Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #33782

Re: Inconsistent behaviour os str.find/str.index when providing optional parameters

Show key headers only | View raw

On 11/21/2012 8:32 AM, MRAB wrote:
> On 2012-11-21 12:43, Giacomo Alzetta wrote:
>> I just came across this:

 >>> 'spam'.find('')
0
 >>> 'spam'.find('', 1)
1
 >>> 'spam'.find('', 4)
4

>>>>> 'spam'.find('', 5)
>> -1
>>
>>
>> Now, reading find's documentation:
>>
>>>>> print(str.find.__doc__)
>> S.find(sub [,start [,end]]) -> int
>>
>> Return the lowest index in S where substring sub is found,
>> such that sub is contained within S[start:end].  Optional
>> arguments start and end are interpreted as in slice notation.

This seems not to be true, as 'spam'[4:] == 'spam'[5:] == ''

>> Return -1 on failure.
>>
>> Now, the empty string is a substring of every string so how can find
>> fail?
>> find, from the doc, should be generally be equivalent to
>> S[start:end].find(substring) + start, except if the substring is not
>> found but since the empty string is a substring of the empty string it
>> should never fail.
>>
> [snip]
> I think that returning -1 is correct (as far as returning -1 instead of
> raising an exception like .index could be considered correct!) because
> otherwise it whould be returning a non-existent index. For the string
> "spam", the range is 0..4.

I tend to agree, but perhaps the doc should be changed. In edge cases 
like this, there sometimes is no 'right' answer. I suspect that the 
current behavior is intentional. You might find a discussion on the tracker.

-- 
Terry Jan Reedy

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Inconsistent behaviour os str.find/str.index when providing optional parameters Giacomo Alzetta <giacomo.alzetta@gmail.com> - 2012-11-21 04:43 -0800
  Re: Inconsistent behaviour os str.find/str.index when providing optional parameters MRAB <python@mrabarnett.plus.com> - 2012-11-21 13:32 +0000
  Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Alister <alister.ware@ntlworld.com> - 2012-11-21 16:59 +0000
    Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Hans Mulder <hansmu@xs4all.nl> - 2012-11-21 20:25 +0100
      Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Giacomo Alzetta <giacomo.alzetta@gmail.com> - 2012-11-21 12:21 -0800
      Re: Inconsistent behaviour os str.find/str.index when providing optional parameters MRAB <python@mrabarnett.plus.com> - 2012-11-21 20:58 +0000
  Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Terry Reedy <tjreedy@udel.edu> - 2012-11-21 22:41 -0500
  Re: Inconsistent behaviour os str.find/str.index when providing optional parameters MRAB <python@mrabarnett.plus.com> - 2012-11-22 04:00 +0000
    Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Giacomo Alzetta <giacomo.alzetta@gmail.com> - 2012-11-21 23:01 -0800
      Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-22 08:44 +0000
        Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Giacomo Alzetta <giacomo.alzetta@gmail.com> - 2012-11-22 10:22 -0800
    Re: Inconsistent behaviour os str.find/str.index when providing optional parameters Giacomo Alzetta <giacomo.alzetta@gmail.com> - 2012-11-21 23:01 -0800

csiph-web