In a compromise with JHy, and to be more consistent with the style now
documented in PEP1, remove the Emacs page breaks. Also, Owner: -> Author:, added Created: and Post-History: headers Changed "Standard For-Loops" section to "Motivation" and shortened considerably; readers already know how for-loops work in Python. Added notes about Guido's veto of lazy evaluation.
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pep-0201.txt
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pep-0201.txt
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PEP: 201
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Title: Parallel Iteration
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Version: $Revision$
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Owner: bwarsaw@beopen.com (Barry A. Warsaw)
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Author: bwarsaw@beopen.com (Barry A. Warsaw)
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Python-Version: 2.0
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Status: Draft
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Created: 13-Jul-2000
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Post-History:
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Introduction
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This PEP describes the `parallel iteration' proposal for Python
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@ -19,42 +20,19 @@ Introduction
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history of this file contains the definitive historical record.
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Standard For-Loops
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Motivation
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Motivation for this feature has its roots in a concept described
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as `parallel for loops'. A standard for-loop in Python iterates
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over every element in the sequence until the sequence is
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exhausted. A `break' statement inside the loop suite causes an
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explicit loop exit. For-loops also have else: clauses which get
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executed when the loop exits normally (i.e. not by execution of a
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break).
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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, "Chinese Menu"
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type of way.
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For-loops can iterate over built-in types such as lists and
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tuples, but they can also iterate over instance types that conform
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to an informal sequence protocol. This protocol states that the
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instance should implement the __getitem__() method, expecting a
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monotonically increasing index starting at 0, and this method
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should raise an IndexError when the sequence is exhausted. This
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protocol is currently undocumented -- a defect in Python's
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documentation hopefully soon corrected.
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For-loops are described in the Python language reference
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manual[1].
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An example for-loop:
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>>> for i in (1, 2, 3): print i
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...
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1
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2
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3
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In this example, the variable `i' is called the `target', and is
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assigned the next element of the list, each time through the loop.
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The common idioms used to accomplish this are unintuitive and
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inflexible. This PEP proposes a standard way of performing such
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iterations by introducing a new builtin function called `zip'.
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Parallel For-Loops
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Parallel for-loops are non-nested iterations over two or more
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@ -70,12 +48,6 @@ Parallel For-Loops
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(1, 4)
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(2, 5)
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(3, 6)
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Here, map() returns a list of N-tuples, where N is the number of
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sequences in map()'s argument list (after the initial `None').
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Each tuple is constructed of the i-th elements from each of the
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argument lists, specifically in this example:
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>>> map(None, a, b)
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[(1, 4), (2, 5), (3, 6)]
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@ -106,7 +78,6 @@ Parallel For-Loops
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comprehensions' (see pep-0202.txt).
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The Proposed Solution
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The proposed solution is to introduce a new built-in sequence
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@ -129,7 +100,6 @@ The Proposed Solution
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Issues below for more discussion.
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Lazy Execution
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For performance purposes, zip() does not construct the list of
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@ -138,8 +108,9 @@ Lazy Execution
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for-loop protocol. This method constructs the individual tuples
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on demand.
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Guido is strongly opposed to lazy execution. See Open Issues.
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Examples
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Here are some examples, based on the reference implementation
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@ -194,7 +165,6 @@ Examples
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not all the same length.
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Reference Implementation
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Here is a reference implementation, in Python of the zip()
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@ -292,7 +262,6 @@ Reference Implementation
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return _Zipper(args, kws)
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Rejected Elaborations
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Some people have suggested that the user be able to specify the
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@ -352,9 +321,12 @@ Rejected Elaborations
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advantages.
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Open Issues
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- Guido opposes lazy evaluation for zip(). He believes zip()
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should return a real list, with an xzip() lazy evaluator added
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later if necessary.
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- What should "zip(a)" do? Given
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a = (1, 2, 3); zip(a)
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@ -427,7 +399,6 @@ Open Issues
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always override pad if both were given.
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References
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[1] http://www.python.org/doc/devel/ref/for.html
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@ -435,7 +406,6 @@ References
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TBD: URL to python-dev archives
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Copyright
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This document has been placed in the public domain.
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