Balanced parenthesis in python

balanced.py

opening = "([{"
closing = ")]}"


def balanced(expr):
    stack = []
    for c in expr:
        if c in opening:
            stack.append(c)
        elif c in closing:
            idx = closing.index(c)
            if stack and stack[-1] == opening[idx]:
                stack.pop()
            else:
                return False
    return not stack