lr_parsing.py

ops = {
    "+": (lambda a, b: a + b),
    "-": (lambda a, b: a - b)
}

def eval(tokens):
    stack = []
    for token in tokens:
        #print token, stack
        if token in ops:
            arg2 = stack.pop()
            arg1 = stack.pop()
            #print '  ', arg2, token, arg1,
            result = ops[token](arg1, arg2)
            stack.append(result)
            #print '->', result
            #print '  ', stack
        else:
            stack.append(int(token))
        #print stack
    return stack.pop()

print eval("7 2 3 + -".split())