read a list backwards in python
August 4th, 2006 by Lawrence David
to iterate over a list in reverse order in python, i use the ‘reverse’ method. i’m sure there are faster ways to do this in terms of running time, but its hard to conceive of anything easier to implement.
here goes: to iterate over ‘my_list’ backwards, write:
my_list.reverse()
for item in my_list:
print item
for i in reversed(my_list):
print i
(new builtin from python 2.4)
Just to show there’s more than one way to do it:
for i in list[::-1]:
print i
correct me if I am wrong, but reversed is the way to go if memory is an issue as the other two solutions first create a copy of the list while rereversed(list) simply returns a reverse iterator?
If you need the index or the sequence is a string you can always:
for i in range(len(my_list) – 1, -1, -1):
print my_list[i]
I would like to emphasize that Zach Dwiel’s solution is side effect free, which he also states.
In the original solution, if you wanted to print out the list and then process the list again with the original order, then you would need:
list.reverse()
# print list in reverse order
for i in list:
print i
list.reverse()
# print list in original order
for i in list:
print i
where as Zach’s solution allows for the more optimal
for i in reversed(list):
print i
for i in list:
print i