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

Noam Raphael noamraph at gmail.com
Sun Sep 16 21:01:29 CEST 2007

On 9/16/07, Guido van Rossum <guido at python.org> wrote:

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

I think that it's still better, at least if you add some newlines:

[friend (Ok, we are talking about a list of friends. From where do these friends come from?) for friend in city.friends if friend.name != "Guido" (Ah, they are all the friends in a city who aren't called Guido. What about the city?) for city in cities if city.name != "Amsterdam"] (Ah, the city is every city which isn't Amsterdam.)

Versus:

[friend (Ok, we are talking about a list of friends. From where do these friends come from?) for city in cities if city.name != "Amsterdam" (What do cities which aren't Amsterdam have to do with my friend?) for friend in city.friends if friend.name != "Guido"] (Ah, we're talking about all the friends in those cities who aren't called Guido. Let's have a look at the first line to remember what we do with them... ah, yes, we just return them...)

Noam

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