[Tutor] Callbacks in Python

[Lie Ryan]

Jramak wrote:
> Hello
>  
> I'm confused by callbacks. I would really appreciate any introduction or 
> help in understanding the concept of callbacks.

By example:

def callback(nums):
     """ The callback function """
     return sum(nums) * 2

def another_callback(nums):
     """ Yet another callback function """
     return sum(nums) * 3

def strange_sum(nums, cb):
     """
     Returns the sum, if less than 10
     else returns the result of calling the callback function cb(),
     which must accepts one list argument
     """
     if sum(nums) > 10:
         return sum(nums)
     else:
         return cb(nums)

print strange_sum([1, 3, 4, 2, 4], callback)
print strange_sum([3, 5, 4, 2], another_callback)

So basically, a callback is a function that you passes as an argument 
(to another function) that may be called when a certain condition 
happens. In GUI-related stuff, callbacks are called when a certain 
events happens (e.g. a button is clicked).

For the note, there are two main reasons why you want to pass a function 
to another function:
1. For callbacks, as explained and reasoned above.
2. To modify the passed functions, this case is known as decorator.