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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)?
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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.
In Python, there is a built-in function called reverse() that is used to reverse the list. This is a simple and quick way to reverse a list that requires little memory. Syntax- list_name. reverse() Here, list_name means you have to write the name of the list which has to be reversed.
Python reversed() The reversed() method computes the reverse of a given sequence object and returns it in the form of a list.
Another way to reverse python list without the use of any build-in methods is using loops. Create an empty list to copy the reversed elements. In the for loop, add the iterator as a list element at the beginning with the new list elements. So in that way, the list elements will be reversed.
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.
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