### 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://www.python.org/doc/current/ref/lists.html#l2h-238