Convert PEP 201 to reST. (#50)
Note that link to Greg's link at the end is wrong: Greg Wilson's questionaire on proposed syntax to some CS grad students http://www.python.org/pipermail/python-dev/2000-July/013139.html It does not exists. While the archive **do** go to this date. I believe that woudl be https://mail.python.org/pipermail/python-dev/2000-July/006427.html [Python-Dev] multi-loop quiz results Thu, 13 Jul 2000 15:17:41 -0400 (EDT) > I gave 20+ grad students in computer engineering and computer > science...
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Matthias Bussonnier
Berker Peksag
parent
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commit
8eff8067cf
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pep-0201.txt
266
pep-0201.txt
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@ -5,47 +5,51 @@ Last-Modified: $Date$
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Author: barry@python.org (Barry Warsaw)
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 13-Jul-2000
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Python-Version: 2.0
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Post-History: 27-Jul-2000
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Introduction
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This PEP describes the `lockstep iteration' proposal. This PEP
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tracks the status and ownership of this feature, slated for
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introduction in Python 2.0. It contains a description of the
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feature and outlines changes necessary to support the feature.
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This PEP summarizes discussions held in mailing list forums, and
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provides URLs for further information, where appropriate. The CVS
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revision history of this file contains the definitive historical
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record.
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Introduction
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============
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This PEP describes the 'lockstep iteration' proposal. This PEP tracks
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the status and ownership of this feature, slated for introduction in
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Python 2.0. It contains a description of the feature and outlines
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changes necessary to support the feature. This PEP summarizes
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discussions held in mailing list forums, and provides URLs for further
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information, where appropriate. The CVS revision history of this file
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contains the definitive historical record.
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Motivation
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==========
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Standard for-loops in Python iterate over every element in a
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sequence until the sequence is exhausted[1]. However, for-loops
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iterate over only a single sequence, and it is often desirable to
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loop over more than one sequence in a lock-step fashion. In other
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words, in a way such that nthe i-th iteration through the loop
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returns an object containing the i-th element from each sequence.
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Standard for-loops in Python iterate over every element in a sequence
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until the sequence is exhausted [1]_. However, for-loops iterate over
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only a single sequence, and it is often desirable to loop over more
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than one sequence in a lock-step fashion. In other words, in a way
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such that the i-th iteration through the loop returns an object
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containing the i-th element from each sequence.
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The common idioms used to accomplish this are unintuitive. This
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PEP proposes a standard way of performing such iterations by
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introducing a new builtin function called `zip'.
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The common idioms used to accomplish this are unintuitive. This PEP
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proposes a standard way of performing such iterations by introducing a
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new builtin function called ``zip``.
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While the primary motivation for zip() comes from lock-step
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iteration, by implementing zip() as a built-in function, it has
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additional utility in contexts other than for-loops.
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While the primary motivation for zip() comes from lock-step iteration,
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by implementing zip() as a built-in function, it has additional
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utility in contexts other than for-loops.
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Lockstep For-Loops
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==================
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Lockstep for-loops are non-nested iterations over two or more
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sequences, such that at each pass through the loop, one element
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from each sequence is taken to compose the target. This behavior
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can already be accomplished in Python through the use of the map()
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built-in function:
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Lockstep for-loops are non-nested iterations over two or more
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sequences, such that at each pass through the loop, one element from
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each sequence is taken to compose the target. This behavior can
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already be accomplished in Python through the use of the map() built-
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in function::
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>>> a = (1, 2, 3)
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>>> b = (4, 5, 6)
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@ -57,63 +61,66 @@ Lockstep For-Loops
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>>> map(None, a, b)
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[(1, 4), (2, 5), (3, 6)]
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The for-loop simply iterates over this list as normal.
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The for-loop simply iterates over this list as normal.
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While the map() idiom is a common one in Python, it has several
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disadvantages:
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While the map() idiom is a common one in Python, it has several
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disadvantages:
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- It is non-obvious to programmers without a functional
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programming background.
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* It is non-obvious to programmers without a functional programming
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background.
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- The use of the magic `None' first argument is non-obvious.
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* The use of the magic ``None`` first argument is non-obvious.
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- It has arbitrary, often unintended, and inflexible semantics
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when the lists are not of the same length: the shorter sequences
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are padded with `None'.
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* It has arbitrary, often unintended, and inflexible semantics when
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the lists are not of the same length: the shorter sequences are
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padded with ``None``::
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>>> c = (4, 5, 6, 7)
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>>> map(None, a, c)
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[(1, 4), (2, 5), (3, 6), (None, 7)]
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For these reasons, several proposals were floated in the Python
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2.0 beta time frame for syntactic support of lockstep for-loops.
