[Python-Dev] Lockstep iteration - eureka!

[Just van Rossum]

At 7:49 AM -0500 09-08-2000, Guido van Rossum wrote:
>In the past there were syntax proposals, e.g. ``for i
>indexing s''.  Maybe you and Just can draft a PEP?

PEP:            1716099-3
Title:          Index-enhanced sequence iteration
Version:        $Revision: 1.1 $
Owner:          Someone-with-commit-rights
Python-Version: 2.0
Status:         Incomplete

Introduction

    This PEP proposes a way to more conveniently iterate over a
    sequence and its indices.

Features

    It adds an optional clause to the 'for' statement:

        for <index> indexing <element> in <seq>:
            ...

    This is equivalent to (see the zip() PEP):

        for <index>, <element> in zip(range(len(seq)), seq):
            ...

    Except no new list is created.

Mechanism

    The index of the current element in a for-loop already
    exists in the implementation, however, it is not reachable
    from Python. The new 'indexing' keyword merely exposes the
    internal counter.

Implementation

    Implementation should be trivial for anyone named Guido,
    Tim or Thomas. Justs better not try.

Advantages:

    Less code needed for this common operation, which is
    currently most often written as:

        for index in range(len(seq)):
            element = seq[i]
            ...

Disadvantages:

    It will break that one person's code that uses "indexing"
    as a variable name.

Copyright

    This document has been placed in the public domain.

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