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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

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