code.py

def add(x=None):
    if x is None:
        return getattr(add, 'result', 0)
    else:
        if hasattr(add, 'result'):
            add.result += x
        else:
            setattr(add, 'result', x)
        return add

print(add(1)(2)(3)()) # 6

Task

You need to make the add function that has the following behavior:

print( add(1)(2)(3)() ) # 1 + 2 + 3 = 6
>>> 6

Disclaimer

I don't know where the original task is, I remember seeing it somewhere in Python chat.