diff --git a/src/drf_yasg/inspectors/field.py b/src/drf_yasg/inspectors/field.py
index 4d456fbf..ec25d7a6 100644
--- a/src/drf_yasg/inspectors/field.py
+++ b/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 typing
 from django.core import validators
@@ -193,7 +192,7 @@ def get_queryset_from_view(view, serializer=None):
 
         if queryset is not None and serializer is not None:
             # make sure the view is actually using *this* serializer
-            assert type(serializer) == call_view_method(view, 'get_serializer_class', 'serializer_class')
+            assert type(serializer) is call_view_method(view, 'get_serializer_class', 'serializer_class')
 
         return queryset
     except Exception:  # pragma: no cover
@@ -617,17 +616,19 @@ 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
 
diff --git a/src/drf_yasg/openapi.py b/src/drf_yasg/openapi.py
index 0a2cfb8f..c4632173 100644
--- a/src/drf_yasg/openapi.py
+++ b/src/drf_yasg/openapi.py
@@ -88,7 +88,7 @@ class SwaggerDict(OrderedDict):
     def __init__(self, **attrs):
         super(SwaggerDict, self).__init__()
         self._extras__ = attrs
-        if type(self) == SwaggerDict:
+        if type(self) is SwaggerDict:
             self._insert_extras__()
 
     def __setattr__(self, key, value):
@@ -441,6 +441,8 @@ def __init__(self, name, in_, description=None, required=None, schema=None,
             # path parameters must always be required
             assert required is not False, "path parameter cannot be optional"
             self.required = True
+        if self['in'] == IN_QUERY and type == TYPE_ARRAY:
+            self.collection_format = 'multi'
         if self['in'] != IN_BODY and schema is not None:
             raise AssertionError("schema can only be applied to a body Parameter, not %s" % type)
         if default and not type:
@@ -516,7 +518,7 @@ def __init__(self, resolver, name, scope, expected_type, ignore_unresolved=False
         :param bool ignore_unresolved: do not throw if the referenced object does not exist
         """
         super(_Ref, self).__init__()
-        assert not type(self) == _Ref, "do not instantiate _Ref directly"
+        assert not type(self) is _Ref, "do not instantiate _Ref directly"
         ref_name = "#/{scope}/{name}".format(scope=scope, name=name)
         if not ignore_unresolved:
             obj = resolver.get(name, scope)
diff --git a/src/drf_yasg/utils.py b/src/drf_yasg/utils.py
index c78a4821..480c35d0 100644
--- a/src/drf_yasg/utils.py
+++ b/src/drf_yasg/utils.py
@@ -442,7 +442,7 @@ def force_real_str(s, encoding='utf-8', strings_only=False, errors='strict'):
     """
     if s is not None:
         s = force_str(s, encoding, strings_only, errors)
-        if type(s) != str:
+        if type(s) is not str:
             s = '' + s
 
         # Remove common indentation to get the correct Markdown rendering
diff --git a/tests/test_schema_generator.py b/tests/test_schema_generator.py
index 063e7c08..85999074 100644
--- a/tests/test_schema_generator.py
+++ b/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 = """\