How to fix mypy error, “Expression has type Any [misc]”, for python type hinting of int __pow__ int?

I have a python function that utilizes the python __pow__ (**) operator with two int’s, Mypy tells me that the expression b ** e has type “Any”, where b and e have type “int”. I have tried to cast the expression to an int with int(b ** e), but I still get the error. How do I correctly type hint this expression?

Also, if the expression b ** e really does return type “Any”, can you explain why?

Error:

temp.py:7: error: Expression has type "Any"  [misc]
        power: Callable[[int, int], int] = lambda b, e: b ** e
                                                        ^

temp.py

from functools import reduce
from typing import Callable, Dict


def factorization_product(fact: Dict[int, int]) -> int:
    '''returns the product which has the prime factorization of fact'''
    power: Callable[[int, int], int] = lambda b, e: b ** e # error on expression "b ** e"
    product: Callable[[int, int], int] = lambda x, y: x * y
    return reduce(product, [power(b, e) for b, e in fact.items()], 1)

Edit:

I realized I can use the builtin pow and operator.mul instead of lambda’s, but I still get the error.

Error:

temp.py:8: error: Expression has type "Any"  [misc]
        return reduce(mul, [pow(b, e) for b, e in fact.items()], 1)
                            ^

revised temp.py

from functools import reduce
from operator import mul
from typing import Dict


def factorization_product(fact: Dict[int, int]) -> int:
    '''returns the product which has the prime factorization of fact'''
    return reduce(mul, [pow(b, e) for b, e in fact.items()], 1)

Answer

Also, if the expression b ** e really does return type “Any”, can you explain why?

Checking typeshed shows that an int is returned only for the specialized case of squaring a number (x of type Literal[2]). This is because even though b and e are ints, e may be negative, in which case the result is a float. Since the result can be a float or int, it looks like typeshed went with Any for the general case.

I’d say this is a language limitation. Ideally we could use @overload on all non-negative integes for x, but Literal only supports specific values.

To get around this while also using --disallow-any-expr, use typing.cast like so:

power: Callable[[int, int], int] = lambda b, e: typing.cast(int, b ** e)

Running mypy --disallow-any-expr temp.py now returns Success: no issues found in 1 source file.

But before you blindly add the cast, consider the scenario I brought up where e is negative, causing type checking to succeed but the runtime to fail if you do int-specific manipulation with the result of factorization_product. You may want to add validation here. For example, with no validation:

factorial_sized_lst = [0] * factorization_product({1: 2, 3: -4})

fails at run-time with can't multiply sequence by non-int of type 'float' despite mypy reporting type-check success.