`difflib`-generated HTML file does not use monospaced font

[tfpf]

Bug report

Bug description:

difflib.HtmlDiff.make_file writes an HTML file with a CSS rule specifying Courier as the font to use. On systems which don't have it installed, browsers may fall back to the system default font, which likely won't be monospaced.

import difflib
import webbrowser

hd = difflib.HtmlDiff()
html = hd.make_file(["a", "aab", "c" + "f" * 20, "eg", "ry"], ["aab", "f" * 19, "ry"])
with open("diff.html", "w") as writer:
    print(html, file=writer)
webbrowser.open("diff.html")

On my system (which does not have Courier), the default font is Signika, which is what Firefox used to render the text.

• Python 3.13.2 (main, Feb 5 2025, 08:05:21) [GCC 14.2.1 20250128]
• glibc 2.41+r9+ga900dbaf70f0-1
• Arch Linux (64-bit)

I think is is sufficient to do:

diff --git a/Lib/difflib.py b/Lib/difflib.py
index bc09aa128aa..11d286a4aa8 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -1633,7 +1633,7 @@ def _line_pair_iterator():
 
 _styles = """
         :root {color-scheme: light dark}
-        table.diff {font-family:Courier; border:medium;}
+        table.diff {font-family:monospace; border:medium;}
         .diff_header {background-color:#e0e0e0}
         td.diff_header {text-align:right}
         .diff_next {background-color:#c0c0c0}

and will follow up with a pull request soon.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-131204: Fix difflib.HtmlDiff may not use monospaced font #131221
• [3.13] gh-131204: Fix difflib.HtmlDiff may not use monospaced font (GH-131221) #131242
• [3.12] gh-131204: Fix difflib.HtmlDiff may not use monospaced font (GH-131221) #131243

[hugovk]

Shall we use the System Font Stack for better cross-platform support?

font-family: Menlo, Consolas, Monaco, Liberation Mono, Lucida Console, monospace;

https://systemfontstack.com/

[picnixz]

(I think we can consider it as a bugfix as it hinders readability)