Bug in string.find; was: Re: Proposed PEP: New style indexing, was Re: Bug in slice type

[Steve Holden]

Bryan Olson wrote:
> Antoon Pardon wrote:
>  > Bryan Olson schreef:
>  >
>  >>Steve Holden asked:
>  >>>And what are you proposing that
>  >>>find() should return if the substring isn't found at all? please don't
>  >>>suggest it should raise an exception, as index() exists to provide that
>  >>>functionality.
>  >>
>  >>There are a number of good options. A legal index is not one of
>  >>them.
>  >
>  > IMO, with find a number of "features" of python come together.
>  > that create an awkward situation.
>  >
>  > 1) 0 is a false value, but indexes start at 0 so you can't
>  >    return 0 to indicate nothing was found.
>  >
>  > 2) -1 is returned, which is both a true value and a legal
>  >    index.
>  >
>  > It probably is too late now, but I always felt, find should
>  > have returned None when the substring isn't found.
> 
> None is certainly a reasonable candidate. The one-past-the-end
> value, len(sequence), would be fine, and follows the preferred
> idiom of C/C++. I don't see any elegant way to arrange for
> successful finds always to return a true value and unsuccessful
> calls to return a false value.
> 
> The really broken part is that unsuccessful searches return a
> legal index.
> 
We might agree, before further discussion, that this isn't the most 
elegant part of Python's design, and it's down to history that this tiny 
little wart remains.

> My suggestion doesn't change what find() returns, and doesn't
> break code. Negative one is a reasonable choice to represent an
> unsuccessful search -- provided it is not a legal index. Instead
> of changing what find() returns, we should heal the
> special-case-when-index-is-negative-in-a-certain-range wart.
> 
> 
What I don't understand is why you want it to return something that 
isn't a legal index. Before using the result you always have to perform 
a test to discriminate between the found and not found cases. So I don't 
really see why this wart has put such a bug up your ass.

regards
  Steve
-- 
Steve Holden       +44 150 684 7255  +1 800 494 3119
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