what is a callback function in python code example

Example: python callback

In this code

if callback != None:
    callback
callback on its own doesn't do anything; it accepts parameters - def callback(a, b):

The fact that you did callback(1, 2) first will call that function, thereby printing Sum = 3.

Since callback returns no explicit value, it is returned as None.

Thus, your code is equivalent to

callback(1, 2)
main()
Solution
You could try not calling the function at first and just passing its handle.

def callback(sum):
    print("Sum = {}".format(sum))

def main(a, b, _callback = None):
    print("adding {} + {}".format(a, b))
    if _callback:
        _callback(a+b)

main(1, 2, callback)