gh-103000: Optimise dataclasses.asdict for the common case (#104364)

Co-authored-by: David Ellis <ducksual@gmail.com>

Lib/dataclasses.py

@@ -1324,11 +1324,18 @@ def _asdict_inner(obj, dict_factory):
  if type(obj) in _ATOMIC_TYPES:
  return obj
  elif _is_dataclass_instance(obj):
- result = []
- for f in fields(obj):
- value = _asdict_inner(getattr(obj, f.name), dict_factory)
- result.append((f.name, value))
- return dict_factory(result)
+ # fast path for the common case
+ if dict_factory is dict:
+ return {
+ f.name: _asdict_inner(getattr(obj, f.name), dict)
+ for f in fields(obj)
+ }
+ else:
+ result = []
+ for f in fields(obj):
+ value = _asdict_inner(getattr(obj, f.name), dict_factory)
+ result.append((f.name, value))
+ return dict_factory(result)
  elif isinstance(obj, tuple) and hasattr(obj, '_fields'):
  # obj is a namedtuple. Recurse into it, but the returned
  # object is another namedtuple of the same type. This is

Misc/NEWS.d/next/Library/2023-05-10-19-33-36.gh-issue-103000.j0KSfD.rst

@@ -0,0 +1,2 @@
+Improve performance of :func:`dataclasses.asdict` for the common case where
+*dict_factory* is ``dict``. Patch by David C Ellis.