Interview question with irritating (but super simple once the pressure was off) edge case

brace_sanity.py

def brace_sanity_check(test_string):
    """Return True if braces brackets and parens are sane

    """
    open_paren = 0
    open_bracket = 0
    open_brace = 0

    for s in test_string:
        if s == "(":
            required_for_close = "paren"
            open_paren += 1
        elif s == ")":
            open_paren -= 1
            if open_paren < 0 or required_for_close != "paren":
                return False
        elif s == "{":
            required_for_close = "brace"
            open_brace += 1
        elif s == "}":
            open_brace -= 1
            if open_brace < 0 or required_for_close != "brace":
                return False
        elif s == "[":
            required_for_close = "bracket"
            open_bracket += 1
        elif s == "]":
            open_bracket -= 1
            if open_bracket < 0 or required_for_close != "bracket":
                return False

    if not any([open_brace, open_bracket, open_paren]):
        return True
    else:
        return False