python postfix conversion

1"""
2Author : ITVoyagers (itvoyagers.in)
3
4Date :31st October 2019
5
6Description : Program to show use of stack in infix to postfix conversion using python.
7"""
8class infix_to_postfix:
9    precedence={'^':5,'*':4,'/':4,'+':3,'-':3,'(':2,')':1}
10    def __init__(self):
11        self.items=[]
12        self.size=-1
13    def push(self,value):
14        self.items.append(value)
15        self.size+=1
16    def pop(self):
17        if self.isempty():
18            return 0
19        else:
20            self.size-=1
21            return self.items.pop()
22    def isempty(self):
23        if(self.size==-1):
24            return True
25        else:
26            return False
27    def seek(self):
28        if self.isempty():
29            return false
30        else:
31            return self.items[self.size]
32    def isOperand(self,i):
33        if i in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
34            return True
35        else:
36            return False
37    def infixtopostfix (self,expr):
38        postfix=""
39        print('postfix expression after every iteration is:')
40        for i in expr:
41            if(len(expr)%2==0):
42                print("Incorrect infix expr")
43                return False
44            elif(self.isOperand(i)):
45                postfix +=i
46            elif(i in '+-*/^'):
47                while(len(self.items)and self.precedence[i]<=self.precedence[self.seek()]):
48                    postfix+=self.pop()
49                self.push(i)
50            elif i is '(':
51                self.push(i)
52            elif i is ')':
53                o=self.pop()
54                while o!='(':
55                    postfix +=o
56                    o=self.pop()
57            print(postfix)
58                #end of for
59        while len(self.items):
60            if(self.seek()=='('):
61                self.pop()
62            else:
63                postfix+=self.pop()
64        return postfix
65s=infix_to_postfix()
66expr=input('enter the expression ')
67result=s.infixtopostfix(expr)
68if (result!=False):
69    print("the postfix expr of :",expr,"is",result)
70