Introduction

A list comprehension is a syntactical tool for creating lists in a natural and concise way, as illustrated in the following code to make a list of squares of the numbers 1 to 10:

[i ** 2 for i in range(1,11)]

The dummy i from an existing list range is used to make a new element pattern. It is used where a for loop would be necessary in less expressive languages.

Syntax

• [i for i in range(10)] # basic list comprehension
• [i for i in xrange(10)] # basic list comprehension with generator object in python 2.x
• [i for i in range(20) if i % 2 == 0] # with filter
• [x + y for x in [1, 2, 3] for y in [3, 4, 5]] # nested loops
• [i if i > 6 else 0 for i in range(10)] # ternary expression
• [i if i > 4 else 0 for i in range(20) if i % 2 == 0] # with filter and ternary expression
• [[x + y for x in [1, 2, 3]] for y in [3, 4, 5]] # nested list comprehension

Remarks

List comprehensions were outlined in PEP 202 and introduced in Python 2.0.

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