How to overload the add operator to return the length of two string when they are added?

Question

I am trying to create a class diffStr that behaves like str except, when a user tries to add or multiply two strings, the output is the length of each string added or multiplied (respectively).

My code -

class diffStr:

  def __init__(self, str1):
    self.str1 = str1

  def __add__(self, other):
   return len(self.str1) + len(other.str1)
  
  def __mul__(self, other):
    return len(self.str1) * len(other.str1)

x = diffStr('hi')
print(x+'hello') # supposed to print 7 (2+5)
print(x*'hello') # supposed to print 10 (2*5)

I am sure I am doing something wrong because I keep getting the following error message when I try to run the code -

AttributeError: 'str' object has no attribute 'str1'

As you can probably tell, I am a noob at programming and trying my best to learn. I would appreciate any help in fixing this block of code. Thank you.

Answer 1

With x+'hello', you're passing a str object of 'hello' as the other argument to x.__add__, which evaluates other.str1, and since a str object has no str1 attribute, you get the said exception.

A better-rounded approach that helps make the + and * operators work for both str and diffStr operands would be to implement the __len__ method for diffStr, so that you can simply call the len function on the other argument regardless of its type:

class diffStr:
    def __init__(self, str1):
        self.str1 = str1

    def __add__(self, other):
        return len(self.str1) + len(other)

    def __mul__(self, other):
        return len(self.str1) * len(other)

    def __len__(self):
        return len(self.str1)

x = diffStr('hi')
print(x + 'hello')
print(x * 'hello')
print(x + diffStr('hello'))
print(x * diffStr('hello'))

This outputs:

7
10
7
10