bpo-35540: Fix dataclasses.asdict support for defaultdict fields

dataclasses.asdict breaks when there are `defaultdict`s in the dataclass
attributes because it assumes `defaultdict` and `dict` are the same thing,
but in reality `defaultdict` takes a default_factory as the first argument
in its constructor. This change adds support for defaultdict fields.

Author: kwsp

Lib/dataclasses.py

@@ -1295,6 +1295,14 @@ def _asdict_inner(obj, dict_factory):
         # generator (which is not true for namedtuples, handled
         # above).
         return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
+    elif isinstance(obj, dict) and hasattr(obj, 'default_factory'):
+        # obj is a defaultdict, which has a different constructor from
+        # dict as it requires the default_factory as its first arg.
+        # https://bugs.python.org/issue35540
+        result = type(obj)(getattr(obj, 'default_factory'))
+        for k, v in obj.items():
+            result[_asdict_inner(k, dict_factory)] = _asdict_inner(v, dict_factory)
+        return result
     elif isinstance(obj, dict):
         return type(obj)((_asdict_inner(k, dict_factory),
                           _asdict_inner(v, dict_factory))

Misc/NEWS.d/next/Library/2022-03-22-18-28-55.bpo-35540.nyijX9.rst

@@ -0,0 +1 @@
+Fix :func:`dataclasses.asdict` crash when :class:`collections.defaultdict`s are present in the attributes.