[Python-ideas] Consider making enumerate a sequence if its argument is a sequence

M.-A. Lemburg mal at egenix.com
Wed Sep 30 20:43:30 CEST 2015



On 30.09.2015 20:26, Andrew Barnert via Python-ideas wrote:
> On Sep 30, 2015, at 11:11, M.-A. Lemburg <mal at egenix.com> wrote:
>>
>>> On 30.09.2015 19:19, Neil Girdhar wrote:
>>> I guess, I'm just asking for enumerate to go through the same change that
>>> range went through.  Why wasn't it a problem for range?
>>
>> range() returns a list in Python 2 and a generator in Python 3.
> 
> No it doesn't. It returns a (lazy) sequence. Not a generator, or any other kind of iterator.

You are right that it's not of a generator type
and more like a lazy sequence. To be exact, it returns
a range object and does implement the iter protocol via
a range_iterator object.

In Python 2 we have the xrange object which has similar
properties, but not the same, e.g. you can't slice it.

> I don't know why so many people seem to believe it returns a generator. (And, when you point out what it returns, most of them say, "Why was that changed from 2.x xrange, which returned a generator?" but xrange never returned a generator either--it returned a lazy almost-a-sequence from the start.)

Perhaps because it behaves like one ? :-)

Unlike an iterator, it doesn't iterate over a sequence, but instead
generates the values on the fly.

FWIW: I don't think many people use the lazy sequence features
of range(), e.g. the slicing or index support. By far most
uses are in for-loops.

-- 
Marc-Andre Lemburg
eGenix.com

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