python-peps/peps/pep-0202.rst

PEP: 202
Title: List Comprehensions
Author: Barry Warsaw <barry@python.org>
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.


References
==========

.. [1] http://docs.python.org/reference/expressions.html#list-displays