string Index for the last position

[Clarendon]

I am trying to get the index for the last occurrance of a sub string.

S = 'dab dab dab'
print S.find('ab')
1

This gives me the index for the first position of 'ab'.
But I need to index the last position of 'ab' here.
Is there a quick option for find that will do this, or do I have to
write a long code to get what I want?

Thank you very much.