Fix type hint support for functools cached_property wrapped funcs

functools added a `cached_property` wrapper in Python 3.8. Support type
hinting for properties wrapped in both Django's `cached_property` and
functool's `cached_property`.

drf_spectacular/plumbing.py

@@ -27,6 +27,7 @@
 )
 from django.utils.functional import Promise, cached_property
 from django.utils.module_loading import import_string
+from django.utils.version import PY38
 from rest_framework import exceptions, fields, mixins, serializers, versioning
 from rest_framework.settings import api_settings
 from rest_framework.test import APIRequestFactory
@@ -46,6 +47,11 @@
     class Choices:  # type: ignore
         pass
 
+if PY38:
+    CACHED_PROPERTY_FUNCS = (functools.cached_property, cached_property)  # type: ignore
+else:
+    CACHED_PROPERTY_FUNCS = (cached_property,)  # type: ignore
+
 T = TypeVar('T')
 
 
@@ -373,7 +379,7 @@ def _follow_field_source(model, path: List[str]):
         # end of traversal
         if isinstance(field_or_property, property):
             return field_or_property.fget
-        elif isinstance(field_or_property, cached_property):
+        elif isinstance(field_or_property, CACHED_PROPERTY_FUNCS):
             return field_or_property.func
         elif callable(field_or_property):
             return field_or_property
@@ -397,10 +403,13 @@ def _follow_field_source(model, path: List[str]):
             else:
                 return field
     else:
-        if isinstance(field_or_property, (property, cached_property)) or callable(field_or_property):
+        if (
+            isinstance(field_or_property, (property,) + CACHED_PROPERTY_FUNCS)
+            or callable(field_or_property)
+        ):
             if isinstance(field_or_property, property):
                 target_model = _follow_return_type(field_or_property.fget)
-            elif isinstance(field_or_property, cached_property):
+            elif isinstance(field_or_property, CACHED_PROPERTY_FUNCS):
                 target_model = _follow_return_type(field_or_property.func)
             else:
                 target_model = _follow_return_type(field_or_property)

tests/test_fields.py

@@ -1,3 +1,4 @@
+import functools
 import tempfile
 import uuid
 from datetime import date, datetime, timedelta
@@ -112,6 +113,10 @@ def field_model_property_float(self) -> float:
     def field_model_cached_property_float(self) -> float:
         return 1.337
 
+    @functools.cached_property  # type: ignore
+    def field_model_py_cached_property_float(self) -> float:
+        return 1.337
+
     @property
     def field_list(self):
         return [1.1, 2.2, 3.3]
@@ -134,6 +139,10 @@ def sub_object(self) -> SubObject:
     def sub_object_cached(self) -> SubObject:
         return SubObject(self)
 
+    @functools.cached_property  # type: ignore
+    def sub_object_py_cached(self) -> SubObject:
+        return SubObject(self)
+
     @property
     def optional_sub_object(self) -> Optional[SubObject]:
         return SubObject(self)
@@ -199,6 +208,8 @@ def get_field_method_object(self, obj) -> dict:
 
     field_model_cached_property_float = serializers.ReadOnlyField()
 
+    field_model_py_cached_property_float = serializers.ReadOnlyField()
+
     field_dict_int = serializers.DictField(
         child=serializers.IntegerField(),
         source='field_json',
@@ -217,6 +228,14 @@ def get_field_method_object(self, obj) -> dict:
     field_sub_object_cached_nested_calculated = serializers.ReadOnlyField(source='sub_object_cached.nested.calculated')
     field_sub_object_cached_model_int = serializers.ReadOnlyField(source='sub_object_cached.model_instance.field_int')
 
+    field_sub_object_py_cached_calculated = serializers.ReadOnlyField(source='sub_object_py_cached.calculated')
+    field_sub_object_py_cached_nested_calculated = serializers.ReadOnlyField(
+        source='sub_object_py_cached.nested.calculated',
+    )
+    field_sub_object_py_cached_model_int = serializers.ReadOnlyField(
+        source='sub_object_py_cached.model_instance.field_int',
+    )
+
     # typing.Optional
     field_optional_sub_object_calculated = serializers.ReadOnlyField(
         source='optional_sub_object.calculated',

tests/test_fields.yml

@@ -167,6 +167,10 @@ components:
           type: number
           format: float
           readOnly: true
+        field_model_py_cached_property_float:
+          type: number
+          format: float
+          readOnly: true
         field_dict_int:
           type: object
           additionalProperties:
@@ -192,6 +196,15 @@ components:
         field_sub_object_cached_model_int:
           type: integer
           readOnly: true
+        field_sub_object_py_cached_calculated:
+          type: integer
+          readOnly: true
+        field_sub_object_py_cached_nested_calculated:
+          type: integer
+          readOnly: true
+        field_sub_object_py_cached_model_int:
+          type: integer
+          readOnly: true
         field_optional_sub_object_calculated:
           type: integer
           readOnly: true
@@ -303,6 +316,7 @@ components:
       - field_method_object
       - field_model_cached_property_float
       - field_model_property_float
+      - field_model_py_cached_property_float
       - field_o2o
       - field_optional_sub_object_calculated
       - field_posint
@@ -324,6 +338,9 @@ components:
       - field_sub_object_model_int
       - field_sub_object_nested_calculated
       - field_sub_object_optional_int
+      - field_sub_object_py_cached_calculated
+      - field_sub_object_py_cached_model_int
+      - field_sub_object_py_cached_nested_calculated
       - field_text
       - field_time
       - field_url