From ec78492ef1ae8db70d6b85c03310fad27af6170d Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Thu, 3 Apr 2025 01:35:47 +0100
Subject: [PATCH 1/6] Optimise import time for ``string``

---
 Lib/string.py                                 | 64 +++++++++++--------
 ...-04-03-01-35-02.gh-issue-118761.VQcj70.rst |  2 +
 2 files changed, 41 insertions(+), 25 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-04-03-01-35-02.gh-issue-118761.VQcj70.rst

diff --git a/Lib/string.py b/Lib/string.py
index c4f05c7223ce8a..e3416135990179 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -49,29 +49,20 @@ def capwords(s, sep=None):
 
 
 ####################################################################
-import re as _re
-from collections import ChainMap as _ChainMap
-
+_sentinel_flags = object()
 _sentinel_dict = {}
 
-class Template:
-    """A string class for supporting $-substitutions."""
 
-    delimiter = '$'
-    # r'[a-z]' matches to non-ASCII letters when used with IGNORECASE, but
-    # without the ASCII flag.  We can't add re.ASCII to flags because of
-    # backward compatibility.  So we use the ?a local flag and [a-z] pattern.
-    # See https://bugs.python.org/issue31672
-    idpattern = r'(?a:[_a-z][_a-z0-9]*)'
-    braceidpattern = None
-    flags = _re.IGNORECASE
-
-    def __init_subclass__(cls):
-        super().__init_subclass__()
-        if 'pattern' in cls.__dict__:
+class _TemplatePattern:
+    def __get__(self, instance, cls=None):
+        if cls is None:
+            return self
+        import re
+        if ('pattern' in cls.__dict__
+            and not isinstance(cls.__dict__['pattern'], _TemplatePattern)):
             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 +73,32 @@ def __init_subclass__(cls):
               (?P<invalid>)             # Other ill-formed delimiter exprs
             )
             """
-        cls.pattern = _re.compile(pattern, cls.flags | _re.VERBOSE)
+        if cls.flags is _sentinel_flags:
+            cls.flags = re.IGNORECASE
+        pattern = re.compile(pattern, cls.flags | re.VERBOSE)
+        # replace this descriptor with the compiled pattern
+        setattr(cls, 'pattern', pattern)
+        return pattern
+
+
+class Template:
+    """A string class for supporting $-substitutions."""
+
+    delimiter = '$'
+    # r'[a-z]' matches to non-ASCII letters when used with IGNORECASE, but
+    # without the ASCII flag.  We can't add re.ASCII to flags because of
+    # backward compatibility.  So we use the ?a local flag and [a-z] pattern.
+    # See https://bugs.python.org/issue31672
+    idpattern = r'(?a:[_a-z][_a-z0-9]*)'
+    braceidpattern = None
+    flags = _sentinel_flags  # default: re.IGNORECASE
+
+    # use a descriptor to be able to defer the import of `re`, for performance
+    pattern = _TemplatePattern()
+
+    def __init_subclass__(cls):
+        super().__init_subclass__()
+        cls.pattern = _TemplatePattern()
 
     def __init__(self, template):
         self.template = template
@@ -105,7 +121,8 @@ 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 +141,8 @@ 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 +188,6 @@ 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
new file mode 100644
index 00000000000000..257ad7ece7d18a
--- /dev/null
+++ 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.

From 2b542c76445ec12ffaa4b6b869d6be8192d3e044 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Thu, 3 Apr 2025 02:52:55 +0100
Subject: [PATCH 2/6] Refactor to a common classmethod

---
 Lib/string.py | 55 +++++++++++++++++++++++++++------------------------
 1 file changed, 29 insertions(+), 26 deletions(-)

diff --git a/Lib/string.py b/Lib/string.py
index e3416135990179..1a62b699dc2ac6 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -49,36 +49,15 @@ def capwords(s, sep=None):
 
 
 ####################################################################
-_sentinel_flags = object()
 _sentinel_dict = {}
+_sentinel_flags = object()
 
 
 class _TemplatePattern:
     def __get__(self, instance, cls=None):
         if cls is None:
             return self
-        import re
-        if ('pattern' in cls.__dict__
-            and not isinstance(cls.__dict__['pattern'], _TemplatePattern)):
-            pattern = cls.pattern
-        else:
-            delim = re.escape(cls.delimiter)
-            id = cls.idpattern
-            bid = cls.braceidpattern or cls.idpattern
-            pattern = fr"""
-            {delim}(?:
-              (?P<escaped>{delim})  |   # Escape sequence of two delimiters
-              (?P<named>{id})       |   # delimiter and a Python identifier
-              {{(?P<braced>{bid})}} |   # delimiter and a braced identifier
-              (?P<invalid>)             # Other ill-formed delimiter exprs
-            )
-            """
-        if cls.flags is _sentinel_flags:
-            cls.flags = re.IGNORECASE
-        pattern = re.compile(pattern, cls.flags | re.VERBOSE)
-        # replace this descriptor with the compiled pattern
-        setattr(cls, 'pattern', pattern)
-        return pattern
+        return cls._compile_pattern()
 
 
 class Template:
@@ -93,12 +72,36 @@ class Template:
     braceidpattern = None
     flags = _sentinel_flags  # default: re.IGNORECASE
 
