PEP 202 – List Comprehensions

PEP

202

Title

List Comprehensions

Version

$Revision$

Last-Modified

$Date$

Author

barry at python.org (Barry Warsaw)

Status

Final

Type

Standards Track

Content-Type

text/x-rst

Created

13-Jul-2000

Python-Version

2.0

Post-History

Introduction

This PEP describes a proposed syntactical extension to Python, list comprehensions.

The Proposed Solution

It is proposed to allow conditional construction of list literals using for and if clauses. They would nest in the same way for loops and if statements nest now.

Rationale

List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

Examples

>>> print [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> print [i for i in range(20) if i%2 == 0]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

>>> nums = [1, 2, 3, 4]
>>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
>>> print [(i, f) for i in nums for f in fruit]
[(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
 (2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
 (3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
 (4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
>>> print [(i, f) for i in nums for f in fruit if f[0] == "P"]
[(1, 'Peaches'), (1, 'Pears'),
 (2, 'Peaches'), (2, 'Pears'),
 (3, 'Peaches'), (3, 'Pears'),
 (4, 'Peaches'), (4, 'Pears')]
>>> print [(i, f) for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
[(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
>>> print [i for i in zip(nums, fruit) if i[0]%2==0]
[(2, 'Peaches'), (4, 'Bananas')]

Reference Implementation

List comprehensions become part of the Python language with release 2.0, documented in 1.

BDFL Pronouncements

  • The syntax proposed above is the Right One.

  • The form [x, y for ...] is disallowed; one is required to write [(x, y) for ...].

  • The form [... for x... for y...] nests, with the last index varying fastest, just like nested for loops.