Preserve type hints in decorator (aio-libs#512)

The @cached decorator did not preserve type hints, breaking static analysis.

Author: elecnix

aiocache/decorators.py

@@ -2,6 +2,14 @@
 import functools
 import inspect
 import logging
+from typing import Callable, TypeVar
+try:
+    from typing import ParamSpec  # Python 3.10+
+except ImportError:
+    from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+R = TypeVar("R")
 
 from aiocache.base import SENTINEL
 from aiocache.lock import RedLock
@@ -43,9 +51,10 @@ def __init__(
         self.noself = noself
         self.cache = cache
 
-    def __call__(self, f):
+    # Use ParamSpec and TypeVar to preserve the decorated function's type signature for static type checkers.
+    def __call__(self, f: Callable[P, R]) -> Callable[P, R]:
         @functools.wraps(f)
-        async def wrapper(*args, **kwargs):
+        async def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
             return await self.decorator(f, *args, **kwargs)
 
         wrapper.cache = self.cache
@@ -228,9 +237,10 @@ def __init__(
         self.skip_cache_func = skip_cache_func
         self.ttl = ttl
 
-    def __call__(self, f):
+    # Use ParamSpec and TypeVar to preserve the decorated function's type signature for static type checkers.
+    def __call__(self, f: Callable[P, R]) -> Callable[P, R]:
         @functools.wraps(f)
-        async def wrapper(*args, **kwargs):
+        async def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
             return await self.decorator(f, *args, **kwargs)
 
         wrapper.cache = self.cache

tests/ut/test_decorators.py

@@ -177,6 +177,21 @@ async def what(self, a, b):
         assert str(inspect.signature(what)) == "(self, a, b)"
         assert inspect.getfullargspec(what.__wrapped__).args == ["self", "a", "b"]
 
+    async def test_cached_preserves_type_hints(self, mock_cache):
+        @cached(cache=mock_cache)
+        async def add(x: int, y: int) -> int:
+            return x + y
+
+        try:
+            from typing import get_type_hints
+        except ImportError:
+            from typing_extensions import get_type_hints
+
+        hints = get_type_hints(add)
+        assert hints['x'] is int
+        assert hints['y'] is int
+        assert hints['return'] is int
+
     async def test_reuses_cache_instance(self, mock_cache):
         @cached(cache=mock_cache)
         async def what():