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	PEP: 202
	Title: List Comprehensions
	Version: $Revision$
	Last-Modified: $Date$
	Author: barry@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.


	References
	==========

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



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peps/pep-0202.txt