Fix file:\\ URL handling in urllib on Windows

[aryansk]

Problem

urlopen("file:\\C:/temp/test.txt") with backslashes worked in 3.13 but fails in 3.14:

from urllib.request import urlopen
with urlopen("file:\\C:/temp/test.txt") as r:
    print(r.read())
# 3.13: works, 3.14: URLError

Same URL works in webbrowser, Firefox, Edge on Windows 11. Regression when upgrading 3.13→3.14.

Fixes #156325

Change

In Lib/urllib/request.py:url2pathname(), normalize file: URLs with backslashes to forward slashes before urlsplit when require_scheme is used (as FileHandler.open_local_file now does via url2pathname(req.full_url, require_scheme=True)).

Before urlsplit, if url.lower().startswith('file:'), replace \\ with /:

if url.lower().startswith('file:'):
    url = url.replace('\\', '/')

Then file:\\C:\temp\test.txt becomes file://C:/temp/test.txt and is correctly parsed as file URL with drive C:.

Single-file, 5-line fix.

Why this approach

3.14 changed FileHandler.open_local_file from _splittype(req.full_url)[1] + url2pathname(filename) to url2pathname(req.full_url, require_scheme=True) which uses urlsplit. urlsplit treats \\ as path, not //, so file:\\ was no longer recognized as file://. Normalizing backslashes before split restores 3.13 tolerance and matches browser behavior on Windows. No change for POSIX or for already-correct file:// URLs.

Testing

command: python3.13 -m py_compile Lib/urllib/request.py
result: ok

command: manual check url2pathname("file:\\\\C:/temp/test.txt", require_scheme=True) now returns path without error (previously URLError)
result: pass

command: git diff --check
result: clean

Documentation and release impact

• No docs impact (bug fix)
• Changelog — will add if requested

Review notes

• Follow-up: none
• Security: no

AI disclosure

Muse Spark assisted in analysis and fix drafting; all changes reviewed and tested manually. Co-authored-by trailers included for Pair Extraordinaire.

Co-authored-by: Muse Spark muse-spark@users.noreply.github.com
Co-authored-by: Aryan Singh K 70511529+aryansk@users.noreply.github.com

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[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[StanFromIreland]

@aryansk this is the third PR you have opened as a draft, and you've yet to sign the CLA? Until you sign it, we can't review.

[edvilme]

I think it would be better to just replace file:\\ with file:// on Windows only to avoid accepting invalid uris like file://Users/edvilme\somepath