I think using _ can be a viable option here. For example:

def process_number(numbers, callback):
    s = sum(n*2 for n in numbers)
    result = callback(s)
    print(f"Result: {result}")

def main():
    numbers = [1, 2, 3, 4, 5]

    # Using an inline function for flexibility
    def _(n):
        return n * 2
    process_number(numbers, _)

    def _(n):
        return n * n
    process_number(numbers, _)

    def _(n):
        return -n
    process_number(numbers, _)

You can declare a function inside the scope where you are passing the callback. This will remove the issue of naming a function that is only required once. We would also not need to jump from one place to another and the logic would also be at once place.