-    # use a descriptor to be able to defer the import of `re`, for performance
-    pattern = _TemplatePattern()
+    pattern = _TemplatePattern()  # use a descriptor to compile the pattern
 
     def __init_subclass__(cls):
         super().__init_subclass__()
-        cls.pattern = _TemplatePattern()
+        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)
+            id = cls.idpattern
+            bid = cls.braceidpattern or cls.idpattern
+            pattern = fr"""
+            {delim}(?:
+              (?P<escaped>{delim})  |   # Escape sequence of two delimiters
+              (?P<named>{id})       |   # delimiter and a Python identifier
+              {{(?P<braced>{bid})}} |   # delimiter and a braced identifier
+              (?P<invalid>)             # Other ill-formed delimiter exprs
+            )
+            """
+        if cls.flags is _sentinel_flags:
+            cls.flags = re.IGNORECASE
+        pat = cls.pattern = re.compile(pattern, cls.flags | re.VERBOSE)
+        return pat
 
     def __init__(self, template):
         self.template = template

From e28e43ff093256eade1fe965e6e72eb352c75477 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Thu, 3 Apr 2025 02:55:36 +0100
Subject: [PATCH 3/6] Add comment

---
 Lib/string.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Lib/string.py b/Lib/string.py
index 1a62b699dc2ac6..af762894667e18 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -57,6 +57,7 @@ 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()
 
 

From 056dd07bb50834ee07601661ff00de6f6bcd72fd Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 6 Apr 2025 18:11:27 +0100
Subject: [PATCH 4/6] Use None as the sentinel

---
 Lib/string.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/Lib/string.py b/Lib/string.py
index af762894667e18..c4fb69c4ec7a04 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -50,7 +50,6 @@ def capwords(s, sep=None):
 
 ####################################################################
 _sentinel_dict = {}
-_sentinel_flags = object()
 
 
 class _TemplatePattern:
@@ -71,7 +70,7 @@ class Template:
     # See https://bugs.python.org/issue31672
     idpattern = r'(?a:[_a-z][_a-z0-9]*)'
     braceidpattern = None
-    flags = _sentinel_flags  # default: re.IGNORECASE
+    flags = None  # default: re.IGNORECASE
 
     pattern = _TemplatePattern()  # use a descriptor to compile the pattern
 
@@ -99,7 +98,7 @@ def _compile_pattern(cls):
               (?P<invalid>)             # Other ill-formed delimiter exprs
             )
             """
-        if cls.flags is _sentinel_flags:
+        if cls.flags is None:
             cls.flags = re.IGNORECASE
         pat = cls.pattern = re.compile(pattern, cls.flags | re.VERBOSE)
         return pat

From bb3605a7d89acb288cabdf5e29dffc11b2567251 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Mon, 7 Apr 2025 21:52:14 +0100
Subject: [PATCH 5/6] Make _TemplatePattern a singleton

---
 Lib/string.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/Lib/string.py b/Lib/string.py
index c4fb69c4ec7a04..9ba0769e93eee3 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -53,11 +53,12 @@ def capwords(s, sep=None):
 
 
 class _TemplatePattern:
+    # This descriptor is overwritten in ``Template._compile_pattern()``.
     def __get__(self, instance, cls=None):
         if cls is None:
             return self
-        # This descriptor is overwritten in ``_compile_pattern()``.
         return cls._compile_pattern()
+_TemplatePattern = _TemplatePattern()
 
 
 class Template:
@@ -72,7 +73,7 @@ class Template:
     braceidpattern = None
     flags = None  # default: re.IGNORECASE
 
-    pattern = _TemplatePattern()  # use a descriptor to compile the pattern
+    pattern = _TemplatePattern  # use a descriptor to compile the pattern
 
     def __init_subclass__(cls):
         super().__init_subclass__()
@@ -83,7 +84,7 @@ 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):
+        if cls_pattern is not None and cls_pattern is not _TemplatePattern:
             # Prefer a pattern defined on the class.
             pattern = cls_pattern
         else:

From 6fb9b5d65c2822012efe47bf7158227bcfde4ce0 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Date: Tue, 8 Apr 2025 10:39:27 +0100
Subject: [PATCH 6/6] Serhiy's suggestion

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
---
 Lib/string.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/Lib/string.py b/Lib/string.py
index 9ba0769e93eee3..eab5067c9b133e 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -83,11 +83,8 @@ def __init_subclass__(cls):
     def _compile_pattern(cls):
         import re  # deferred import, for performance
 
-        cls_pattern = cls.__dict__.get('pattern')
-        if cls_pattern is not None and cls_pattern is not _TemplatePattern:
-            # Prefer a pattern defined on the class.
-            pattern = cls_pattern
-        else:
+        pattern = cls.__dict__.get('pattern', _TemplatePattern)
+        if pattern is _TemplatePattern:
             delim = re.escape(cls.delimiter)
             id = cls.idpattern
             bid = cls.braceidpattern or cls.idpattern

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