Expert Python Programming - Third Edition

List comprehensions

As you probably know, writing a piece of code such as this can be tedious:

>>> evens = []
>>> for i in range(10):
...     if i % 2 == 0:
...         evens.append(i)
...   
>>> evens
[0, 2, 4, 6, 8] 

This may work for C, but it actually makes things slower for Python for the following reasons:

• It makes the interpreter work on each loop to determine what part of the sequence has to be changed
• It makes you keep a counter to track what element has to be processed
• It requires additional function lookups to be performed at every iteration because append() is a list's method

A list comprehension is a better pattern for these kind of situations. It allows us to define a list by using a single line of code:

>>> [i for i in range(10) if i % ...