Comment the three states of pattern and note the documented-behavior fix in NEWS

Author: tonghuaroot

Lib/string/__init__.py

@@ -83,9 +83,13 @@ def __init_subclass__(cls):
     def _compile_pattern(cls):
         import re  # deferred import, for performance
 
+        # `pattern` may be the `_TemplatePattern` sentinel (not yet compiled), an
+        # already-compiled regular expression object (as documented), or a string
+        # regular expression.  An already-compiled object is returned as-is; the
+        # other two are compiled and cached back on the class.
         pattern = cls.__dict__.get('pattern', _TemplatePattern)
         if isinstance(pattern, re.Pattern):
-            # Already compiled; reuse it (re.compile() rejects flags on a Pattern).
+            # re.compile() rejects flags on an already-compiled pattern.
             return pattern
         if pattern is _TemplatePattern:
             delim = re.escape(cls.delimiter)

Misc/NEWS.d/next/Library/2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst

@@ -1,2 +1,4 @@
-Fix a race in :class:`string.Template` where concurrent first use on the
-free-threaded build could raise a spurious :exc:`ValueError`.
+Fix :class:`string.Template` raising a spurious :exc:`ValueError` when the
+*pattern* attribute is a compiled regular expression object, which the
+documentation allows.  On the free-threaded build this also occurred as a data
+race on the first concurrent use.