Support for Counter objects (#471)

dataclasses_json/core.py

@@ -21,7 +21,7 @@
                                     _is_collection, _is_mapping, _is_new_type,
                                     _is_optional, _isinstance_safe,
                                     _get_type_arg_param,
-                                    _get_type_args,
+                                    _get_type_args, _is_counter,
                                     _NO_ARGS,
                                     _issubclass_safe, _is_tuple)
 
@@ -271,7 +271,7 @@ def _decode_generic(type_, value, infer_missing):
         res = type_(value)
     # FIXME this is a hack to fix a deeper underlying issue. A refactor is due.
     elif _is_collection(type_):
-        if _is_mapping(type_):
+        if _is_mapping(type_) and not _is_counter(type_):
             k_type, v_type = _get_type_args(type_, (Any, Any))
             # a mapping type has `.keys()` and `.values()`
             # (see collections.abc)
@@ -284,6 +284,8 @@ def _decode_generic(type_, value, infer_missing):
                 xs = _decode_items(types[0], value, infer_missing)
             else:
                 xs = _decode_items(_get_type_args(type_) or _NO_ARGS, value, infer_missing)
+        elif _is_counter(type_):
+            xs = dict(zip(_decode_items(_get_type_arg_param(type_, 0), value.keys(), infer_missing), value.values()))
         else:
             xs = _decode_items(_get_type_arg_param(type_, 0), value, infer_missing)
 

dataclasses_json/utils.py

@@ -1,6 +1,7 @@
 import inspect
 import sys
 from datetime import datetime, timezone
+from collections import Counter
 from typing import (Collection, Mapping, Optional, TypeVar, Any, Type, Tuple,
                     Union, cast)
 
@@ -142,6 +143,10 @@ def _is_optional(type_):
             type_ is Any)
 
 
+def _is_counter(type_):
+    return _issubclass_safe(_get_type_origin(type_), Counter)
+
+
 def _is_mapping(type_):
     return _issubclass_safe(_get_type_origin(type_), Mapping)
 

tests/entities.py

@@ -1,3 +1,4 @@
+import sys
 from collections import deque
 from dataclasses import dataclass, field
 from datetime import datetime
@@ -15,6 +16,10 @@
                     Union,
                     Any)
 from uuid import UUID
+if sys.version_info >= (3, 9):
+    from collections import Counter
+else:
+    from typing import Counter
 
 from marshmallow import fields
 
@@ -370,3 +375,9 @@ class DataClassWithNestedOptional:
 @dataclass
 class DataClassWithNestedDictWithTupleKeys:
     a: Dict[Tuple[int], int]
+
+
+@dataclass_json
+@dataclass
+class DataClassWithCounter:
+    c: Counter[str]

tests/test_collections.py

@@ -1,4 +1,4 @@
-from collections import deque
+from collections import Counter, deque
 
 from tests.entities import (DataClassIntImmutableDefault,
                             DataClassMutableDefaultDict,
@@ -19,7 +19,8 @@
                             DataClassWithFrozenSetUnbound,
                             DataClassWithDequeCollections,
                             DataClassWithTuple, DataClassWithTupleUnbound,
-                            DataClassWithUnionIntNone, MyCollection)
+                            DataClassWithUnionIntNone, MyCollection,
+                            DataClassWithCounter)
 
 
 class TestEncoder:
@@ -112,6 +113,10 @@ def test_mutable_default_list(self):
     def test_mutable_default_dict(self):
         assert DataClassMutableDefaultDict().to_json() == '{"xs": {}}'
 
+    def test_counter(self):
+        assert DataClassWithCounter(
+            c=Counter('foo')).to_json() == '{"c": {"f": 1, "o": 2}}'
+
 
 class TestDecoder:
     def test_list(self):
@@ -235,3 +240,7 @@ def test_mutable_default_dict(self):
                 == DataClassMutableDefaultDict())
         assert (DataClassMutableDefaultDict.from_json('{}', infer_missing=True)
                 == DataClassMutableDefaultDict())
+
+    def test_counter(self):
+        assert DataClassWithCounter.from_json('{"c": {"f": 1, "o": 2}}') == \
+               DataClassWithCounter(c=Counter('foo'))