bpo-35540: Add collections.defaultdict support to dataclasses.{asdict,astuple}

Lib/dataclasses.py

@@ -6,6 +6,7 @@
 import keyword
 import builtins
 import functools
+import collections
 import _thread
 
 
@@ -1077,6 +1078,12 @@ 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, collections.defaultdict):
+        # defaultdict does not have the same constructor than dict and must be
+        # handled separately
+        return type(obj)(obj.default_factory, ((_asdict_inner(k, dict_factory),
+                                               _asdict_inner(v, dict_factory))
+                                               for k, v in obj.items()))
     elif isinstance(obj, dict):
         return type(obj)((_asdict_inner(k, dict_factory),
                           _asdict_inner(v, dict_factory))
@@ -1129,6 +1136,12 @@ def _astuple_inner(obj, tuple_factory):
         # generator (which is not true for namedtuples, handled
         # above).
         return type(obj)(_astuple_inner(v, tuple_factory) for v in obj)
+    elif isinstance(obj, collections.defaultdict):
+        # defaultdict does not have the same constructor than dict and must be
+        # handled separately
+        return type(obj)(obj.default_factory, ((_asdict_inner(k, dict_factory),
+                                               _asdict_inner(v, dict_factory))
+                                               for k, v in obj.items()))
     elif isinstance(obj, dict):
         return type(obj)((_astuple_inner(k, tuple_factory), _astuple_inner(v, tuple_factory))
                           for k, v in obj.items())

Lib/test/test_dataclasses.py

@@ -1493,6 +1493,17 @@ class C:
         self.assertIsNot(d['f'], t)
         self.assertEqual(d['f'].my_a(), 6)
 
+    def test_helper_asdict_defaultdict(self):
+        @dataclass
+        class C:
+            d: dict
+
+        from collections import defaultdict
+        c = C(defaultdict(int))
+        self.assertEqual(asdict(c), {
+            'd': defaultdict(int)
+        })
+
     def test_helper_astuple(self):
         # Basic tests for astuple(), it should return a new tuple.
         @dataclass
@@ -1620,6 +1631,15 @@ class C:
         t = astuple(c, tuple_factory=list)
         self.assertEqual(t, ['outer', T(1, ['inner', T(11, 12, 13)], 2)])
 
+    def test_helper_astuple_defaultdict(self):
+        @dataclass
+        class C:
+            d: dict
+
+        from collections import defaultdict
+        c = C(defaultdict(int))
+        self.assertEqual(astuple(c), (defaultdict(int),))
+
     def test_dynamic_class_creation(self):
         cls_dict = {'__annotations__': {'x': int, 'y': int},
                     }

Misc/NEWS.d/next/Library/2018-12-30-01-53-59.bpo-35540.sL32q2.rst

@@ -0,0 +1,2 @@
+`dataclasses.asdict` and `dataclasses.astuple` now accept instances with
+`collections.defaultdict` attributes. Patch by Rémi Lapeyre.