Calculate stack value's

[GFreenD]

would this then mean that i take poped element then try to add to with a symbol who is for example + and add it with the other poped element and get result ? and where exactly should i write this command ?
meanwhile i was trying to upgrade my own:

class stack():
    def __init__(self):
        self.items = []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def get_stack(self):
        return self.items

    def is_empty(self):
        return self.items == []

    def is_full(self):
        return self.items == [1]

    def size(self):
        return len(self.items)



    def eval(self):
        s = stack()
        for symbol in self:
            if symbol in "0123456789":
                s.append(int(symbol))

            plus = None
            elif not s.is_empty():
            if symbol == "+":
                plus = s.push() + s.push()
            elif symbol == "-":
                plus = s.push() - s.push()
            elif symbol == "*":
                plus = s.push() * s.push()
            elif symbol == "/":
                plus = s.push() / s.push()

        if plus is not None:
            s.append(plus)
        else:
            raise Exception("unknown value %s" % symbol)

    return s.pop()


s = stack()
print("Is stack empty?")
print(s.is_empty())
print("Is stack Full?")
print(s.is_full())
s.push(9)
s.push('+')
s.push(5)
s.push('+')
s.push(8)
s.push('+')
s.push(7)

s.push('+')
s.push(3)
print(s.size())
print(s.get_stack())
print(s.eval())

s.pop()
s.pop()
print(s.get_stack())

but ran into a problem where it does not recognize this comand and i can seem to figure out why:

Error:
    
elif not s.is_empty():
       ^
SyntaxError: invalid syntax