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PEP 279 -- The enumerate() built-in function

PEP: 279
Title: The enumerate() built-in function
Author: python at (Raymond Hettinger)
Status: Final
Type: Standards Track
Created: 30-Jan-2002
Python-Version: 2.3


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.


Python 2.2 introduced the concept of an iterable interface as proposed in PEP 234 [3] . 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 [1] , 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 [2] . 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,
      i += 1

Note A: PEP 212 Loop Counter Iteration [2] 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.