How to Type Hint a Decorator in Python

Decorators are a concept that can trip up new Python users. You may find this definition helpful: A decorator is a function that takes in another function and adds new functionality to it without modifying the original function.

Functions can be used just like any other data type in Python. A function can be passed to a function or returned from a function, just like a string or integer.

If you have jumped on the type-hinting bandwagon, you will probably want to add type hints to your decorators. That has been difficult until fairly recently.

Let’s see how to type hint a decorator!

Here’s an example:

from functools import wraps
from typing import Any, Callable, TypeVar


Generic_function = TypeVar("Generic_function", bound=Callable[..., Any])

def info(func: Generic_function) -> Generic_function:
    @wraps(func)
    def wrapper(*args: Any, **kwargs: Any) -> Any:
        print('Function name: ' + func.__name__)
        print('Function docstring: ' + str(func.__doc__))
        result = func(*args, **kwargs)
        return result
    return wrapper

@info
def doubler(number: int) -> int:
    """Doubles the number passed to it"""
    return number * 2

print(doubler(4))

If you run mypy —strict info_decorator.py you will get the following output:

info_decorator.py:14: error: Incompatible return value type (got "_Wrapped[[VarArg(Any), KwArg(Any)], Any, [VarArg(Any), KwArg(Any)], Any]", expected "Generic_function")  [return-value]
Found 1 error in 1 file (checked 1 source file)