Fix union type hint checks (#318)

Fix some obscure edge cases related to typing.Union type args.

Fixes #304.

src/drf_yasg/inspectors/field.py

@@ -490,6 +490,20 @@ if typing:
     hinting_type_info.append(((typing.Sequence, typing.AbstractSet), inspect_collection_hint_class))
 
 
+def _get_union_types(hint_class):
+    if typing:
+        origin_type = get_origin_type(hint_class)
+        if origin_type is typing.Union:
+            return hint_class.__args__
+        try:
+            # python 3.5.2 and lower compatibility
+            if issubclass(origin_type, typing.Union):
+                return hint_class.__union_params__
+        except TypeError:
+            pass
+    return None
+
+
 def get_basic_type_info_from_hint(hint_class):
     """Given a class (eg from a SerializerMethodField's return type hint,
     return its basic type information - ``type``, ``format``, ``pattern``,
@@ -499,11 +513,11 @@ def get_basic_type_info_from_hint(hint_class):
     :return: the extracted attributes as a dictionary, or ``None`` if the field type is not known
     :rtype: OrderedDict
     """
-    if typing and get_origin_type(hint_class) == typing.Union:
+    union_types = _get_union_types(hint_class)
+    if typing and union_types:
         # Optional is implemented as Union[T, None]
-        if len(hint_class.__args__) == 2 and hint_class.__args__[1] == type(None):  # noqa: E721
-            child_type = hint_class.__args__[0]
-            result = get_basic_type_info_from_hint(child_type)
+        if len(union_types) == 2 and isinstance(None, union_types[1]):
+            result = get_basic_type_info_from_hint(union_types[0])
             result['x-nullable'] = True
             return result
 

tests/test_get_basic_type_info_from_hint.py

@@ -33,6 +33,10 @@ if typing:
         # Following cases are not 100% correct, but it should work somehow and not crash.
         (Union[int, float], None),
         (List, {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_STRING)}),
+        ('SomeType', None),
+        (type('SomeType', (object,), {}), None),
+        (None, None),
+        (6, None),
     ])
     def test_get_basic_type_info_from_hint(hint_class, expected_swagger_type_info):
         type_info = get_basic_type_info_from_hint(hint_class)