How do I reverse an itertools.chain object?

Question

My function creates a chain of generators:

def bar(num):
    import itertools
    some_sequence = (x*1.5 for x in range(num))
    some_other_sequence = (x*2.6 fo         


        

Answer 1

itertools.chain would need to implement __reversed__() (this would be best) or __len__() and __getitem__()

Since it doesn't, and there's not even a way to access the internal sequences you'll need to expand the entire sequence to be able to reverse it.

reversed(list(CHAIN_INSTANCE))

It would be nice if chain would make __reversed__() available when all the sequences are reversable, but currently it does not do that. Perhaps you can write your own version of chain that does

Answer 2

reversed only works on objects that support len and indexing. You have to first generate all results of a generator before wrapping reversed around them.

However, you could easily do this:

def bar(num):
    import itertools
    some_sequence = (x*1.5 for x in range(num, -1, -1))
    some_other_sequence = (x*2.6 for x in range(num, -1, -1))
    chained = itertools.chain(some_other_sequence, some_sequence)
    return chained

Answer 3

In theory you can't because chained objects may even contain infinite sequences such as itertools.count(...).

You should try to reverse your generators/sequences or use reversed(iterable) for each sequence if applicable and then chain them together last-to-first. Of course this highly depends on your use case.

Answer 4

def reversed2(iter):
    return reversed(list(iter))

Answer 5

Does this work in you real app?

def bar(num):
    import itertools
    some_sequence = (x*1.5 for x in range(num))
    some_other_sequence = (x*2.6 for x in range(num))
    list_of_chains = [some_sequence, some_other_sequence]
    if num < 0:
        list_of_chains.reverse()
    chained = itertools.chain(*list_of_chains)
    return chained

Answer 6

if num < 0:
    lst = list(chained)
    lst.reverse()
    return lst
else:
    return chained

reversed() needs an actual sequence, because it iterates it backwards by index, and that wouldn't work for a generator (which only has the notion of "next" item).

Since you will need to unroll the whole generator anyway for reversing, the most efficient way is to read it to a list and reverse the list in-place with the .reverse() method.