how to control list index with button in python code example

Example: how to index python

#you can index in a string like:
string='cupcake'
string[0]#returns 'c'
#you can index in a list like:
list = ['red', 'blue', 'green']
list[0]#returns 'red'
list[0][0]#returns[0] of 'red' wich is r