PEP: 202
Title: List Comprehensions
Version: $Revision$
Owner: tpeters@beopen.com (Tim Peters)
Python-Version: 2.0
Status: Incomplete


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

    Please refer to 

        https://sourceforge.net/patch/?func=detailpatch&patch_id=100654&group_id=5470

    for a patch that adds list comprehensions to Python.


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 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.


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