Fix type order relevance for union wrapping 2 types (#419)

dataclasses_json/mm.py

@@ -303,7 +303,7 @@ def schema(cls, mixin, infer_missing):
                 options['allow_none'] = True
                 if len(type_.__args__) == 2:
                     # Union[str, int, None] is optional too, but it has more than 1 typed field.
-                    type_ = type_.__args__[0]
+                    type_ = [tp for tp in type_.__args__ if tp is not type(None)][0]
 
             if metadata.letter_case is not None:
                 options['data_key'] = metadata.letter_case(field.name)

tests/test_union.py

@@ -73,6 +73,47 @@ class C9:
     f1: List[Union[Aux1, Aux2]]
 
 
+try:
+    @dataclass_json
+    @dataclass
+    class C10:
+        """
+         Use py3.10+ pipe notation for optionals
+         Check if passing None as the first type option doesn't break type resolution
+
+         Instantiate with None default so NoneType is returned for runtime field value type.
+        """
+        f1: None | str = None
+except TypeError:
+    @dataclass_json
+    @dataclass
+    class C10:
+        """
+         Replace test case on versions prior to 3.10
+        """
+        f1: Union[None, str] = None
+
+
+try:
+    @dataclass_json
+    @dataclass
+    class C11:
+        """
+         Use py3.10+ pipe notation for optionals
+         Check if passing None as the second type option doesn't break type resolution
+
+         Instantiate with None default so NoneType is returned for runtime field value type.
+        """
+        f1: str | None = None
+except TypeError:
+    @dataclass_json
+    @dataclass
+    class C11:
+        """
+         Replace test case on versions prior to 3.10
+        """
+        f1: Union[str, None] = None
+
 params = [
     (C1(f1=12), {"f1": 12}, '{"f1": 12}'),
     (C1(f1="str1"), {"f1": "str1"}, '{"f1": "str1"}'),
@@ -106,7 +147,12 @@ class C9:
 
     (C9([Aux1(12), Aux2("str3")]),
      {"f1": [{"f1": 12, "__type": "Aux1"}, {"f1": "str3", "__type": "Aux2"}]},
-     '{"f1": [{"f1": 12, "__type": "Aux1"}, {"f1": "str3", "__type": "Aux2"}]}')
+     '{"f1": [{"f1": 12, "__type": "Aux1"}, {"f1": "str3", "__type": "Aux2"}]}'),
+
+    (C10(f1=None), {"f1": None}, '{"f1": null}'),
+    (C10(f1='str1'), {"f1": 'str1'}, '{"f1": "str1"}'),
+
+    (C11(f1=None), {"f1": None}, '{"f1": null}')
 ]