how to obtain the content of brackets

1"""
2This will try to split the argument into 2, using the square brackets, then
3will split the index one, so, the variable foo just is the content of
4any group of characters. like ()[]{}
5"""
6try:
7  foo = arg.split("[")[1].split("]")[0]
8except:
9# The argument doesn't contain a class.
10    pass