urllib.request.pathname2url() and url2pathname() remove slashes on Windows

[barneygale]

Bug report

Bug description:

urllib.request.pathname2url() unexpectedly removes doubled and trailing slashes, but only on Windows, and only for DOS device paths (not UNC paths):

>>> from urllib.request import pathname2url, url2pathname
>>> pathname2url('C:\\')
'///C:'  # This is *not* the same path!
>>> pathname2url('C:\\foo\\')
'///C:/foo'
>>> pathname2url('C:\\foo\\\\bar')
'///C:/foo/bar'

url2pathname() does almost the same thing, except that it specially preserves a slash immediately after a DOS drive (but not elsewhere):

>>> url2pathname('///c:/')
'C:\\'  # OK
>>> url2pathname('///c:/foo/')
'C:\\foo'
>>> url2pathname('///c:/foo//bar')
'C:\\foo\\bar'

This behaviour makes no sense and has no equivalent on POSIX, where all slashes are preserved.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• GH-126212: Fix removal of slashes in file URIs on Windows #126214
• [3.13] GH-126212: Fix removal of slashes in file URIs on Windows (GH-126214) #126590
• [3.12] GH-126212: Fix removal of slashes in file URIs on Windows (GH-126214) #126591