How to reverse tuples in Python? [duplicate]

Question

Possible Duplicate:
Traverse a list in reverse order in Python

Is this possible? Doesn't have to be in place, just looking for a way to reverse a tuple so I can iterate on it backwards.

Answer 1

There are two idiomatic ways to do this:

reversed(x)  # returns an iterator 

or

x[::-1]  # returns a new tuple 

Since tuples are immutable, there is no way to reverse a tuple in-place.

---

Edit: Building on @lvc's comment, the iterator returned by reversed would be equivalent to

def myreversed(seq):     for i in range(len(x) - 1, -1, -1):         yield seq[i] 

i.e. it relies on the sequence having a known length to avoid having to actually reverse the tuple.

As to which is more efficient, i'd suspect it'd be the seq[::-1] if you are using all of it and the tuple is small, and reversed when the tuple is large, but performance in python is often surprising so measure it!

Answer 2

You can use the reversed builtin function.

>>> x = (1, 2, 3, 4) >>> x = tuple(reversed(x)) >>> x (4, 3, 2, 1) 

If you just want to iterate over the tuple, you can just use the iterator returned by reversed directly without converting it into a tuple again.

>>> for k in reversed(x): ...     print(k) ...  4 3 2 1