Python functions are first-class elements of the language. That means that they can be treated like any other values. One common use of first-class functions is to use them as parameters to the higher-order functions map and filter. The function map takes a function as its first parameter and a list as its second parameter and returns a list for which the function has been applied to every element in the original list. For example i def square (x return x x A for element in map( range (5) (square print (element) will print 0, 1,4,9, 16 (on separate lines And filter takes a function that returns a boolean as its first argument and a list as its second and returns a list of all of the elements of the original list for which the filter function returns true. For example i def even (x) return (x 2) 0 for element in filter (even range (5)) print (element) will print 0,2, (on separate lines). And they can be combined like so 1 for element in filter (even map( range (5))) (square print (element) which will print 0,4, 16 (on separate lines)