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(callback=None):
    print('Add any two digits.')
    if callback != None:
        callback

main(callback(1, 2))

I receive this when I execute it:

Sum = 3
Add any two digits.

Why Add any two digits is after Sum = 3? I guess it is because the callback function executes first. How to execute the callback function after all other code in main() executed?

Answer 1

Here's what you wanted to do :

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

def main(a,b,f=None):
    print('Add any two digits.')
    if f != None:
        f(a,b)

main(1, 2, callback)

Answer 2

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)

Answer 3

As mentioned in the comments, your callback is called whenever it's suffixed with open and close parens; thus it's called when you pass it.

You might want to use a lambda and pass in the values.

#!/usr/bin/env python3

def main(callback=None, x=None, y=None):
    print('Add any two digits.')
    if callback != None and x != None and y != None:
        print("Result of callback is {0}".format(callback(x,y)))
    else:
        print("Missing values...")

if __name__ == "__main__":
    main(lambda x, y: x+y, 1, 2)

Answer 4

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)

Answer 5

The problem is that you're evaluating the callback before you pass it as a callable. One flexible way to solve the problem would be this:

def callback1(a, b):
    print('Sum = {0}'.format(a+b))

def callback2(a):
    print('Square = {0}'.format(a**2))

def callback3():
    print('Hello, world!')

def main(callback=None, cargs=()):
    print('Calling callback.')
    if callback != None:
        callback(*cargs)

main(callback1, cargs=(1, 2))
main(callback2, cargs=(2,))
main(callback3)

Optionally you may want to include a way to support keyword arguments.