Antoon Pardon wrote: > On 2007-04-11, Terry Reedy <[EMAIL PROTECTED]> wrote: >> "BJrn Lindqvist" <[EMAIL PROTECTED]> wrote in message >> news:[EMAIL PROTECTED] >> On 4/10/07, Steve Holden <[EMAIL PROTECTED]> wrote: >>> One might perversely allow extension to lists and tuples to allow >>> >>> [3, 4] in [1, 2, 3, 4, 5, 6] >>> >>> to succeed, but that's forcing the use case beyond normal limits. >> I'd love to have that! There are at least one million use cases for >> finding a sequence in a sequence and implementing it yourself is >> non-trivial. Plus then both list and tuple's index methods would work >> *exactly* like string's. It would be easier to document and more >> useful. A big win. >> >> ======================= >> It would be ambiguous: [3,4] in [[1,2], [3,4], [5,6]] is True now. >> >> Strings are special in that s[i] can only be a (sub)string of length 1. >> 'b' in 'abc' is True. This makes looking for longer substrings easy. >> >> However, [2] in [1,2,3] is False. IE, list[i] is not normally a list. So >> looking for sublists is different from looking for items. > > Well I think this illustrates nicely what can happen if you design by > use cases. > > Let us assume for a moment that finding out if one list is a sublist of > a second list gets considered something usefull enough to be included > in Python. Now the in operator can't be used for this because it > would create ambiguities. So it would become either a new operator > or a new method. But whatever the solution it would be different > from the string solution. > That's because strings are different from other sequences. See below.
> Now if someone would have thought about how "st1 in st2" would > generalize to other sequemce if st1 contained more than one > character they probably would have found the possible inconsistency > that could create and though about using an other way than using > the in-operator for this with strings. A way that wouldn't create > ambiguities when it was considered to be extended to other sequences. > The fact is that strings are the only sequences composed of subsequences of length 1 - in other words the only sequences where type(s) == type(s[0:1]) is an invariant condition. This was discussed (at my instigation, IIRC) on python-dev when Python (2.4?) adopted the enhanced semantics for "in" on strings - formerly only tests for single characters were allowed - but wasn't thought significant enough to deny what was felt to be a "natural" usage for strings only. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden Recent Ramblings http://holdenweb.blogspot.com -- http://mail.python.org/mailman/listinfo/python-list
