Group Checks

###THIS WORKS--FINALLY!!!
def group_check(s):
    stack = []
    iparens = iter('(){}[]<>')
    parens = dict(zip(iparens, iparens))
    closing = parens.values()
    for char in s:
        d = parens.get(char, None)
        if d:  # opening char
            stack.append(d)
        elif char in closing:  # closing char
            if not stack or char != stack.pop():
                return False
    return not stack