infix evaluation

infix.py

# infix calculation
# infix -> postfix -> eval

NUMS = map(lambda x: str(x), range(10))

def toponlist(l):
    return l[len(l)-1]

def dop(op, arg1, arg2):
    if op == '-':
        t = int(arg2) - int(arg1)
        return str(t)
    if op == '+':
        t = int(arg1) + int(arg2)
        return str(t)

def infix2postfix(expr):
    prec = { 
    "+": 2,
    "-": 2,
    "(": 1,
    }
    plist = []
    stack = []
    
    for t in list(expr):
        if t in NUMS:
            plist.append(t)
        elif t == "(":
            stack.append(t)
        elif t == ")":
            top = stack.pop()
            while top != "(":
                plist.append(top)
                top = stack.pop()
        else:
            while ((len(stack) != 0) and (prec[toponlist(stack)] >= prec[t])):
                plist.append(stack.pop())
            stack.append(t)

    while (len(stack) != 0):
        plist.append(stack.pop())
    return plist

def evalexp(expr):
    infix = infix2postfix(expr)
    print infix
    stack = []

    for t in infix:
        if t in NUMS:
            stack.append(t)
        else:
            op = t
            arg1 = stack.pop()
            arg2 = stack.pop()
            stack.append(dop(op, arg1,arg2))

    return stack.pop()

print evalexp("1+3-(2+9)")
print evalexp("(2-(6+1+3)-5)+(8+7)")