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Here are two suggestions:
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For these reasons, several proposals were floated in the Python 2.0
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beta time frame for syntactic support of lockstep for-loops. Here are
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two suggestions::
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for x in seq1, y in seq2:
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# stuff
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::
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for x, y in seq1, seq2:
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# stuff
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Neither of these forms would work, since they both already mean
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something in Python and changing the meanings would break existing
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code. All other suggestions for new syntax suffered the same
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problem, or were in conflict with other another proposed feature
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called `list comprehensions' (see PEP 202).
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Neither of these forms would work, since they both already mean
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something in Python and changing the meanings would break existing
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code. All other suggestions for new syntax suffered the same problem,
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or were in conflict with other another proposed feature called 'list
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comprehensions' (see PEP 202).
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The Proposed Solution
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=====================
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The proposed solution is to introduce a new built-in sequence
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generator function, available in the __builtin__ module. This
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function is to be called `zip' and has the following signature:
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The proposed solution is to introduce a new built-in sequence
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generator function, available in the ``__builtin__`` module. This
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function is to be called ``zip`` and has the following signature::
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zip(seqa, [seqb, [...]])
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zip() takes one or more sequences and weaves their elements
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together, just as map(None, ...) does with sequences of equal
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length. The weaving stops when the shortest sequence is
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exhausted.
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``zip()`` takes one or more sequences and weaves their elements
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together, just as ``map(None, ...)`` does with sequences of equal
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length. The weaving stops when the shortest sequence is exhausted.
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Return Value
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============
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zip() returns a real Python list, the same way map() does.
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``zip()`` returns a real Python list, the same way ``map()`` does.
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Examples
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========
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Here are some examples, based on the reference implementation
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below.
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Here are some examples, based on the reference implementation below::
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>>> a = (1, 2, 3, 4)
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>>> b = (5, 6, 7, 8)
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@ -129,8 +136,8 @@ Examples
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>>> zip(a, b, c, d)
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[(1, 5, 9, 12), (2, 6, 10, 13)]
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Note that when the sequences are of the same length, zip() is
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reversible:
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Note that when the sequences are of the same length, ``zip()`` is
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reversible::
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>>> a = (1, 2, 3)
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>>> b = (4, 5, 6)
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@ -146,15 +153,16 @@ Examples
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>>> x == z
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1
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It is not possible to reverse zip this way when the sequences are
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not all the same length.
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It is not possible to reverse zip this way when the sequences are not
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all the same length.
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Reference Implementation
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========================
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Here is a reference implementation, in Python of the zip()
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built-in function. This will be replaced with a C implementation
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after final approval.
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Here is a reference implementation, in Python of the zip() built-in
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function. This will be replaced with a C implementation after final
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approval::
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def zip(*args):
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if not args:
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@ -172,110 +180,120 @@ Reference Implementation
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return ret
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BDFL Pronouncements
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===================
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Note: the BDFL refers to Guido van Rossum, Python's Benevolent
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Dictator For Life.
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Note: the BDFL refers to Guido van Rossum, Python's Benevolent
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Dictator For Life.
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- The function's name. An earlier version of this PEP included an
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open issue listing 20+ proposed alternative names to zip(). In
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* The function's name. An earlier version of this PEP included an
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open issue listing 20+ proposed alternative names to ``zip()``. In
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the face of no overwhelmingly better choice, the BDFL strongly
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prefers zip() due to its Haskell[2] heritage. See version 1.7
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prefers ``zip()`` due to its Haskell [2]_ heritage. See version 1.7
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of this PEP for the list of alternatives.
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- zip() shall be a built-in function.
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* ``zip()`` shall be a built-in function.
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- Optional padding. An earlier version of this PEP proposed an
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optional `pad' keyword argument, which would be used when the
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* Optional padding. An earlier version of this PEP proposed an
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optional ``pad`` keyword argument, which would be used when the
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argument sequences were not the same length. This is similar
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behavior to the map(None, ...) semantics except that the user
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would be able to specify pad object. This has been rejected by
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the BDFL in favor of always truncating to the shortest sequence,
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because of the KISS principle. If there's a true need, it is
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easier to add later. If it is not needed, it would still be
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impossible to delete it in the future.
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behavior to the ``map(None, ...)`` semantics except that the user
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would be able to specify pad object. This has been rejected by the
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BDFL in favor of always truncating to the shortest sequence, because
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of the KISS principle. If there's a true need, it is easier to add
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later. If it is not needed, it would still be impossible to delete
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it in the future.
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- Lazy evaluation. An earlier version of this PEP proposed that
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zip() return a built-in object that performed lazy evaluation
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using __getitem__() protocol. This has been strongly rejected
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* Lazy evaluation. An earlier version of this PEP proposed that
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``zip()`` return a built-in object that performed lazy evaluation
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using ``__getitem__()`` protocol. This has been strongly rejected
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by the BDFL in favor of returning a real Python list. If lazy
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evaluation is desired in the future, the BDFL suggests an xzip()
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evaluation is desired in the future, the BDFL suggests an ``xzip()``
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function be added.
