python · AA-Turner · Apr 8, 2025 · Apr 3, 2025 · Apr 3, 2025 · Apr 3, 2025
diff --git a/Lib/string.py b/Lib/string.py
@@ -49,11 +49,17 @@ def capwords(s, sep=None):


 ####################################################################
-import re as _re
-from collections import ChainMap as _ChainMap
-
 _sentinel_dict = {}

+
+class _TemplatePattern:
+    def __get__(self, instance, cls=None):
+        if cls is None:
+            return self
+        # This descriptor is overwritten in ``_compile_pattern()``.
+        return cls._compile_pattern()
+
+
 class Template:
    """A string class for supporting $-substitutions."""

@@ -64,14 +70,24 @@ class Template:
    # See https://bugs.python.org/issue31672
    idpattern = r'(?a:[_a-z][_a-z0-9]*)'
    braceidpattern = None
-    flags = _re.IGNORECASE
+    flags = None  # default: re.IGNORECASE
+
+    pattern = _TemplatePattern()  # use a descriptor to compile the pattern

    def __init_subclass__(cls):
        super().__init_subclass__()
-        if 'pattern' in cls.__dict__:
-            pattern = cls.pattern
+        cls._compile_pattern()
+
+    @classmethod
+    def _compile_pattern(cls):
+        import re  # deferred import, for performance
+
+        cls_pattern = cls.__dict__.get('pattern')
+        if cls_pattern and not isinstance(cls_pattern, _TemplatePattern):
+            # Prefer a pattern defined on the class.
+            pattern = cls_pattern
        else:
-            delim = _re.escape(cls.delimiter)
+            delim = re.escape(cls.delimiter)
            id = cls.idpattern
            bid = cls.braceidpattern or cls.idpattern
            pattern = fr"""
@@ -82,7 +98,10 @@ def __init_subclass__(cls):
              (?P<invalid>)             # Other ill-formed delimiter exprs
            )
            """
-        cls.pattern = _re.compile(pattern, cls.flags | _re.VERBOSE)
+        if cls.flags is None:
+            cls.flags = re.IGNORECASE
+        pat = cls.pattern = re.compile(pattern, cls.flags | re.VERBOSE)
+        return pat

    def __init__(self, template):
        self.template = template
@@ -105,7 +124,8 @@ def substitute(self, mapping=_sentinel_dict, /, **kws):
        if mapping is _sentinel_dict:
            mapping = kws
        elif kws:
-            mapping = _ChainMap(kws, mapping)
+            from collections import ChainMap
+            mapping = ChainMap(kws, mapping)
        # Helper function for .sub()
        def convert(mo):
            # Check the most common path first.
@@ -124,7 +144,8 @@ def safe_substitute(self, mapping=_sentinel_dict, /, **kws):
        if mapping is _sentinel_dict:
            mapping = kws
        elif kws:
-            mapping = _ChainMap(kws, mapping)
+            from collections import ChainMap
+            mapping = ChainMap(kws, mapping)
        # Helper function for .sub()
        def convert(mo):
            named = mo.group('named') or mo.group('braced')
@@ -170,10 +191,6 @@ def get_identifiers(self):
                    self.pattern)
        return ids

-# Initialize Template.pattern.  __init_subclass__() is automatically called
-# only for subclasses, not for the Template class itself.
-Template.__init_subclass__()
-

 ########################################################################
 # the Formatter class

diff --git a/Misc/NEWS.d/next/Library/2025-04-03-01-35-02.gh-issue-118761.VQcj70.rst b/Misc/NEWS.d/next/Library/2025-04-03-01-35-02.gh-issue-118761.VQcj70.rst
@@ -0,0 +1,2 @@
+Improve import times by up to 27x for the :mod:`string` module.
+Patch by Adam Turner.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Improve import times by up to 27x for the :mod:`string` module.
		Patch by Adam Turner.