gh-129843: fix pure Python implementation of warnings.warn_explicit (#129848)

The pure Python implementation of `warnings.warn_explicit` constructs a `WarningMessage`
with an incorrect source (it incorrectly sets the WarningMessage's line to the given `source`).

Author: srinivasreddy

Lib/warnings.py

@@ -422,7 +422,7 @@ def warn_explicit(message, category, filename, lineno,
     linecache.getlines(filename, module_globals)
 
     # Print message and context
-    msg = WarningMessage(message, category, filename, lineno, source)
+    msg = WarningMessage(message, category, filename, lineno, source=source)
     _showwarnmsg(msg)
 
 

Misc/NEWS.d/next/Library/2025-03-17-15-45-36.gh-issue-129843.NPdpXL.rst

@@ -0,0 +1 @@
+Fix incorrect argument passing in :func:`warnings.warn_explicit`.