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

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)