List comprehensions are used to derive a new list from an existing list. For example, you have a list of numbers and you want to get a corresponding list with all the numbers multiplied by 2 but only when the number itself is more than 2, then list comprehensions are ideal for such situations.
Example 15.1. Using List Comprehensions
#!/usr/bin/python # Filename: list_comprehensions.py listone = [2, 3, 4] listtwo = [2*i for i in listone if i > 2] print listtwo
Here, we derive a new list by specifying the manipulation to be done (2*i) when some condition is satisfied (if i > 2). Note that the original list remains unmodified. Many a time, we use loops to process each element of a list - the same can be achieved using list comprehensions in a more precise, compact and explicit manner.