List Comprehensions

The manipulation of lists is at the heart of a lot of programming problems.

A list of squares

def listofsquares(alist):
    squares = []
    for i in alist:
        squares.append(i * i)
    return squares

>>> listofsquares([1, 2, 3, 4])
[1, 4, 9, 16]

A list of squares using a list comprehension

List comprehensions introduce a terse style for manipulating lists.

def listofsquares(alist):
    return [ i * i for i in alist ]

>>> listofsquares([1, 2, 3, 4])
[1, 4, 9, 16]

A list of even squares

def listofevensquares(alist):
    return [ i * i for i in alist if i % 2 == 0]

>>> listofevensquares([1, 2, 3, 4])
[4, 16]

If you are interested in learning more about list comprehensions, Udacity’s Design of Computer Programs has a nice introduction. You may also want to look at the Python documentation or Python for Beginners.