def infix_to_postfix(self):
    """
    function to generate postfix expression from infix expression
    """
    postfix = []
    for i in range(len(self.expression)):
        if self._isOperand(self.expression[i]):
            postfix.append(self.expression[i])
        elif self.expression[i] == '(':
            self.my_stack.push(self.expression[i])
        elif self.expression[i] == ')':
            top_operator = self.my_stack.pop()
            while not self.my_stack.is_empty() and top_operator != '(':
                postfix.append(top_operator)
                top_operator = self.my_stack.pop()
        else:
            while not self.my_stack.is_empty() and self._precedence(self.expression[i]) <= self._precedence(self.my_stack.peek()):
                postfix.append(self.my_stack.pop())
            self.my_stack.push(self.expression[i])

    while not self.my_stack.is_empty():
        postfix.append(self.my_stack.pop())
    return ' '.join(postfix)