Split a list into two parts based on a filter?

[Terry Reedy]

On 6/12/2013 7:39 AM, Roy Smith wrote:

> starts.  But, somewhat more seriously, I wonder what, exactly, it is
> that freaks people out about:
>
>>>>> [(new_songs if s.is_new() else old_songs).append(s) for s in songs]
>
> Clearly, it's not the fact that it build and immediately discards a
> list, because that concern is addressed with the generator hack, and I
> think everybody (myself included) agrees that's just horrible.

It is an example of comprehension abuse. Comprehensions express and 
condense a stylized pattern of creating collections from another 
collection or collections, possibly filtered. They were not mean to 
replace for statements and turn Python into an fp languages. Indeed, 
they do replace and expand upon the fp map function. Python for loops 
are not evil.

> Or, is it the use of the conditional to create the target for append()?
> Would people be as horrified if I wrote:
>
> for s in songs:
>      (new_songs if s.is_new() else old_songs).append(s)


No. That succinctly expresses and implements the idea 'append each song 
to one of two lists.

> or even:
>
> for s in songs:
>      the_right_list = new_songs if s.is_new() else old_songs
>      the_right_list.append(s)
>


-- 
Terry Jan Reedy