How to write a simple callback function?

Question

Python 2.7.10

I wrote the following code to test a simple callback function.

def callback(a, b):
    print(\'Sum = {0}\'.format(a+b))

def main(         


        

Answer 1

Your code is executed as follows:

main(callback(1, 2))

callback function is called with (1, 2) and it returns None (Without return statement, your function prints Sum = 3 and returns None)

main function is called with None as argument (So callback != None will always be False)