Mypy: lru_cache annotation doesn't work as a property

[takeda]

So I originally reported it in typeshed/#2659 but was told this can't be fixed there and it is mypy limitation so repporting it here.

---

I have following code:

#! /usr/bin/env python

import time
from functools import lru_cache

class MyClass:
    @property
    @lru_cache(1)
    def expensive_operation(self) -> int:
        time.sleep(2)
        return 42

    def another_operation(self) -> int:
        return self.expensive_operation + 1

if __name__ == '__main__':
    o = MyClass()
    print(o.expensive_operation)
    print(o.expensive_operation)
    print(o.another_operation())

It works as expected:

$ ./test.py
42
42
43

Yet, mypy is showing errors:

$ mypy test.py
test.py:7: error: Decorated property not supported
test.py:14: error: Unsupported operand types for + ("_lru_cache_wrapper[int]" and "int")

The lru_cache works just fine when decorated as property, and the returned type should be int and not _lru_cache_wrapper[int].

This is on python 3.7.0 and mypy 0.641.

[probablyodd]

Duplicates https://github.com/python/mypy/issues/1362 and https://github.com/python/mypy/issues/1608

[sashgorokhov]

@probablyodd well its a bit different. If you set # type: ignore to @property in this case,
mypy will completely ignore existence of this decorator and treat such methods as common callable methods. Mypy will just say somethingh like "lru_cache" has no attribute "..." if complex object such as dict returned from property method.

from functools import lru_cache

class Foo:
    @property  # type: ignore
    @lru_cache
    def bar(self) -> dict:
        return dict(key='value')

Foo().bar.get('key')

test.py:11: error: "lru_cache" has no attribute "get"

Edit:

Possible adhoc solution:

T = TypeVar('T')

def cache(func: Callable[..., T]) -> T:
    return functools.lru_cache()(func)  # type: ignore

...
    @property  # type: ignore
    @cache
    def foo(self) -> dict:
        return {}

[pgilad]

Besides @sashgorokhov awesome workaround (that adds some complexity to code) - is there any plan to support this ?