[Python-Dev] Re: string.find() again (was Re: timsort for jython)

[Raymond Hettinger]

GvR:
> > I think we've argued about '' in 'abc' long enough.  Tim has failed to
> > convince me, so '' in 'abc' returns True.  Barry has checked it all
> > in.

Ka-Ping:
> My personal opinion sides with Tim -- i think an exception is definitely
> the right choice.  (I still haven't seen convincing examples where True
> is a more useful result than an exception, and the fact that there is
> doubt suggests that it is an exceptional case.)

I think Barry and GvR are on the right track.

My gut feeling is that it is best to stay with the mathematical view that
the null set is a subset of every other set.  It doesn't seem to have hurt the
world of regular expressions where re.match('', 'abc') returns a match
object.  Likewise, the truth of "abc" ~ "" is not on the wart list for AWK.  
Excel and Lotus have both return non-zero for FIND("","abc").

Though errors should not pass silently, we are talking about an error
that is possibly very far upstream from the membership check:

   potentialsub = complicatedfunction(*manyvars) #semantic error here
   <much other computation here ...>
   if potentialsub in astring:  # why raise an exception way down here
      handle_inclusion()
   else:
      handle_exclusion()

'in' should not be responsible for suggesting that complicatedfunction()
doesn't know what it is doing.  If there is an error, it isn't the membership
check; rather, it is a semantic problem with the function.  Accordingly, 
the postcondition for the function belongs at the tail of the function and 
not as a precondition for the use of the result.  Otherwise, the exception 
and its cause are too far apart (as in the example above).


Raymond Hettinger