[Python-3000] The order of list comprehensions and generator expressions (original) (raw)

Guido van Rossum guido at python.org
Sun Sep 16 20:42:42 CEST 2007

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.isreallygood for city in cities if city.isclosetome ] 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/)

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