Is this decorator type-annotated correctly? [closed]

Question

from functools import wraps
from typing import Any, Callable


def timer(func: Callable[..., Any]) -> Callable[..., Any]:
    """Calculates the runtime of a function, and outputs it to logging.DEBUG."""

    @wraps(func)
    def wrapper(*args, **kwargs):
        start = perf_counter()
        value = func(*args, **kwargs)
        end = perf_counter()
        _logger = logging.getLogger(__name__ + "." + func.__name__)
        _logger.debug(" runtime: {:.4f} seconds".format(end - start))
        return value

    return wrapper

Answer 1

The problem with this approach is that now, MyPy loses the return type, or rather, it degenerates into Any, so consider:

import logging
from typing import Callable, Any
from time import perf_counter
from functools import wraps

def timer(func: Callable[..., Any]) -> Callable[..., Any]:
    """Calculates the runtime of a function, and outputs it to logging.DEBUG."""

    @wraps(func)
    def wrapper(*args, **kwargs):
        start = perf_counter()
        value = func(*args, **kwargs)
        end = perf_counter()
        _logger = logging.getLogger(__name__ + '.' + func.__name__)
        _logger.debug(' runtime: {:.4f} seconds'.format(end - start))
        return value
    return wrapper

@timer
def func(x:int) -> int:
    return x


def string_func(s: str):
    return s[:]

x = 42 * func(42)

reveal_type(x)

string_func(x)

Using:

(py37) Juans-MacBook-Pro:tempdata juan$ mypy --version
mypy 0.641

If I try to type-check this, it passes!

(py37) Juans-MacBook-Pro:tempdata juan$ mypy typing_decorators.py
typing_decorators.py:29: error: Revealed type is 'Any'

I've found one solution in the case where you want to preserve exactly the arguments, which is to use a TypeVar and to cast the wrapper, that way MyPy can know exactly the type (assuming the original function is annoated):

import logging
from typing import Callable, Any, TypeVar, cast
from time import perf_counter
from functools import wraps


F = TypeVar('F', bound=Callable[..., Any])

def timer(func: F) -> F:
    """Calculates the runtime of a function, and outputs it to logging.DEBUG."""

    @wraps(func)
    def wrapper(*args, **kwargs):
        start = perf_counter()
        value = func(*args, **kwargs)
        end = perf_counter()
        _logger = logging.getLogger(__name__ + '.' + func.__name__)
        _logger.debug(' runtime: {:.4f} seconds'.format(end - start))
        return value
    return cast(F, wrapper)

@timer
def func(x:int) -> int:
    return x


def string_func(s: str):
    return s[:]

x = 42 * func(42)

reveal_type(x)

string_func(x)

In this case:

(py37) Juans-MacBook-Pro:tempdata juan$ mypy typing_decorators.py
typing_decorators.py:32: error: Revealed type is 'builtins.int'
typing_decorators.py:34: error: Argument 1 to "string_func" has incompatible type "int"; expected "str"