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- zip() with no arguments. the BDFL strongly prefers this raise a
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* ``zip()`` with no arguments. the BDFL strongly prefers this raise a
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TypeError exception.
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- zip() with one argument. the BDFL strongly prefers that this
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* ``zip()`` with one argument. the BDFL strongly prefers that this
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return a list of 1-tuples.
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- Inner and outer container control. An earlier version of this
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PEP contains a rather lengthy discussion on a feature that some
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people wanted, namely the ability to control what the inner and
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outer container types were (they are tuples and list
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respectively in this version of the PEP). Given the simplified
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API and implementation, this elaboration is rejected. For a
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more detailed analysis, see version 1.7 of this PEP.
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* Inner and outer container control. An earlier version of this PEP
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contains a rather lengthy discussion on a feature that some people
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wanted, namely the ability to control what the inner and outer
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container types were (they are tuples and list respectively in this
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version of the PEP). Given the simplified API and implementation,
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this elaboration is rejected. For a more detailed analysis, see
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version 1.7 of this PEP.
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Subsequent Change to zip()
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Subsequent Change to ``zip()``
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==============================
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In Python 2.4, zip() with no arguments was modified to return an
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empty list rather than raising a TypeError exception. The rationale
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for the original behavior was that the absence of arguments was
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thought to indicate a programming error. However, that thinking
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did not anticipate the use of zip() with the * operator for unpacking
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variable length argument lists. For example, the inverse of zip
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could be defined as: unzip = lambda s: zip(*s). That transformation
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also defines a matrix transpose or an equivalent row/column swap for
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tables defined as lists of tuples. The latter transformation is
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commonly used when reading data files with records as rows and fields
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as columns. For example, the code:
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In Python 2.4, zip() with no arguments was modified to return an empty
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list rather than raising a TypeError exception. The rationale for the
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original behavior was that the absence of arguments was thought to
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indicate a programming error. However, that thinking did not
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anticipate the use of zip() with the ``*`` operator for unpacking
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variable length argument lists. For example, the inverse of zip could
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be defined as: ``unzip = lambda s: zip(*s)``. That transformation
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also defines a matrix transpose or an equivalent row/column swap for
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tables defined as lists of tuples. The latter transformation is
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commonly used when reading data files with records as rows and fields
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as columns. For example, the code::
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date, rain, high, low = zip(*csv.reader(file("weather.csv")))
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rearranges columnar data so that each field is collected into
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individual tuples for straightforward looping and summarization:
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rearranges columnar data so that each field is collected into
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individual tuples for straightforward looping and summarization::
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print "Total rainfall", sum(rain)
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Using zip(*args) is more easily coded if zip(*[]) is handled as an
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allowable case rather than an exception. This is especially helpful
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when data is either built up from or recursed down to a null case
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with no records.
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Using ``zip(*args)`` is more easily coded if ``zip(*[])`` is handled
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as an allowable case rather than an exception. This is especially
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helpful when data is either built up from or recursed down to a null
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case with no records.
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Seeing this possibility, the BDFL agreed (with some misgivings) to
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have the behavior changed for Py2.4.
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Seeing this possibility, the BDFL agreed (with some misgivings) to
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have the behavior changed for Py2.4.
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Other Changes
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=============
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- The xzip() function discussed above was implemented in Py2.3 in
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the itertools module as itertools.izip(). This function provides
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lazy behavior, consuming single elements and producing a single
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tuple on each pass. The "just-in-time" style saves memory and
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runs faster than its list based counterpart, zip().
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* The ``xzip()`` function discussed above was implemented in Py2.3 in
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the ``itertools`` module as ``itertools.izip()``. This function
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provides lazy behavior, consuming single elements and producing a
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single tuple on each pass. The "just-in-time" style saves memory
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and runs faster than its list based counterpart, ``zip()``.
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- The itertools module also added itertools.repeat() and
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itertools.chain(). These tools can be used together to pad
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sequences with None (to match the behavior of map(None, seqn)):
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* The ``itertools`` module also added ``itertools.repeat()`` and
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``itertools.chain()``. These tools can be used together to pad
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sequences with ``None`` (to match the behavior of ``map(None,
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seqn)``)::
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zip(firstseq, chain(secondseq, repeat(None)))
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References
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==========
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[1] http://docs.python.org/reference/compound_stmts.html#for
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[2] http://www.haskell.org/onlinereport/standard-prelude.html#$vzip
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.. [1] http://docs.python.org/reference/compound_stmts.html#for
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Greg Wilson's questionaire on proposed syntax to some CS grad students
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http://www.python.org/pipermail/python-dev/2000-July/013139.html
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.. [2] http://www.haskell.org/onlinereport/standard-prelude.html#$vzip
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Greg Wilson's questionaire on proposed syntax to some CS grad students
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http://www.python.org/pipermail/python-dev/2000-July/013139.html
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Copyright
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=========
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This document has been placed in the public domain.
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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