问题
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)?
回答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.
回答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.
回答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).
来源：https://stackoverflow.com/questions/1561214/python-reverse-generator