167 lines
4.8 KiB
Plaintext
167 lines
4.8 KiB
Plaintext
PEP: 212
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Title: Loop Counter Iteration
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Version: $Revision$
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Last-Modified: $Date$
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Author: nowonder@nowonder.de (Peter Schneider-Kamp)
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Status: Deferred
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Type: Standards Track
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Created: 22-Aug-2000
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Python-Version: 2.1
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Introduction
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This PEP describes the often proposed feature of exposing the loop
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counter in for-loops. This PEP tracks the status and ownership of
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this feature. It contains a description of the feature and
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outlines changes necessary to support the feature. This PEP
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summarizes discussions held in mailing list forums, and provides
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URLs for further information, where appropriate. The CVS revision
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history of this file contains the definitive historical record.
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Motivation
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Standard for-loops in Python iterate over the elements of a
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sequence[1]. Often it is desirable to loop over the indices or
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both the elements and the indices instead.
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The common idioms used to accomplish this are unintuitive. This
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PEP proposes two different ways of exposing the indices.
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Loop counter iteration
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The current idiom for looping over the indices makes use of the
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built-in 'range' function:
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for i in range(len(sequence)):
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# work with index i
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Looping over both elements and indices can be achieved either by the
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old idiom or by using the new 'zip' built-in function[2]:
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for i in range(len(sequence)):
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e = sequence[i]
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# work with index i and element e
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or
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for i, e in zip(range(len(sequence)), sequence):
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# work with index i and element e
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The Proposed Solutions
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There are three solutions that have been discussed. One adds a
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non-reserved keyword, the other adds two built-in functions.
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A third solution adds methods to sequence objects.
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Non-reserved keyword 'indexing'
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This solution would extend the syntax of the for-loop by adding
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an optional '<variable> indexing' clause which can also be used
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instead of the '<variable> in' clause..
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Looping over the indices of a sequence would thus become:
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for i indexing sequence:
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# work with index i
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Looping over both indices and elements would similarly be:
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for i indexing e in sequence:
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# work with index i and element e
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Built-in functions 'indices' and 'irange'
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This solution adds two built-in functions 'indices' and 'irange'.
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The semantics of these can be described as follows:
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def indices(sequence):
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return range(len(sequence))
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def irange(sequence):
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return zip(range(len(sequence)), sequence)
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These functions could be implemented either eagerly or lazily and
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should be easy to extend in order to accept more than one sequence
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argument.
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The use of these functions would simplify the idioms for looping
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over the indices and over both elements and indices:
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for i in indices(sequence):
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# work with index i
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for i, e in irange(sequence):
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# work with index i and element e
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Methods for sequence objects
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This solution proposes the addition of 'indices', 'items'
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and 'values' methods to sequences, which enable looping over
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indices only, both indices and elements, and elements only
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respectively.
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This would immensely simplify the idioms for looping over indices
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and for looping over both elements and indices:
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for i in sequence.indices():
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# work with index i
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for i, e in sequence.items():
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# work with index i and element e
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Additionally it would allow to do looping over the elements
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of sequences and dicitionaries in a consistent way:
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for e in sequence_or_dict.values():
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# do something with element e
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Implementations
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For all three solutions some more or less rough patches exist
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as patches at SourceForge:
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'for i indexing a in l': exposing the for-loop counter[3]
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add indices() and irange() to built-ins[4]
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add items() method to listobject[5]
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All of them have been pronounced on and rejected by the BDFL.
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Note that the 'indexing' keyword is only a NAME in the
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grammar and so does not hinder the general use of 'indexing'.
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Backward Compatibility Issues
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As no keywords are added and the semantics of existing code
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remains unchanged, all three solutions can be implemented
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without breaking existing code.
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Copyright
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This document has been placed in the public domain.
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References
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[1] http://www.python.org/doc/current/ref/for.html
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[2] Lockstep Iteration, PEP 201
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[3] http://sourceforge.net/patch/download.php?id=101138
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[4] http://sourceforge.net/patch/download.php?id=101129
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[5] http://sourceforge.net/patch/download.php?id=101178
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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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