Antoon Pardon wrote: > On 2007-04-12, Carsten Haese <[EMAIL PROTECTED]> wrote: >> On Thu, 2007-04-12 at 14:10 +0000, Antoon Pardon wrote: >>> People are always defending duck-typing in this news group and now python >>> has chosen to choose the option that makes duck-typing more difficult. >> Au contraire! The "inconsistent" behavior of "in" is precisely what >> duck-typing is all about: Making the operator behave in a way that makes >> sense in its context. > > No it isn't. Ducktyping is about similar objects using a similar > interface to invoke similar behaviour and getting similar result. > > So that if you write a function you don't concern yourself with > the type of the arguments but depend on the similar behaviour. > Please note that "similar" does not mean "exact".
The behavior of str.__contains__ and list.__contains__ is similar. Duck-typing allows natural access to polymorphism. You appear to be making semantic distinctions merely for the sake of continuing this rather fatuous thread. > Suppose someone writes a function that acts on a sequence. > The algorithm used depending on the following invariant. > > i = s.index(e) => s[i] = e > > Then this algorithm is no longer guaranteed to work with strings. > Because strings have different properties than other sequences. I can't help pointing out that your invariant is invalid for tuples also, because tuples don't have a .index() method. > > On the other hand I subclass list and add a sub method > to check for the argument being a sublist of the object. > Now I write a function that depends on this functionality. > But although strings have the functionality I can't use > them as argument because the functionality is invoked > in a different way. > >> Nobody seems to be complaining about "+" behaving >> "inconsistently" depending on whether you're adding numbers or >> sequences. > > You are wrong. I already mentioned problems with it. The > problem is that there are structures that are numbers and > sequences at the same time. So I have a choice. Either I > overload the "+" to get an addition or to get a concatanation. > > In the first case I can't trust my structure to work with > functions that expect a general sequence because they > may depend on the fact that "+" concatenates. In the > other case I can't trust my structure to work with > numbers because they may depend on the fact that "+" > behaves like an addition. > Good grief. -- 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
