Guido van Rossum:
... mostly-late binding -- which is late binding, except that the iterable in the leftmost for-clause is evaluated at once.
Did you really mean leftmost, or all-but-the-rightmost? Special-casing the left-most causes the two-generator version to behave as expected, but it makes the three-generator case even more confusing. Perhaps the entire early/late binding problem comes from trying to treat multiple generators as a special case. For a single-generator expression, late binding makes sense (or why use a free variable), but early binding would also work most of the time. For multiple generators, the current behavior is that the final generator is exhausted by the first loop, so the other generators really only need one value each. The only problem with that is that it isn't usually what people want. If users do want a cross-product, then either the values have to be stored somewhere, or python has to create a whole series of "equivalent" generators. Python can't do this any better than the user, so why not make the storage explicit? (x,y,z for x in e1 for y in e2 for z in e3) <==> for x in e1.__iter__(): for x in e2.__iter__() for z in e3.__iter__() yield (x,y,z) It is the user's responsibility to ensure that e2 and e3 can return an iterator multiple times, perhaps by wrapping them in a list or tuple. -jJ