How to find the last occurrence of an item in a Python list

Question

Say I have this list:

li = ["a", "b", "a", "c", "x", "d", "a", "6"] 

As far as help showed me, there is not a builtin function that returns the last occurrence of a string (like the reverse of index). So basically, how can I find the last occurrence of "a" in the given list?

Answer 1

If you are actually using just single letters like shown in your example, then str.rindex would work handily. This raises a ValueError if there is no such item, the same error class as list.index would raise. Demo:

>>> li = ["a", "b", "a", "c", "x", "d", "a", "6"] >>> ''.join(li).rindex('a') 6 

For the more general case you could use list.index on the reversed list:

>>> len(li) - 1 - li[::-1].index('a') 6 

The slicing here creates a copy of the entire list. That's fine for short lists, but for the case where li is very large, efficiency can be better with a lazy approach:

def list_rindex(li, x):     for i in reversed(range(len(li))):         if li[i] == x:             return i     raise ValueError("{} is not in list".format(x)) 

One-liner version:

next(i for i in reversed(range(len(li))) if li[i] == 'a')