Python Reverse Generator

Question

I'm looking for a way to reverse a generator object. I know how to reverse sequences:

foo = imap(seq.__getitem__, xrange(len(seq)-1, -1, -1))

But is something similar possible with a generator as the input and a reversed generator as the output (len(seq) stays the same, so the value from the original sequence can be used)?

Solution 1

You cannot reverse a generator in any generic way except by casting it to a sequence and creating an iterator from that. Later terms of a generator cannot necessarily be known until the earlier ones have been calculated.

Even worse, you can't know if your generator will ever hit a StopIteration exception until you hit it, so there's no way to know what there will even be a first term in your sequence.

The best you could do would be to write a reversed_iterator function:

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

EDIT: You could also, of course, replace reversed in this with your imap based iterative version, to save one list creation.

Solution 2

reversed(list(input_generator)) is probably the easiest way.

There's no way to get a generator's values in "reverse" order without gathering all of them into a sequence first, because generating the second item could very well rely on the first having been generated.

Solution 3

You have to walk through the generator anyway to get the first item so you might as well make a list. Try

reversed(list(g))

where g is a generator.

reversed(tuple(g))

would work as well (I didn't check to see if there is a significant difference in performance).