Fixed #29478 -- Added support for mangled names to cached_property.

Co-Authored-By: Sergey Fedoseev <fedoseev.sergey@gmail.com>

django/utils/functional.py

@@ -3,6 +3,8 @@ import itertools
 import operator
 from functools import total_ordering, wraps
 
+from django.utils.version import PY36, get_docs_version
+
 
 # You can't trivially replace this with `functools.partial` because this binds
 # to classes and returns bound instances, whereas functools.partial (on
@@ -18,13 +20,54 @@ class cached_property:
     Decorator that converts a method with a single self argument into a
     property cached on the instance.
 
-    Optional ``name`` argument allows you to make cached properties of other
-    methods. (e.g.  url = cached_property(get_absolute_url, name='url') )
+    A cached property can be made out of an existing method:
+    (e.g. ``url = cached_property(get_absolute_url)``).
+    On Python < 3.6, the optional ``name`` argument must be provided, e.g.
+    ``url = cached_property(get_absolute_url, name='url')``.
     """
+    name = None
+
+    @staticmethod
+    def func(instance):
+        raise TypeError(
+            'Cannot use cached_property instance without calling '
+            '__set_name__() on it.'
+        )
+
+    @staticmethod
+    def _is_mangled(name):
+        return name.startswith('__') and not name.endswith('__')
+
     def __init__(self, func, name=None):
-        self.func = func
+        if PY36:
+            self.real_func = func
+        else:
+            func_name = func.__name__
+            name = name or func_name
+            if not (isinstance(name, str) and name.isidentifier()):
+                raise ValueError(
+                    "%r can't be used as the name of a cached_property." % name,
+                )
+            if self._is_mangled(name):
+                raise ValueError(
+                    'cached_property does not work with mangled methods on '
+                    'Python < 3.6 without the appropriate `name` argument. See '
+                    'https://docs.djangoproject.com/en/%s/ref/utils/'
+                    '#cached-property-mangled-name' % get_docs_version(),
+                )
+            self.name = name
+            self.func = func
         self.__doc__ = getattr(func, '__doc__')
-        self.name = name or func.__name__
+
+    def __set_name__(self, owner, name):
+        if self.name is None:
+            self.name = name
+            self.func = self.real_func
+        elif name != self.name:
+            raise TypeError(
+                "Cannot assign the same cached_property to two different names "
+                "(%r and %r)." % (self.name, name)
+            )
 
     def __get__(self, instance, cls=None):
         """