Add python/copy_type.py

Author: rec

python/copy_type.py

@@ -0,0 +1,41 @@
+from abc import ABC, abstractmethod
+from typing import Any, TypeVar, Callable
+from typing_extensions import override, reveal_type
+
+
+_T = TypeVar("_T")
+
+
+class Module:
+    def __call__(self, *args: Any, **kwargs: Any) -> Any:
+        return self.forward(*args, **kwargs)
+
+    @abstractmethod
+    def forward(self, *args: Any, **kwargs: Any) -> Any:
+        raise NotImplementedError
+
+
+def copy_type(f: _T) -> Callable[[Any], _T]:
+    # From https://github.com/python/typing/issues/769#issuecomment-903760354
+    return lambda x: x
+
+
+class Implementation(Module):
+    @override
+    def forward(self, x: int, /, y: str) -> int:
+        return x * int(y)
+
+    # This boilerplate would need to be pasted into any class derived from Module
+    # to get the right typing for `__call__`
+    @copy_type(forward)
+    def __call__(self, *a: Any, **k: Any) -> Any:
+        return super().__call__(*a, **k)
+
+
+imp = Implementation()
+print(imp(2, "3"))
+print(imp("2", "3"))
+
+# mypy says:
+# copy_type.py:34: error: Argument 1 to "__call__" of "Implementation" has incompatible type "str"; expected "int"  [arg-type]
+# Found 1 error in 1 file (checked 1 source file)