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

[Steve Holden]

Antoon Pardon wrote:
> Op 2005-08-29, Steve Holden schreef <steve at holdenweb.com>:
> 
>>Antoon Pardon wrote:
>>
>>>Op 2005-08-27, Steve Holden schreef <steve at holdenweb.com>:
>>>
>>>
>>>>If you want an exception from your code when 'w' isn't in the string you 
>>>>should consider using index() rather than find.
>>>
>>>
>>>Sometimes it is convenient to have the exception thrown at a later
>>>time.
>>>
>>>
>>>
>>>>Otherwise, whatever find() returns you will have to have an "if" in 
>>>>there to handle the not-found case.
>>>
>>>
>>>And maybe the more convenient place for this "if" is in a whole different
>>>part of your program, a part where using -1 as an invalid index isn't
>>>at all obvious.
>>>
>>>
>>>
>>>>This just sounds like whining to me. If you want to catch errors, use a 
>>>>function that will raise an exception rather than relying on the 
>>>>invalidity of the result.
>>>
>>>
>>>You always seem to look at such things in a very narrow scope. You never
>>>seem to consider that various parts of a program have to work together.
>>>
>>
>>Or perhaps it's just that I try not to mix parts inappropriately.
> 
> 
> I didn't know it was inappropriately to mix certain parts. Can you
> give a list of modules in the standard list I shouldn't mix.
> 
> 
>>>So what happens if you have a module that is collecting string-index
>>>pair, colleted from various other parts. In one part you
>>>want to select the last letter, so you pythonically choose -1 as
>>>index. In an other part you get a result of find and are happy
>>>with -1 as an indictation for an invalid index. Then these
>>>data meet.
>>>
>>
>>That's when debugging has to start. Mixing data of such types is 
>>somewhat inadvisable, don't you agree?
> 
> 
> The type of both data is the same, it is a string-index pair in
> both cases. The problem is that a module from the standard lib
> uses a certain value to indicate an illegal index, that has
> a very legal value in python in general.
> 
Since you are clearly feeling pedantic enough to beat this one to death 
with a 2 x 4 please let me substitute "usages" for "types".

In the case of a find() result -1 *isn't* a string index, it's a failure 
flag. Which is precisely why it should be filtered out of any set of 
indexes. once it's been inserted it can no longer be distinguished as a 
failure indication.

> 
>>I suppose I can't deny that people do things like that, myself included, 
> 
> 
> It is not about what people do. If this was about someone implementing
> find himself and using -1 as an illegal index, I would certainly agree
> that it was inadvisable to do so. Yet when this is what python with
> its libary offers the programmer, you seem reluctant find fault with
> it.
> 
I've already admitted that the choice of -1 as a return value wasn't 
smart. However you appear to be saying that it's sensible to mix return 
values from find() with general-case index values. I'm saying that you 
should do so only with caution. The fact that the naiive user will often 
not have the wisdom to apply such caution is what makes a change desirable.

> 
>>but mixing data sets where -1 is variously an error flag and a valid 
>>index is only going to lead to trouble when the combined data is used.
> 
> 
> Yet this is what python does. Using -1 variously as an error flag and
> a valid index and when  people complain about that, you say it sounds like
> whining.
> 
What I am trying to say is that this doesn't make sense: if you want to 
combine find() results with general-case indexes (i.e. both positive and 
negative index values) it behooves you to strip out the -1's before you 
do so. Any other behaviour is asking for trouble.

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