How to trim a string in python to a desired character from the end

Given a string:

a = 'First - second - third'

How can I trim the string from the end to ' -' to get:

'First - second'

Previously, when the string contained only one ' - ', I did it via .split(' - ') and removed the last element at index [1], but as soon as strings with two ' - ' appeared, it stopped working correctly. Or maybe I can also use .split(' - '), but remove not specifically [1], but simply the last element in the resulting list. Does anyone know how to do this? Please advise.