Check that brackets are balanced

daves_bracket_challenge.py

friend = {'}':'{', ']':'[', ')':'('}
close = friend.keys()
open = friend.values()

def check(input):
    stack = []
    for c in input:
        if c in open:
            stack.append(c)
        elif c in close:
            if len(stack) > 0 and stack.pop() == friend[c]:
                continue
            else:
                return False
    return len(stack) == 0

assert check('()') == True
assert check('[()]') == True
assert check('[(])') == False
assert check("(a)") == True
assert check("(sdfas") == False
assert check("7(z)))") == False
assert check('{thi(s[is]some)stuff}') == True