Valid Parentheses

{ { } [ ] [ [ [ ] ] ] } is VALID expression
          [ [ [ ] ] ]    is VALID sub-expression
  { } [ ]                is VALID sub-expression

import sys

def error(c, line_number, column_number):
    print 'Error: unmatched', c, 'line', line_number, 'column', column_number

def check(stack, wanted, c, line_number, column_number):
    if stack[-1] != wanted:
        error(c, line_number, column_number)
    else:
        stack.pop()

def check_parentheses(f):
    stack = list()
    line_number = 0
    for line in f:
        line_number = line_number + 1
        column_number = 0
        for c in line:
            column_number = column_number + 1
            if c == '(' or c == '[' or c == '{':
                stack.append(c)
            elif c == ')':
                check(stack, '(', ')', line_number, column_number)
            elif c == ']':
                check(stack, '[', ']', line_number, column_number)
            elif c == '}':
                check(stack, '{', '}', line_number, column_number)

def main():
    filename = sys.argv[1]
    try:
        f = file(filename)
    except IOError:
        sys.stderr.write('Error: Cannot open file %s' % filename)
        sys.exit(1)
    check_parentheses(f)
    f.close()

main()