parens.py

strings = [
    '(()',
    ')()())',
    '()())))((())((()())',
    '()())))(())()((()()))(((()(())(()())'
]

def longestParen(string):
    
    max_start  = 0
    max_length = 0
    
    for i in range(len(string)):
        
        count = 0
        for j, char in enumerate(string[i:]):
            count += (1 if char == '(' else -1)
            
            if count < 0:
                break
            
            if not count:
                if j > max_length:
                    max_start, max_length = i, j + 1
                    
    return string[max_start:max_start+max_length]

for string in strings:
    print string, '->', longestParen(string)