class Stack:
    def __init__(self):
        self.TopP=0
        self.S = list()
    def Push(self, x):
        self.TopP=self.TopP+1
        self.S.append(x)
    def Pop(self):
        if TopP>0:
            c=self.S[TopP]
            self.TopP=self.TopP-1
        else:
            c=int(c)
            c=0
        return c
    def Top():
        return self.S[TopP]
    def IsEmpty():
        return self.TopP==0

St=Stack()
St.Push(x[i])

Traceback (most recent call last):
  File "C:\PythonTProg\Stack.py", line 71, in <module>
    c=Polish(X)
  File "C:\PythonTProg\Stack.py", line 42, in Polish
    St.Push(X[i])
TypeError: unbound method Push() must be called with Stack instance as first argument (got str instance instead)

# coding: utf8

# ...

s = 'hello'
st = Stack()
st.Push(s[1])

print st.S
# ['e']

def Polish(X):
    St=Stack
    for i in range(0,len(X)):
        if (X[i]>='0')or(X[i]<='9'):
            St.Push(X[i])
        elif X[i]=='+':
            a=St.Pop()
            b=St.Pop()
            St.Push(int(a)+int(b))
        elif X[i]=='-':
            a=St.Pop()
            b=St.Pop()
            St.Push(int(a)-int(b))
        elif X[i]=='*':
            a=St.Pop()
            b=St.Pop()
            St.Push(int(a)*int(b))
return St.Top

St=Stack

Stack()

>>> a = []
>>> a.append(1)
>>> a.append(2)
>>> a.pop()
2
>>> a.pop()
1