[Python-Dev] Adding start to enumerate() (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue May 13 20:53:06 CEST 2008

On Wed, 14 May 2008 12:25:01 am Guido van Rossum wrote:

However I see no use for skipping items from the start,

You've never had to deal with data where the first N items were special in some way? e.g. skipping over a header line in a file?

I know I've written code like this before:

it = iter(whatever()) for i in xrange(N): # skip the first N items it.next() for item in it: process(item)

and if that use case ever came up, passing a slice to enumerate() would be the appropriate thing to do.

While slices are wonderfully useful things, they aren't panaceas. They're not so useful with iterators, and they make a copy of the data, which can be problematic if there's a lot of it.

[tongue firmly in cheek] Perhaps what we need is a more flexible enumerate function? enumerate(iterable, start_at_index=0, count_from=0)

Having a consistent index provided by enumerate reduces the amount of thought I have to put into the loop structure, and avoids the temptation to do this:

for i, item in enumerate(seq, start=1): print "line %d: %s" % (i, item) # subtract one from the index to adjust for one-based counting seq[i-1] = foo(item)

I never need to think about whether it is zero-based, one-based, or some other N-based counting, because it is always zero-based.

-0 on any change to enumerate.

-- Steven D'Aprano

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