str.index() and str.find() versus only list.index()

Ian Kelly ian.g.kelly at
Tue Jul 14 06:07:27 CEST 2015

On Mon, Jul 13, 2015 at 9:23 PM, Steven D'Aprano <steve at> wrote:
> On Tue, 14 Jul 2015 01:12 pm, Ian Kelly wrote:
>> On Mon, Jul 13, 2015 at 10:56 AM, Roel Schroeven <roel at>
>> wrote:
>>> Hi,
>>> Quick question: why does str have both index() and find(), while list
>>> only has index()? Is there a reason for that, or is it just an historical
>>> accident?
>> Historical accident, I think. If it were to be redone, I doubt that
>> str.find would exist. The problem with it is that it returns -1 to
>> indicate that the argument was not found, but -1 is a valid index into
>> the string. This is a potential source of hard-to-find bugs.
> Correct. But rather than removing it, it would be better to take a leaf out
> of re.match's book and return None as the sentinel. That would eliminate
> the "using -1 as a valid index" bug.

I disagree on both counts.

>>> s = 'abc'
>>> s[None:None]

Better IMO to just have the one non-redundant method that raises an
exception rather than returning anything that could possibly be
interpreted as a string index.

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