curious about slice behaviour

[Stephan Diehl]

On Mon, 05 Sep 2005 14:26:14 -0400, Terry Reedy wrote:

> 
> "Stephan Diehl" <stephan.diehl at gmx.net> wrote in message 
> news:pan.2005.09.05.17.06.41.434180 at gmx.net...
>>I just found out by accident, that slice indices can be larger than
>> the length of the object. For example
>>>>> 'test'[:50]
>> 'test'
[...]
>> Does anybody know, why this is preferred to just raising an error?
> 
> Slicing was intentially designed to always give an answer (given int 
> coords) and never say 'can't answer' (whether by exception or a None 
> return).  This avoids having to call len() when you don't care and avoids 
> having to use try:...except:... or conditionalize the code when it is not 
> needed.  For  instance c=s[0:1] is equivalent to
> 
> c=s[0:min(1,len(s))]  # if slice had to be exact, or
> 
> c = s and s[0] or '' # or
> 
> if s:
>   c = s[0]
> else:
>   c = ''  # or
> 
> try:
>   c = s[0]
> except IndexError:
>   c = ''
> 
> People occasionally post buggy code which simply needs s[0] changed to 
> s[0:1].
> 
> The form s[i:], which I am sure you agree is useful, is effectively 
> equivalent to eithers[i:len(s)] or s[i:<maxint>].   The latter view 
> generalizes to iterables without a knowable length.

I do think that this is useful and can save some lines of code.
Just never expected this.

> 
> Terry J. Reedy