From 204c7dada412b0ca4ce22315d2acae640adb128f Mon Sep 17 00:00:00 2001
From: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Date: Wed, 5 May 2021 17:25:58 +0200
Subject: [PATCH] Add support for functools.cached_property (#10408)

Add support for functools.cached_property decorator:
https://docs.python.org/3/library/functools.html#functools.cached_property

Closes #8913.
Closes python/typeshed#3963.
---
 mypy/semanal.py                      |  5 ++++-
 test-data/unit/check-functools.test  | 20 ++++++++++++++++++++
 test-data/unit/fixtures/property.pyi |  1 +
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/mypy/semanal.py b/mypy/semanal.py
index 0927e465b07c..f4702635fb9d 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -999,12 +999,15 @@ def visit_decorator(self, dec: Decorator) -> None:
                 dec.var.is_classmethod = True
                 self.check_decorated_function_is_method('classmethod', dec)
             elif (refers_to_fullname(d, 'builtins.property') or
-                  refers_to_fullname(d, 'abc.abstractproperty')):
+                  refers_to_fullname(d, 'abc.abstractproperty') or
+                  refers_to_fullname(d, 'functools.cached_property')):
                 removed.append(i)
                 dec.func.is_property = True
                 dec.var.is_property = True
                 if refers_to_fullname(d, 'abc.abstractproperty'):
                     dec.func.is_abstract = True
+                elif refers_to_fullname(d, 'functools.cached_property'):
+                    dec.var.is_settable_property = True
                 self.check_decorated_function_is_method('property', dec)
                 if len(dec.func.arguments) > 1:
                     self.fail('Too many arguments', dec.func)
diff --git a/test-data/unit/check-functools.test b/test-data/unit/check-functools.test
index 416006591425..33653c8d3fbc 100644
--- a/test-data/unit/check-functools.test
+++ b/test-data/unit/check-functools.test
@@ -107,3 +107,23 @@ Ord() > 1
 Ord() >= 1  # E: Unsupported left operand type for >= ("Ord")
 [builtins fixtures/ops.pyi]
 [builtins fixtures/dict.pyi]
+
+[case testCachedProperty]
+# flags: --python-version 3.8
+from functools import cached_property
+class Parent:
+    @property
+    def f(self) -> str: pass
+class Child(Parent):
+    @cached_property
+    def f(self) -> str: pass
+    @cached_property
+    def g(self) -> int: pass
+    @cached_property
+    def h(self, arg) -> int: pass  # E: Too many arguments
+reveal_type(Parent().f)  # N: Revealed type is "builtins.str"
+reveal_type(Child().f)  # N: Revealed type is "builtins.str"
+reveal_type(Child().g)  # N: Revealed type is "builtins.int"
+Child().f = "Hello World"
+Child().g = "invalid"  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+[builtins fixtures/property.pyi]
diff --git a/test-data/unit/fixtures/property.pyi b/test-data/unit/fixtures/property.pyi
index 5dc785da2364..b3f60abaf8a0 100644
--- a/test-data/unit/fixtures/property.pyi
+++ b/test-data/unit/fixtures/property.pyi
@@ -12,6 +12,7 @@ class function: pass
 
 property = object()  # Dummy definition
 
+class dict: pass
 class int: pass
 class str: pass
 class bytes: pass