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.