bracket_checker.py

openers_to_closers_map = {
    '(' : ')',
    '{' : '}',
    '[' : ']'
}
openers = openers_to_closers_map.keys()
closers = openers_to_closers_map.values()

def is_valid(code):
    openers_stack = []
    for char in code:

        if char in openers:
            openers_stack.append(char)

        elif char in closers:
            if not openers_stack:
                return False
            else:
                last_unclosed_opener = openers_stack.pop()
                # if this closer doesn't correspond to the most recently
                # seen unclosed opener, short-circuit, returning false
                if not openers_to_closers_map[last_unclosed_opener] == char:
                    return False

    return openers_stack == []