python · warsaw · Jul 8, 2026
diff --git 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
@@ -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
@@ -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
@@ -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.