fix type hints when using postponed evaluation of annotations (PEP-563)

src/drf_yasg/inspectors/field.py

@@ -6,7 +6,6 @@
 from contextlib import suppress
 from collections import OrderedDict
 from decimal import Decimal
-from inspect import signature as inspect_signature
 
 import pkg_resources
 import typing
@@ -613,17 +612,19 @@ def field_to_swagger_object(self, field, swagger_object_type, use_references, **
             return self.probe_field_inspectors(serializer, swagger_object_type, use_references, read_only=True)
         else:
             # look for Python 3.5+ style type hinting of the return value
-            hint_class = inspect_signature(method).return_annotation
-
-            if not inspect.isclass(hint_class) and hasattr(hint_class, '__args__'):
-                hint_class = hint_class.__args__[0]
-            if inspect.isclass(hint_class) and not issubclass(hint_class, inspect._empty):
-                type_info = get_basic_type_info_from_hint(hint_class)
-
-                if type_info is not None:
-                    SwaggerType, ChildSwaggerType = self._get_partial_types(field, swagger_object_type,
-                                                                            use_references, **kwargs)
-                    return SwaggerType(**type_info)
+            hint_class = typing.get_type_hints(method).get('return')
+
+            # annotations such as typing.Optional have an __instancecheck__
+            # hook and will not look like classes, but `issubclass` needs
+            # a class as its first argument, so only in that case abort
+            if inspect.isclass(hint_class) and issubclass(hint_class, inspect._empty):
+                return NotHandled
+
+            type_info = get_basic_type_info_from_hint(hint_class)
+            if type_info is not None:
+                SwaggerType, ChildSwaggerType = self._get_partial_types(field, swagger_object_type,
+                                                                        use_references, **kwargs)
+                return SwaggerType(**type_info)
 
         return NotHandled
 

tests/test_schema_generator.py

@@ -345,7 +345,7 @@ def retrieve(self, request, pk=None):
     )
     swagger = generator.get_schema(None, True)
     property_schema = swagger["definitions"]["OptionalMethod"]["properties"]["x"]
-    assert property_schema == openapi.Schema(title='X', type=expected_type, readOnly=True)
+    assert property_schema == openapi.Schema(title='X', type=expected_type, readOnly=True, x_nullable=True)
 
 
 EXPECTED_DESCRIPTION = """\