Python function “sum”，语法如下：sum() -> 0 sum(1)(2)(3) -> 6

Question

I am new to Python and I need to write function sum with similar syntax:

print(sum())
print(sum(1)(2)(3))
print(sum(1)(2)(3)(-4)(-5))

The output is:

0
6
-3

I tried to write decorator like this:

def sum(a=0):
    res = a
    def helper(b=0):
        nonlocal res
        res += b
        return res
    return helper

But it helps only with fixed quantity of (). So this code works only for: sum(1)(2) -> 3 and doesn't for sum() -> <function sum.<locals>.helper at 0x7fca6de44160> or sum(1)()(3) -> TypeError: 'int' object is not callable I think there should be a decorator with recursion, but i don't know how to realize it

Answer 1

That syntax choice looks very odd to me - should the result of sum(foo) be a number or a function? The built-in sum just takes an iterable and returns a number, which feels much less surprising.

However, assuming that you are certain you indeed want to create something that looks like an integer, walks like an integer, swims like an integer but is also callable, the language does let you create it:

class sum(int):
    def __call__(self, x=0):
        return sum(self + x)

The output is as you specified:

print(sum())
print(sum(1)(2)(3))
print(sum(1)(2)(3)(-4)(-5))

0
6
-3

Answer 2

Finaly, I found my way of solving this task (using only functions):

def sum(x=None):
    value = 0

    def helper(y=None):
        nonlocal value
        if y is None:
            return value
        value += y
        return helper

return helper(x)