Python Programming for Biology: Bioinformatics and Beyond

List comprehension

Sometimes loops are used to go through one collection of items simply to generate another

collection of (different) items. For example, here the function range(1,8) gives a sequence

[1,2,3,4,5,6,7], which is used to make a list of square numbers:

squares = []

for x in range(1,8):

squares.append(x*x)

# squares becomes [1, 4, 9, 16, 25, 36, 49]

However,  when  mapping  one  collection  to  another  in  this  way,  there  is  a  neater

alternative syntax called list comprehension. And often this will allow you to map one list

of items onto another in a single line of code. In essence a list comprehension is a way of

using a loop to build a list from the inside. Here is a simple example:

squares = [x*x for x in range(1,8)] # [1, 4, 9, 16, 25, 36, 49]

The first expression inside the square brackets (x*x) is what is placed in the resulting

list, and the remainder, from the for to the closing bracket, is what generates the values for

the loop variable x. Not only is this shorter to write than a conventional for loop, it is also

computationally quicker.