`pathlib.Path.expanduser()` does not call `os.path.expanduser()`

[barneygale]

BPO	39899
Nosy	@serhiy-storchaka, @eryksun, @zooba, @barneygale, @Fidget-Spinner, @akulakov
PRs	bpo-39899: Make pathlib use os.path.expanduser() to expand home directories #18841 bpo-39899: ntpath.expanduser(): don't check the basename of the directory matches the username if we're requesting the current user's home directory #25277

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-04-09.22:03:46.934>
created_at = <Date 2020-03-08.05:06:12.759>
labels = ['library', '3.10']
title = '`pathlib.Path.expanduser()` does not call `os.path.expanduser()`'
updated_at = <Date 2021-09-06.17:52:12.404>
user = 'https://github.com/barneygale'

bugs.python.org fields:

activity = <Date 2021-09-06.17:52:12.404>
actor = 'andrei.avk'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-09.22:03:46.934>
closer = 'steve.dower'
components = ['Library (Lib)']
creation = <Date 2020-03-08.05:06:12.759>
creator = 'barneygale'
dependencies = []
files = []
hgrepos = []
issue_num = 39899
keywords = ['patch']
message_count = 19.0
messages = ['363635', '363636', '363637', '363639', '363644', '363647', '363652', '390449', '390452', '390461', '390477', '390494', '390497', '390504', '390507', '390528', '390669', '401167', '401168']
nosy_count = 6.0
nosy_names = ['serhiy.storchaka', 'eryksun', 'steve.dower', 'barneygale', 'kj', 'andrei.avk']
pr_nums = ['18841', '25277']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39899'
versions = ['Python 3.10']

[barneygale]

pathlib.Path.expanduser() does not call os.path.expanduser(), but instead re-implements it. The implementations look pretty similar and I can't see a good reason for the duplication. The only difference is that pathlib.Path.expanduser() raises RuntimeError when a home directory cannot be resolved, whereas os.path.expanduser() returns the path unchanged.

[serhiy-storchaka]

There are two reasons:

1. os.path.expanduser() returns the path unchanged when a home directory cannot be resolved, pathlib.Path.expanduser() raises an error. The latter behavior looks more robust, but we can't change os.path.expanduser().

2. os.path.expanduser() needs to split the path on components while pathlib.Path.expanduser() already has ready components. In some cases it may be more efficient.

[barneygale]

We can check whether os.path.expanduser() returned a path beginning with "~" and raise a RuntimeError if so, right? On point #2, I'm not sure this optimization alone justifies the duplication. PR incoming...

[serhiy-storchaka]

I see no reason to change the current code.

[barneygale]

I see no reason for the duplication, and I can point to one concrete bug affecting your re-implementation of expanduser that doesn't affect the original, i.e. that a KeyError is raised on Windows when "USERNAME" is not present in os.environ, whereas all similar cases raise RuntimeError. These sorts of issues sneak in when you duplicate code - better to stick with the battle-hardened version rather than an inherently risky rewrite.

[serhiy-storchaka]

It was discussed in bpo-19776.

Having separate implementation we can avoid design flaws of os.path.expanduser().

[barneygale]

The only design flaw mentioned in that thread is that os.path.expanduser() returns the input unchanged if expansion fails, which is not very pythonic. However, such a problem doesn't necessitate a rewrite of os.path.expanduser(). Checking result[:1] is enough.

I think there's also a difference in the Windows heuristic in that pathlib checks whether basename(%HOMEPATH%) == %USERNAME% whereas ntpath.expanduser doesn't. But if that's really an issue it should probably be fixed in ntpath IMO, rather than having divergent implementations.