python callback

1In this code
2
3if callback != None:
4    callback
5callback on its own doesn't do anything; it accepts parameters - def callback(a, b):
6
7The fact that you did callback(1, 2) first will call that function, thereby printing Sum = 3.
8
9Since callback returns no explicit value, it is returned as None.
10
11Thus, your code is equivalent to
12
13callback(1, 2)
14main()
15Solution
16You could try not calling the function at first and just passing its handle.
17
18def callback(sum):
19    print("Sum = {}".format(sum))
20
21def main(a, b, _callback = None):
22    print("adding {} + {}".format(a, b))
23    if _callback:
24        _callback(a+b)
25
26main(1, 2, callback)