In article <5221352b$0$6599$c3e8da3$54964...@news.astraweb.com>, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
> Heh, everybody has one of two reactions: > > "This is awesome!" [[i.e. what I said]] > > "You'll add type checking to my Python code over my dead body!!!" Duck typing is a funny thing. Sure, I don't have to give you a Duck, I just have to give you something that quacks like a Duck. But, at some point, you and I need to agree on what that means. If you're expecting a https://en.wikipedia.org/wiki/Duck and I give you a https://en.wikipedia.org/wiki/DUKW, we've had a failure to communicate. To take a quasi-realistic example, let's say I've got this musical masterpiece in my database: { "_id" : ObjectId("4ccb7052e5f37551d479add6"), "album" : "My World", "album_comp_id" : NumberLong(34732133), "artist" : "Justin Bieber", "avail_date_aac" : ISODate("1970-01-01T00:00:00Z"), "avail_date_mp3" : ISODate("1970-01-01T00:00:00Z"), "duration" : 192, "genre" : "Pop", "mn_comp_id" : NumberLong(34732147), "seq_num" : 1297, "song_id" : 6544798, "title" : "Love Me", "track" : -1 } If I want to ask you, "Please return to me a url from which I can stream this song as an mp3", I could look at your Song class and find: @staticmethod def get_stream_url(song): [some code goes here] but that would leave me wondering what you mean by "song". You could legitimately mean 6544798, "4ccb7052e5f37551d479add6", ObjectId("4ccb7052e5f37551d479add6"), an instance of the Song class itself, or possibly even 34732147. Depending on the context, any of those might very well be the right answer. And, we haven't even gotten to describing what I should expect to get back. So, to clear things up, you had to go and write something in the doc string: @staticmethod def get_stream_url(song): """Song is an instance of class Song. This will return an absolute URL as a string.""" But, why not just embed that information in some way which is both compact and machine readable? Of course, when I say, "Song is an instance of class Song", what I really (in full duck typing glory) mean is, "Song is something which has a "mn_comp_id" attribute whose value is something which I can pass to str() and get back a properly formatted decimal integer string. So, this would work, wouldn't it? class Platypus: def __getattr__(self, name): return 34732147 duck = Platypus() Song.get_stream_url(duck) Hey, it met the description you gave me, didn't it? And, sure enough, if you do with duck what I expect you will, we will soon hear Justin Bieber's, "Love Me" coming out the speaker. But, in reality, I suspect we would quickly get into an argument about just what exactly did you mean when you said "Duck". -- http://mail.python.org/mailman/listinfo/python-list
