6e0d9e98a9a7f9c9edfdf1311


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[issue39682] pathlib.Path objects can be used as context managers

[Antoine Pitrou]

Change by Antoine Pitrou :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
type:  -> behavior

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[issue39682] pathlib.Path objects can be used as context managers

[Barney Gale]

Change by Barney Gale :


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keywords: +patch
pull_requests: +18203
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18846

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[issue39682] pathlib.Path objects can be used as context managers

[Isaac Muse]

Isaac Muse  added the comment:

Wrong thread sorry

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[issue39682] pathlib.Path objects can be used as context managers

[Isaac Muse]

Isaac Muse  added the comment:

Brace expansion does not currently exist in Python's glob. You'd have to use a 
third party module to expand the braces and then run glob on each returned 
pattern, or use a third party module that implements a glob that does it for 
you.

Shameless plug:

Brace expansion: https://github.com/facelessuser/bracex

Glob that does it for you (when the feature is enabled): 
https://github.com/facelessuser/wcmatch

Now whether Python should integrate such behavior by default is another 
question.

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[issue39682] pathlib.Path objects can be used as context managers

[Brett Cannon]

Brett Cannon  added the comment:

@Antoine I just quite follow what you mean. Are you saying ditch _closed and 
just leave the context manager to be a no-op?

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[issue39682] pathlib.Path objects can be used as context managers

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Note you could simply remove the "closed" flag and the context manager live. 
That way, you don't have to deprecate anything.

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[issue39682] pathlib.Path objects can be used as context managers

[Brett Cannon]

Brett Cannon  added the comment:

> Can I ask what sort of backwards-compatibility guarantees Python provides for 
> these cases?

A deprecation warning for two releases (i.e. two years). Then it can be 
removed. So if people want to move forward with removing this then a 
DeprecationWarning would need to be raised in all places where _closed is set 
with a message stating the functionality is deprecated in 3.9 and slated for 
removal in 3.11 (along with appropriate doc updates, both for the module and 
What's New).

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[issue39682] pathlib.Path objects can be used as context managers

[Barney Gale]

Barney Gale  added the comment:

Also, thank you Antoine for your explanation :-)

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[issue39682] pathlib.Path objects can be used as context managers

[Barney Gale]

Barney Gale  added the comment:

Can I ask what sort of backwards-compatibility guarantees Python provides for 
these cases? In the case of using a Path object as a context manager, I think 
we can say:

- It's easy to do - there's no need to call any underscore-prefixed methods for 
example
- It's undocumented
- It's pretty hard to determine existing usage statically - grepping for `with 
p:` is years of work.

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[issue39682] pathlib.Path objects can be used as context managers

[Antoine Pitrou]

Antoine Pitrou  added the comment:

As with the Accessor abstraction, the original idea was to support Path objects 
backed by a directory file descriptor (for use with openat() and friends).  
That idea was abandoned but it looks like the context manager stayed.  It's 
certainly not useful currently.

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[issue39682] pathlib.Path objects can be used as context managers

[Antony Lee]

Antony Lee  added the comment:

Immutability and hashability are listed first among "general properties" of 
paths (https://docs.python.org/3/library/pathlib.html#general-properties), and 
in the PEP proposing pathlib 
(https://www.python.org/dev/peps/pep-0428/#immutability).  Looking at it again 
I *guess* the docs version could be read as only guaranteeing this for 
PurePaths (because that's in the PurePath section) rather than Path, but the 
PEP version clearly implies that this is true for both PurePaths and concrete 
Paths.

It may be tricky to check usage in third-party packages, given that one would 
need to look for `with : ...` rather than, say, a method name 
which can be grepped.  Do you have any suggestions here?

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[issue39682] pathlib.Path objects can be used as context managers

[Brett Cannon]

Brett Cannon  added the comment:

I guess a question is whether we want immutability guarantees (I for one didn't 
even know you could hash Path objects until now).

I'm personally fine with that idea as it mentally makes sense to not need paths 
to be mutable. But as I said, someone needs to take at least some stab at 
seeing if any preexisting code out there will break if the _closed flag is 
removed before we can continue this discussion.

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[issue39682] pathlib.Path objects can be used as context managers

[Antony Lee]

Antony Lee  added the comment:

A problem of having this bit of state in paths is that it violates assumptions 
based on immutability/hashability, e.g. with

from pathlib import Path

p = Path("foo")
q = Path("foo")

with p: pass

unique_paths = {p, q}

print(len(unique_paths))  # == 1, as p == q
for path in unique_paths:
print(path.resolve())  # Fails if the path is closed.

The last line fails with `unique_paths = {p, q}` as p has been closed (and 
apparently sets keep the first element when multiple "equal" elements are 
passed in), but not with `unique_paths = {q, p}`.

It would also prevent optimizations based on immutability as proposed in 
https://bugs.python.org/issue39783.

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[issue39682] pathlib.Path objects can be used as context managers

[Brett Cannon]

Brett Cannon  added the comment:

A use-case of "closing" a path is to cheaply flag that a path object is no 
longer valid, e.g. you deleted the underlying file and so you don't want others 
using the object anymore without paying the penalty of having to do the I/O to 
trigger the failure due to the path no longer existing.

Now whether anyone is using this functionality I don't know, but removing it 
will inevitably break code for someone, so someone will need to make at least 
some form of an attempt to see if there's any use in the wild to justify 
simplifying the code by removing the functionality.

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[issue39682] pathlib.Path objects can be used as context managers

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue39682] pathlib.Path objects can be used as context managers

[Barney Gale]

New submission from Barney Gale :

`pathlib.Path` objects can be used as context managers, but this functionality 
is undocumented and makes little sense. Example:

>>> import pathlib
>>> root = pathlib.Path("/")
>>> with root:
... print(1)
... 
1
>>> with root:
... print(2)
... 
Traceback (most recent call last):
  File "", line 1, in 
  File "/home/barney/.pyenv/versions/3.7.3/lib/python3.7/pathlib.py", line 
1028, in __enter__
self._raise_closed()
  File "/home/barney/.pyenv/versions/3.7.3/lib/python3.7/pathlib.py", line 
1035, in _raise_closed
raise ValueError("I/O operation on closed path")
ValueError: I/O operation on closed path

`Path` objects don't acquire any resources on __new__/__init__/__enter__, nor 
do they release any resources on __exit__. The whole notion of the path being 
`_closed` seems to exist purely to make impure `Path` methods unusable after 
exiting from the context manager. I can't personally think of a compelling use 
case for this, and suggest that it be removed.

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components: Library (Lib)
messages: 362244
nosy: barneygale
priority: normal
severity: normal
status: open
title: pathlib.Path objects can be used as context managers
versions: Python 3.8

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