Generator question

[Pierre Reinbold]

Le 14/03/19 à 10:45, Peter Otten a écrit :
> Pierre Reinbold wrote:
> 
>> Wow, thank you Ian for this very detailed answer, and thank you for taking
>> the time for that! Much appreciated!
>>
>> If I get this right, I have to somehow fix the value of a_list during the
>> loop, like when you use the classic default value argument trick with
>> lambdas (damn, I should have thought of that!). So if I "unfold" the
>> generator expression, using default values for both iterables, I get this
>> :
>>
>> def flat_gen_cat_prod(lists):
>>     solutions = [[]]
>>     for a_list in lists:
>>         def new_solutions(l=a_list, s=solutions):
>>             for part_sol in s:
>>                 for el in l:
>>                     yield part_sol+[el]
>>         solutions = new_solutions()
>>     return solutions
>>
>> With this I get the right behavior! Thanks again!
>>
>> Doest that mean that there is no possibility to use a generator expression
>> in this case ? (gen. exp. are sooo much more elegant :-))
> 
> The obvious approach is to put the genexpr into a function:
> 
> def prod(lists):
>     solutions = [[]]
>     def new_solutions(a_list):
>         return (part_sol + [el] for part_sol in solutions for el in a_list)
> 
>     for a_list in lists:
>         solutions = new_solutions(a_list)
>     return solutions
> 
> If you want to avoid the function you can take advantage of the fact that 
> the outermost iterable is bound early:
> 
> def prod(lists):
>     solutions = [[]]
> 
>     for a_list in lists:
>         solutions = (
>             part_sol + [el]
>             for solutions, a_list in [(solutions, a_list)]
>             for part_sol in solutions for el in a_list
>         )
>     return solutions
> 
> 

Thank you Peter, very clever suggestions !


πr