python-peps/peps/pep-0279.rst

PEP: 279
Title: The enumerate() built-in function
Author: Raymond Hettinger <python@rcn.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Jan-2002
Python-Version: 2.3
Post-History:


Abstract
========

This PEP introduces a new built-in function, ``enumerate()`` to
simplify a commonly used looping idiom.  It provides all iterable
collections with the same advantage that ``iteritems()`` affords to
dictionaries -- a compact, readable, reliable index notation.


Rationale
=========

Python 2.2 introduced the concept of an iterable interface as
proposed in :pep:`234`.  The ``iter()`` factory function was provided
as common calling convention and deep changes were made to use
iterators as a unifying theme throughout Python.  The unification
came in the form of establishing a common iterable interface for
mappings, sequences, and file objects.

Generators, as proposed in :pep:`255`, were introduced as a means
for making it easier to create iterators, especially ones with
complex internal execution or variable states.  The availability
of generators makes it possible to improve on the loop counter
ideas in :pep:`212`.  Those ideas provided a clean syntax for
iteration with indices and values, but did not apply to all
iterable objects.  Also, that approach did not have the memory
friendly benefit provided by generators which do not evaluate the
entire sequence all at once.

The new proposal is to add a built-in function, ``enumerate()`` which
was made possible once iterators and generators became available.
It provides all iterables with the same advantage that ``iteritems()``
affords to dictionaries -- a compact, readable, reliable index
notation.  Like ``zip()``, it is expected to become a commonly used
looping idiom.

This suggestion is designed to take advantage of the existing
implementation and require little additional effort to
incorporate.  It is backwards compatible and requires no new
keywords.  The proposal will go into Python 2.3 when generators
become final and are not imported from ``__future__``.


BDFL Pronouncements
===================

The new built-in function is ACCEPTED.


Specification for a new built-in
================================

::

    def enumerate(collection):
       'Generates an indexed series:  (0,coll[0]), (1,coll[1]) ...'
       i = 0
       it = iter(collection)
       while 1:
          yield (i, it.next())
          i += 1

Note A: :pep:`212` Loop Counter Iteration discussed several
proposals for achieving indexing.  Some of the proposals only work
for lists unlike the above function which works for any generator,
xrange, sequence, or iterable object.  Also, those proposals were
presented and evaluated in the world prior to Python 2.2 which did
not include generators.  As a result, the non-generator version in
:pep:`212` had the disadvantage of consuming memory with a giant list
of tuples.  The generator version presented here is fast and
light, works with all iterables, and allows users to abandon the
sequence in mid-stream with no loss of computation effort.

There are other PEPs which touch on related issues: integer
iterators, integer for-loops, and one for modifying the arguments
to ``range`` and ``xrange``.  The ``enumerate()`` proposal does not preclude
the other proposals and it still meets an important need even if
those are adopted -- the need to count items in any iterable.  The
other proposals give a means of producing an index but not the
corresponding value.  This is especially problematic if a sequence
is given which doesn't support random access such as a file
object, generator, or sequence defined with ``__getitem__``.

Note B: Almost all of the PEP reviewers welcomed the function but
were divided as to whether there should be any built-ins.  The
main argument for a separate module was to slow the rate of
language inflation.  The main argument for a built-in was that the
function is destined to be part of a core programming style,
applicable to any object with an iterable interface.  Just as
``zip()`` solves the problem of looping over multiple sequences, the
``enumerate()`` function solves the loop counter problem.

If only one built-in is allowed, then ``enumerate()`` is the most
important general purpose tool, solving the broadest class of
problems while improving program brevity, clarity and reliability.

Note C:  Various alternative names were discussed:

=================  =============================================================
``iterindexed()``  five syllables is a mouthful
``index()``        nice verb but could be confused the ``.index()`` method
``indexed()``      widely liked however adjectives should be avoided
``indexer()``      noun did not read well in a for-loop
``count()``        direct and explicit but often used in other contexts
``itercount()``    direct, explicit and hated by more than one person
``iteritems()``    conflicts with key:value concept for dictionaries
``itemize()``      confusing because ``amap.items()`` != ``list(itemize(amap))``
``enum()``         pithy; less clear than enumerate; too similar to enum
                   in other languages where it has a different meaning
=================  =============================================================

All of the names involving 'count' had the further disadvantage of
implying that the count would begin from one instead of zero.

All of the names involving 'index' clashed with usage in database
languages where indexing implies a sorting operation rather than
linear sequencing.

Note D: This function was originally proposed with optional start
and stop arguments.  GvR pointed out that the function call
``enumerate(seqn,4,6)`` had an alternate, plausible interpretation as
a slice that would return the fourth and fifth elements of the
sequence.  To avoid the ambiguity, the optional arguments were
dropped even though it meant losing flexibility as a loop counter.
That flexibility was most important for the common case of
counting from one, as in::

    for linenum, line in enumerate(source,1):  print linenum, line


Comments from GvR:
    ``filter`` and ``map`` should die and be subsumed into list
    comprehensions, not grow more variants. I'd rather introduce
    built-ins that do iterator algebra (e.g. the iterzip that I've
    often used as an example).

    I like the idea of having some way to iterate over a sequence
    and its index set in parallel.  It's fine for this to be a
    built-in.

    I don't like the name "indexed"; adjectives do not make good
    function names.  Maybe ``iterindexed()``?


Comments from Ka-Ping Yee:
    I'm also quite happy with everything  you
    proposed ... and the extra built-ins (really 'indexed' in
    particular) are things I have wanted for a long time.


Comments from Neil Schemenauer:
    The new built-ins sound okay.  Guido
    may be concerned with increasing the number of built-ins too
    much.  You might be better off selling them as part of a
    module.  If you use a module then you can add lots of useful
    functions (Haskell has lots of them that we could steal).


Comments for Magnus Lie Hetland:
    I think indexed would be a useful and
    natural built-in function. I would certainly use it a lot.  I
    like ``indexed()`` a lot; +1. I'm quite happy to have it make PEP
    281 obsolete. Adding a separate module for iterator utilities
    seems like a good idea.


Comments from the Community:
    The response to the ``enumerate()`` proposal
    has been close to 100% favorable.  Almost everyone loves the
    idea.


Author response:
    Prior to these comments, four built-ins were proposed.
    After the comments, ``xmap``, ``xfilter`` and ``xzip`` were withdrawn.  The
    one that remains is vital for the language and is proposed by
    itself.  ``Indexed()`` is trivially easy to implement and can be
    documented in minutes.  More importantly, it is useful in
    everyday programming which does not otherwise involve explicit
    use of generators.

    This proposal originally included another function ``iterzip()``.
    That was subsequently implemented as the ``izip()`` function in
    the ``itertools`` module.


Copyright
=========

This document has been placed in the public domain.