From 66e8e92fe30b36b22821cc892f6368127c8bb4f0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?tonghuaroot=20=28=E7=AB=A5=E8=AF=9D=29?=
 <tonghuaroot@gmail.com>
Date: Wed, 8 Jul 2026 08:40:54 +0800
Subject: [PATCH] gh-153056: Backport the relevant bits of (#153057)

The data race for compiling the string.Template pattern in free-threading builds is not relevant for
3.13, due to the older string.Template and string.py module implementation.  However, the secondary
bug identified by that isue is still relevant here, and fixed in this branch:

    As a side effect, a subclass that supplies an already-compiled pattern now works too; previously
    it raised the same ValueError at class definition.

* Document that the pattern attribute accepts a string or a compiled regex
* Comment the three states of pattern and note the documented-behavior fix in NEWS
* Update Doc/library/string.rst

---------

Co-authored-by: Barry Warsaw <barry@python.org>
(cherry picked from commit 45729033bff28f8abc36c42e802cb2853c205737)
---
 Doc/library/string.rst                             |  3 ++-
 Lib/string.py                                      |  5 +++++
 Lib/test/test_string.py                            | 14 ++++++++++++++
 .../2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst |  3 +++
 4 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index b7b92ecb14f8c81..153cbce7d832266 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -943,7 +943,8 @@ attributes:
 
 Alternatively, you can provide the entire regular expression pattern by
 overriding the class attribute *pattern*.  If you do this, the value must be a
-regular expression object with four named capturing groups.  The capturing
+regular expression pattern string, or a compiled regular expression
+object, with four named capturing groups.  The capturing
 groups correspond to the rules given above, along with the invalid placeholder
 rule:
 
diff --git a/Lib/string.py b/Lib/string.py
index 2eab6d4f595c4e1..f6babf337e8e658 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -70,6 +70,11 @@ def __init_subclass__(cls):
         super().__init_subclass__()
         if 'pattern' in cls.__dict__:
             pattern = cls.pattern
+            if isinstance(pattern, _re.Pattern):
+                # An already-compiled pattern (which the documentation allows)
+                # is used as-is; re.compile() rejects flags on a compiled
+                # pattern.
+                return
         else:
             delim = _re.escape(cls.delimiter)
             id = cls.idpattern
diff --git a/Lib/test/test_string.py b/Lib/test/test_string.py
index 824b89ad517c120..537587aa9ad66df 100644
--- a/Lib/test/test_string.py
+++ b/Lib/test/test_string.py
@@ -272,6 +272,20 @@ def test_SafeTemplate(self):
         eq(s.safe_substitute(dict(who='tim', what='ham', meal='dinner')),
            'tim likes ham for dinner')
 
+    def test_precompiled_pattern(self):
+        # A subclass may supply an already-compiled pattern; it must be reused,
+        # not recompiled (re.compile() rejects flags on a compiled pattern).
+        import re
+        compiled = re.compile(
+            r'\$(?:(?P<escaped>\$)|(?P<named>[a-z]+)|'
+            r'\{(?P<braced>[a-z]+)\}|(?P<invalid>))')
+        class MyTemplate(Template):
+            pattern = compiled
+        self.assertIs(MyTemplate.pattern, compiled)
+        self.assertEqual(
+            MyTemplate('$who likes $what').substitute(who='tim', what='ham'),
+            'tim likes ham')
+
     def test_invalid_placeholders(self):
         raises = self.assertRaises
         s = Template('$who likes $')
diff --git a/Misc/NEWS.d/next/Library/2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst b/Misc/NEWS.d/next/Library/2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst
new file mode 100644
index 000000000000000..483340d35042159
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-05-12-00-00.gh-issue-153056.tMpLat.rst
@@ -0,0 +1,3 @@
+Fix :class:`string.Template` raising a spurious :exc:`ValueError` when the
+*pattern* attribute is a compiled regular expression object, which the
+documentation allows.