Data Structures and Searching UnitTests Failing about pygorithm HOT 9 CLOSED

  
# OLD VERSION
return ord(char) >= ord('a') and ord(char) <= ord('z') or ord(char) >= ord('A') and ord(char) <= ord('Z')

# NEW VERISON
return True if ord(char) in [ord(c) for c in list(ascii_letters)] else False

Error
Traceback (most recent call last):
  File "C:\Users\doarni\AppData\Local\Continuum\Anaconda3\lib\unittest\case.py", line 58, in testPartExecutor
    yield
  File "C:\Users\doarni\AppData\Local\Continuum\Anaconda3\lib\unittest\case.py", line 600, in run
    testMethod()
  File "C:\Users\doarni\Dev\pygorithm\tests\test_data_structure.py", line 39, in test_infix_to_postfix
    resultString = result.infix_to_postfix()
  File "C:\Users\doarni\Dev\pygorithm\pygorithm\data_structures\stack.py", line 131, in infix_to_postfix
    and self.__precedence(self.expression[i] <= self.__precedence(self.my_stack.peek())):
TypeError: unorderable types: str() <= int()

    def infix_to_postfix(self):
        """
        function to generate postfix expression from infix expression
        """
        postfix = []
        for i in range(len(self.expression)):
            if self.__is_operand(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)