[issue46733] pathlib.Path methods can raise NotImplementedError

[Eryk Sun]

Eryk Sun  added the comment:

> pathlib does not allow to distinguish "path" from "path/".

os.path.normpath() and os.path.abspath() don't retain a trailing slash -- or a 
leading dot component for that matter. Are you referring to os.path.join()? For 
example:

>>> os.path.join('./spam', 'eggs/')
'./spam/eggs/'
>>> os.path.normpath('./spam/eggs/')
'spam/eggs'
>>> PurePath('./spam') / PurePath('eggs/')
PurePosixPath('spam/eggs')

A leading dot component is significant in a context that searches a set of 
paths -- usually PATH. A trailing slash is significant in a context that has to 
distinguish a device or file path from a directory path, of which there are 
several cases in Windows.

I think it's a deficiency in pathlib that it lacks a way to require 
conservative normalization in these cases. Path and PurePath objects could gain 
a keyword-only parameter, and internal attribute if needed, that enables a more 
conservative normalization.

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

> In the long run, it would be better to migrate the implementations in the 
> other direction. Rewrite genericpath, ntpath, posixpath, and parts of shutil 
> to use PurePath and Path objects.

pathlib does not allow to distinguish "path" from "path/".

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Barney Gale  added the comment:

Thanks both. I've adjusted PR 31338 to leave is_mount() unchanged. I've opened 
a new pull request that implements is_mount() on Windows using 
ntpath.ismount(): PR 31458

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Terry J. Reedy]

Terry J. Reedy  added the comment:

I think pathlib should be raising TypeError, ValueError, or something else as 
appropriate.  Or not raising in situations Eryk indicated.  x/0 *could* be +- 
float('inf'), but we don't raise NotImplementedError for it.

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nosy: +terry.reedy

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Eryk Sun]

Eryk Sun  added the comment:

> I'm planning to learn more heavily on posixpath + ntpath in 
> pathlib once bpo-44136 is done. I think that would be a good 
> time to introduce is_mount() support on Windows.

In the long run, it would be better to migrate the implementations in the other 
direction. Rewrite genericpath, ntpath, posixpath, and parts of shutil to use 
PurePath and Path objects. Using path objects instead of generic strings should 
improve the implementation of os.path and shutil, in addition to ensuring 
consistency with pathlib. However, that's a long-term goal that doesn't 
preclude using os.path and shutil as needed now, such as relying on 
os.path.ismount().

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Barney Gale  added the comment:

I'm planning to learn more heavily on posixpath + ntpath in pathlib once 
bpo-44136 is done. I think that would be a good time to introduce is_mount() 
support on Windows.

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Eryk Sun]

Eryk Sun  added the comment:

WindowsPath.is_mount() should call ntpath.ismount(). This function needs a 
significant redesign, but it's fine to use it as long as it's documented that 
is_mount() is equivalent to os.path.ismount().

Since the owner() and group() methods return names instead of numeric IDs, 
technically we could implement them in WindowsPath. That said, in Windows, 
st_mode and chmod() are unrelated to a file's owner and group, plus object 
ownership is fundamentally different from POSIX. Unless chown() is also 
implemented, I don't think we would gain much from knowing the owner and group, 
other than making it easier to display them.

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nosy: +eryksun

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Barney Gale  added the comment:

Thanks very much Alex. I've added some PRs:

PR 31338 addresses owner(), group() and is_mount(). It moves those methods to 
PosixPath, and adds stubs in WindowsPath that raise deprecation warnings. I 
agree with your analysis that, for static typing purposes, these methods 
shouldn't even exist on WindowsPath!

PR 31339 addresses readlink(), symlink_to() and hardlink_to(). In this case I'm 
working towards making those methods unavailable if os.readlink(), symlink() 
and link() are unavailable. Not totally sold on this - thoughts?

PR 31340 addresses glob() and rglob(), switching the exception type to 
ValueError. I think this is a legitimate bugfix with minimal adverse effects.

PR 31341 addresses the Path constructor. This is a backwards incompatible 
change, and /probably/ not worth doing. I add it for completeness sake, as 
these four PRs cover all cases where pathlib raises NotImplementedError.

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Change by Barney Gale :


--
pull_requests: +29491
pull_request: https://github.com/python/cpython/pull/31340

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Change by Barney Gale :


--
pull_requests: +29492
pull_request: https://github.com/python/cpython/pull/31341

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Change by Barney Gale :


--
pull_requests: +29490
pull_request: https://github.com/python/cpython/pull/31339

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Change by Barney Gale :


--
keywords: +patch
pull_requests: +29489
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31338

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Éric Araujo]

Change by Éric Araujo :


--
nosy: +eric.araujo, pitrou, serhiy.storchaka

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Alex Waygood]

Alex Waygood  added the comment:

I suppose it might also be worth considering moving `owner()` and `group()` to 
PosixPath. In practice, these unconditionally raise NotImplementedError on 
Windows, since the pwd and grp modules are not available on Windows. So, in an 
ideal world, they probably wouldn't exist on the common base class.

Again, though, that needs to be weighed against backwards-compatibility 
considerations.

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Alex Waygood]

Alex Waygood  added the comment:

Here's my two cents, as a non-expert when it comes to pathlib:

I'm not really sure why `is_mount()` exists on WindowsPath objects, given that 
it unconditionally raises `NotImplementedError` on WindowsPath objects -- that 
seems *very* strange to me. It seems to me like it should probably be a method 
on the `PosixPath` class rather than on the `Path` class.

The other methods that raise NotImplementedError don't seem as egregious, 
because none of them *unconditionally* raise NotImplementedError. We could 
debate whether some of them would raise different errors in an ideal world, but 
changing them now might have implications for backwards compatibility, and it 
doesn't seem like a *major* issue to me.

--
nosy: +AlexWaygood

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Alex Waygood]

Change by Alex Waygood :


--
keywords:  -patch
stage: patch review -> 

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Alex Waygood]

Change by Alex Waygood :


--
pull_requests:  -29461

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

Change by Barney Gale :


--
keywords: +patch
pull_requests: +29461
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31085

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[issue46733] pathlib.Path methods can raise NotImplementedError

[Barney Gale]

New submission from Barney Gale :

The docs for NotImplementedError say:

> In user defined base classes, abstract methods should raise this exception 
> when they require derived classes to override the method, or while the class 
> is being developed to indicate that the real implementation still needs to be 
> added.

pathlib's use of NotImplementedError appears to be more broad. It can be raised 
in the following circumstances:

1. When attempting to construct a WindowsPath from a non-Windows system, and 
vice-versa. This is the only case where NotImplementedError is mentioned in the 
pathlib docs (in a repl example)
2. In glob() and rglob() when an absolute path is supplied as a pattern
3. In owner() if the pwd module isn't available
4. In group() if the grp module isn't available
5. In readlink() if os.readlink() isn't available
6. In symlink_to() if os.symlink() isn't available
7. In hardlink_to() if os.hardlink() isn't available
8. In WindowsPath.is_mount(), unconditionally

I suspect there are better choices for exception types in all these cases.

--
components: Library (Lib)
messages: 413142
nosy: barneygale
priority: normal
severity: normal
status: open
title: pathlib.Path methods can raise NotImplementedError
type: behavior
versions: Python 3.11

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