python-peps/pep-0201.txt

PEP: 201
Title: Parallel Iteration
Version: $Revision$
Author: bwarsaw@beopen.com (Barry A. Warsaw)
Python-Version: 2.0
Status: Draft
Created: 13-Jul-2000
Post-History: 27-Jul-2000


Introduction

    This PEP describes the `parallel iteration' proposal for Python
    2.0, previously known as `parallel for loops'.  This PEP tracks
    the status and ownership of this feature, slated for introduction
    in Python 2.0.  It contains a description of the feature and
    outlines changes necessary to support the feature.  This PEP
    summarizes discussions held in mailing list forums, and provides
    URLs for further information, where appropriate.  The CVS revision
    history of this file contains the definitive historical record.


Motivation

    Standard for-loops in Python iterate over every element in a
    sequence until the sequence is exhausted[1].  However, for-loops
    iterate over only a single sequence, and it is often desirable to
    loop over more than one sequence, in a lock-step, "Chinese Menu"
    type of way.

    The common idioms used to accomplish this are unintuitive and
    inflexible.  This PEP proposes a standard way of performing such
    iterations by introducing a new builtin function called `zip'.


Parallel For-Loops

    Parallel for-loops are non-nested iterations over two or more
    sequences, such that at each pass through the loop, one element
    from each sequence is taken to compose the target.  This behavior
    can already be accomplished in Python through the use of the map()
    built-in function:

    >>> a = (1, 2, 3)
    >>> b = (4, 5, 6)
    >>> for i in map(None, a, b): print i
    ... 
    (1, 4)
    (2, 5)
    (3, 6)
    >>> map(None, a, b)
    [(1, 4), (2, 5), (3, 6)]

    The for-loop simply iterates over this list as normal.

    While the map() idiom is a common one in Python, it has several
    disadvantages:

    - It is non-obvious to programmers without a functional
      programming background.

    - The use of the magic `None' first argument is non-obvious.

    - It has arbitrary, often unintended, and inflexible semantics
      when the lists are not of the same length: the shorter sequences
      are padded with `None'.

      >>> c = (4, 5, 6, 7)
      >>> map(None, a, c)
      [(1, 4), (2, 5), (3, 6), (None, 7)]

    For these reasons, several proposals were floated in the Python
    2.0 beta time frame for providing a better spelling of parallel
    for-loops.  The initial proposals centered around syntactic
    changes to the for statement, but conflicts and problems with the
    syntax were unresolvable, especially when parallel for-loops were
    combined with another proposed feature called `list
    comprehensions' (see pep-0202.txt).


The Proposed Solution

    The proposed solution is to introduce a new built-in sequence
    generator function, available in the __builtin__ module.  This
    function is to be called `zip' and has the following signature:

    zip(seqa, [seqb, [...]])

    zip() takes one or more sequences and weaves their elements
    together, just as map(None, ...) does with sequences of equal
    length.  The weaving stops when the shortest sequence is
    exhausted.


Return Value

    zip() returns a real Python list, the same way map() does.


Examples

    Here are some examples, based on the reference implementation
    below.

    >>> a = (1, 2, 3, 4)
    >>> b = (5, 6, 7, 8)
    >>> c = (9, 10, 11)
    >>> d = (12, 13)

    >>> zip(a, b)
    [(1, 5), (2, 6), (3, 7), (4, 8)]

    >>> zip(a, d)
    [(1, 12), (2, 13)]

    >>> zip(a, b, c, d)
    [(1, 5, 9, 12), (2, 6, 10, 13)]

    Note that when the sequences are of the same length, zip() is
    reversible:

    >>> a = (1, 2, 3)
    >>> b = (4, 5, 6)
    >>> x = zip(a, b)
    >>> y = zip(*x) # alternatively, apply(zip, x)
    >>> z = zip(*y) # alternatively, apply(zip, y)
    >>> x
    [(1, 4), (2, 5), (3, 6)]
    >>> y
    [(1, 2, 3), (4, 5, 6)]
    >>> z
    [(1, 4), (2, 5), (3, 6)]
    >>> x == z
    1

    It is not possible to reverse zip this way when the sequences are
    not all the same length.


Reference Implementation

    Here is a reference implementation, in Python of the zip()
    built-in function.  These would ultimately be replaced by
    equivalent C code.

    def zip(*args):
        if not args:
            raise TypeError('zip() expects one or more sequence arguments')
        ret = []
        # find the length of the shortest sequence
        shortest = min(*map(len, args))
        for i in range(shortest):
            item = []
            for s in args:
                item.append(s[i])
            ret.append(tuple(item))
        return ret


BDFL Pronouncements

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

    - The function's name.  An earlier version of this PEP included an
      open issue listing 20+ proposed alternative names to zip().  In
      the face of no overwhelmingly better choice, the BDFL strongly
      prefers zip() due to it's Haskell[2] heritage.  See version 1.7
      of this PEP for the list of alteratives.

    - zip() shall be a built-in function.

    - Optional padding.  An earlier version of this PEP proposed an
      optional `pad' keyword argument, which would be used when the
      argument sequences were not the same length.  This is similar
      behavior to the map(None, ...) semantics except that the user
      would be able to specify pad object.  This has been rejected by
      the BDFL in favor of always truncating to the shortest sequence.

    - Lazy evaluation.  An earlier version of this PEP proposed that
      zip() return a built-in object that performed lazy evaluation
      using __getitem__() protocol.  This has been strongly rejected
      by the BDFL in favor of returning a real Python list.  If lazy
      evaluation is desired in the future, the BDFL suggests an xzip()
      function be added.

    - zip() with no arguments.  the BDFL strongly prefers this raise a
      TypeError exception.

    - zip() with one argument.  the BDFL strongly prefers that this
      return a list of 1-tuples.

    - Inner and outer container control.  An earlier version of this
      PEP contains a rather lengthy discussion on a feature that some
      people wanted, namely the ability to control what the inner and
      outer container types were (they are tuples and list
      respectively in this version of the PEP).  Given the simplified
      API and implementation, this elaboration is rejected.  For a
      more detailed analysis, see version 1.7 of this PEP.


References

    [1] http://www.python.org/doc/current/ref/for.html
    [2] http://www.haskell.org/onlinereport/standard-prelude.html#$vzip

    TBD: URL to python-dev archives


Copyright

    This document has been placed in the public domain.



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