I think it's not so obvious that reversing the order is any better when you throw in some if clauses:
[friend for city in cities if city.name != "Amsterdam" for friend in city.friends if friend.name != "Guido"] vs. [friend for friend in city.friends if friend.name != "Guido" for city in cities if city.name != "Amsterdam"] --Guido On 9/16/07, Noam Raphael <[EMAIL PROTECTED]> wrote: > Hello, > > I had a thought about syntax I want to share with you. > > Say you want to get a list of all the phone numbers of your friends. > You'll write something like this: > telephones = [friend.telephone for friend in friends] > > Now suppose that, unfortunately, you have many friends, and they are > grouped by city. Now, you'll probably write: > telephones = [friend.telephone for friend in city.friends for city in cities] > > and you'll (hopefully) get an exception, and change your line to: > telephones = [friend.telephone for city in cities for friend in city.friends] > > and say, "Ah, I should've remembered this from the last time it > happened to me", and forget it until the next time it happens to you. > > The reason is that the code: > for city in cities: > for friend in city.friends: > yield friend.telephone > > makes sense if you read it from the first line to the last line, and > makes sense if you read it from the last line to the first line, but > doesn't make a lot of sense if you start from the last line and then > jump to the first line and read it from there. In other words, you can > go from the general to the specific, and you can go from the specific > to the general, but jumping from the most specific to the most general > and back again up to the second-most specific is strange. > > All this is to say that I think that the "for" parts in list > comprehensions and generator expressions should, in a perfect world, > be evaluated in the other way round. > > The question remains, what should be done with the "if" parts. A > possible solution is this: only one "if" part will be allowed after > each "for" part (you don't need more than that, since you can always > use the "and" operator). So, if I want to limit the list, my line will > look like this: > > telephones = [ > friend.telephone > for friend in city.friends if friend.is_really_good > for city in cities if city.is_close_to_me > ] > > What do you think? > Noam > > (P.S. Please don't be annoyed at me. The answer "this will break too > much code and isn't worth it" is, of course, very sensible. I just > thought that such thoughts can be posted to this list without causing > too much harm.) > _______________________________________________ > Python-3000 mailing list > Python-3000@python.org > http://mail.python.org/mailman/listinfo/python-3000 > Unsubscribe: > http://mail.python.org/mailman/options/python-3000/guido%40python.org > -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-3000 mailing list Python-3000@python.org http://mail.python.org/mailman/listinfo/python-3000 Unsubscribe: http://mail.python.org/mailman/options/python-3000/archive%40mail-archive.com
