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gh-137855: Improve import time of dataclasses by lazy importing `re` and `copy` modules by danielhollas · Pull Request #148379 · python/cpython · GitHub

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gh-137855: Improve import time of dataclasses by lazy importing re and copy modules #148379

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Merged
hugovk merged 7 commits into from
Apr 11, 2026
+16 −8
Conversation 8 Commits 7 Checks 53 Files changed 2
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gh-137855: Improve import time of dataclasses by lazy importing re and copy modules #148379

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5206a52
Lazy import copy module
danielhollas Feb 16, 2026
dca8fc3
Lazy import re module
danielhollas Feb 16, 2026
91473a5
Add blurb
danielhollas Apr 11, 2026
8194309
Update Lib/dataclasses.py
danielhollas Apr 11, 2026
ab4bc95
Reorder lazy imports
danielhollas Apr 11, 2026
933a9c5
Hugo's regex-fu
danielhollas Apr 11, 2026
51fecc5
One more match
danielhollas Apr 11, 2026
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22 changes: 14 additions & 8 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
@@ -1,13 +1,13 @@
import re
import sys
import copy
import types
import inspect
import keyword
import itertools
import annotationlib
import abc
from reprlib import recursive_repr
lazy import copy
lazy import re


__all__ = ['dataclass',
Expand Down Expand Up @@ -217,9 +217,8 @@ def __repr__(self):
_POST_INIT_NAME = '__post_init__'

# String regex that string annotations for ClassVar or InitVar must match.
# Allows "identifier.identifier[" or "identifier[".
# https://bugs.python.org/issue33453 for details.
_MODULE_IDENTIFIER_RE = re.compile(r'^(?:\s*(\w+)\s*\.)?\s*(\w+)')
# This regular expression is compiled on demand so that 're' module can be imported lazily
_MODULE_IDENTIFIER_RE = None

# Atomic immutable types which don't require any recursive handling and for which deepcopy
# returns the same object. We can provide a fast-path for these types in asdict and astuple.
Expand Down Expand Up @@ -804,10 +803,17 @@ def _is_type(annotation, cls, a_module, a_type, is_type_predicate):
# a eval() penalty for every single field of every dataclass
# that's defined. It was judged not worth it.

match = _MODULE_IDENTIFIER_RE.match(annotation)
# String regex that string annotations for ClassVar or InitVar must match.
# Allows "identifier.identifier[" or "identifier[".
# https://github.com/python/cpython/issues/77634 for details.
global _MODULE_IDENTIFIER_RE
if _MODULE_IDENTIFIER_RE is None:
_MODULE_IDENTIFIER_RE = re.compile(r'(?:\s*(\w+)\s*\.)?\s*(\w+)')

match = _MODULE_IDENTIFIER_RE.prefixmatch(annotation)
if match:
ns = None
module_name = match.group(1)
module_name = match[1]
if not module_name:
# No module name, assume the class's module did
# "from dataclasses import InitVar".
Expand All @@ -817,7 +823,7 @@ def _is_type(annotation, cls, a_module, a_type, is_type_predicate):
module = sys.modules.get(cls.__module__)
if module and module.__dict__.get(module_name) is a_module:
ns = sys.modules.get(a_type.__module__).__dict__
if ns and is_type_predicate(ns.get(match.group(2)), a_module):
if ns and is_type_predicate(ns.get(match[2]), a_module):
return True
return False

Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-04-11-12-32-38.gh-issue-137855.tsVny_.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve import time of :mod:`dataclasses` module by lazy importing :mod:`re`
and :mod:`copy` modules.
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