Tin/defaultdicts

src/cattrs/cols.py

@@ -2,10 +2,13 @@
 
 from __future__ import annotations
 
+from collections import defaultdict
+from functools import partial
 from sys import version_info
 from typing import (
     TYPE_CHECKING,
     Any,
+    DefaultDict,
     Iterable,
     Literal,
     NamedTuple,
@@ -16,7 +19,16 @@
 
 from attrs import NOTHING, Attribute
 
-from ._compat import ANIES, is_bare, is_frozenset, is_mapping, is_sequence, is_subclass
+from ._compat import (
+    ANIES,
+    get_args,
+    get_origin,
+    is_bare,
+    is_frozenset,
+    is_mapping,
+    is_sequence,
+    is_subclass,
+)
 from ._compat import is_mutable_set as is_set
 from .dispatch import StructureHook, UnstructureHook
 from .errors import IterableValidationError, IterableValidationNote
@@ -289,3 +301,23 @@ def namedtuple_dict_unstructure_factory(
         working_set.remove(cl)
         if not working_set:
             del already_generating.working_set
+
+
+def is_defaultdict(type: Any) -> bool:
+    """Is this type a defaultdict?
+
+    Bare defaultdicts (defaultdicts with no type arguments) are not supported
+    since there's no way to discover their _default_factory_.
+    """
+    return is_subclass(get_origin(type), (defaultdict, DefaultDict))
+
+
+def defaultdict_struct_factory(
+    type: type[defaultdict], converter: BaseConverter
+) -> StructureHook:
+    """A structure hook factory for defaultdicts.
+
+    The value type parameter will be used as the _default factory_.
+    """
+    value_type = get_args(type)[1]
+    return mapping_structure_factory(type, converter, partial(defaultdict, value_type))

src/cattrs/converters.py

@@ -54,6 +54,8 @@
     signature,
 )
 from .cols import (
+    defaultdict_struct_factory,
+    is_defaultdict,
     is_namedtuple,
     iterable_unstructure_factory,
     list_structure_factory,
@@ -1174,6 +1176,7 @@ def __init__(
         self.register_structure_hook_factory(is_annotated, self.gen_structure_annotated)
         self.register_structure_hook_factory(is_mapping, self.gen_structure_mapping)
         self.register_structure_hook_factory(is_counter, self.gen_structure_counter)
+        self.register_structure_hook_factory(is_defaultdict, defaultdict_struct_factory)
         self.register_structure_hook_factory(is_typeddict, self.gen_structure_typeddict)
         self.register_structure_hook_factory(
             lambda t: get_newtype_base(t) is not None, self.get_structure_newtype

src/cattrs/gen/__init__.py

@@ -919,11 +919,14 @@ def mapping_structure_factory(
     structure_to: type = dict,
     key_type=NOTHING,
     val_type=NOTHING,
-    detailed_validation: bool = True,
+    detailed_validation: bool | Literal["from_converter"] = "from_converter",
 ) -> MappingStructureFn[T]:
     """Generate a specialized structure function for a mapping."""
     fn_name = "structure_mapping"
 
+    if detailed_validation == "from_converter":
+        detailed_validation = converter.detailed_validation
+
     globs: dict[str, type] = {"__cattr_mapping_cl": structure_to}
 
     lines = []

tests/test_defaultdicts.py

@@ -0,0 +1,24 @@
+"""Tests for defaultdicts."""
+
+from collections import defaultdict
+from typing import DefaultDict
+
+from cattrs import Converter
+
+
+def test_typing_defaultdicts(genconverter: Converter):
+    """`typing.DefaultDict` works."""
+    res = genconverter.structure({"a": 1}, DefaultDict[str, int])
+
+    assert isinstance(res, defaultdict)
+    assert res["a"] == 1
+    assert res["b"] == 0
+
+
+def test_collection_defaultdicts(genconverter: Converter):
+    """`collections.defaultdict` works."""
+    res = genconverter.structure({"a": 1}, defaultdict[str, int])
+
+    assert isinstance(res, defaultdict)
+    assert res["a"] == 1
+    assert res["b"] == 0