PEP: 202
Title: List Comprehensions
Version: $Revision$
Author: tim@digicool.com (Tim Peters)
Status: Draft
Type: Standards Track
Python-Version: 2.0
Created: 13-Jul-2000
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')]
    >>> def zip(*args):
    ...    return apply(map, (None,)+args)
    ... 
    >>> print [i for i in zip(nums,fruit) if i[0]%2==0]
    [(2, 'Peaches'), (4, 'Bananas')]


Reference Implementation

    SourceForge contains a patch that adds list comprehensions to Python[1].


BDFL Pronouncements

    Note: the BDFL refers to Guido van Rossum, Python's Benevolent
    Dictator For Life.

    - The syntax proposed above is the Right One.

    - The form [x, y for ...] should be disallowed; one should be
      required to write [(x, y) for ...].

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


Open Issues

    Syntax

        Several people proposed connecting or separating syntax
        between the various clauses, for example, requiring a
        semicolon between them to set them apart:

            [i,f; for i in nums; for f in fruit; if f[0]=="P"; if i%2==1]

        To minimize strain on the Python parser, Guido has suggested
        requiring parentheses around the initial tuple:

            [(i,f) for i in nums for f in fruit if f[0]=="P" if i%2==1]

    Semantics

        The semantics of multiple for clauses is not obvious to many
        people.  Currently, it nests, so that

            [i,f for i in nums for f in fruit]

        is functionally equivalent to

            tmp = []
            for i in nums:
                for f in fruit:
                    tmp.append((i,f))

        Other people would read it as if it executed

            map(None, nums, fruit)

        It's not clear that this is necessary.  The newly proposed
        zip() builtin[2] takes care of that case.

    Stability of the Implementation

        The current reference implementation is simply an adaptation
        of Greg Ewing's original demonstration of the concept.  Other
        than tracking changes to the source code to keep it a valid
        patch, reindenting the code and switching to function
        prototypes, nothing has been done to it.  This obviously
        raises some questions about how stable the code is.  It has
        not had a lot of exercise, though the patch does include a few
        test cases.


References

    [1] https://sourceforge.net/patch/?func=detailpatch&patch_id=100654&group_id=5470
    [2] Lockstep Iteration, pep-0201.txt


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