[Python-3000] The order of list comprehensions and generator expressions

[Guido van Rossum]

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 <noamraph at gmail.com> 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.)
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)