Adventure in Overriding Python Str

Recently I have given a project where we are overriding the python str object, I rather not say the reason because it is job related. But the process of it is pretty general, and is pretty interesting.

So here is an example.

A few thing to take note here.

  1. Notice that there is a __new__ method? In python object lifecycle, the first thing to be called is actually __new__ not __init__. __new__ will choose whether whether to replace __init__ or use a new one. In this case, I set the current object cls to str, with value bleeped
  2. Now if object is not setup in __new__ yet, how do object access the bleep method? Introducing, classmethod. Classmethod is a method the bind to a class, not an object. This is useful if you want to do call a function before a class is initiated.
  3. Classmethod binds to the child class, not the parent class it inherited from, and you need the first parameter to be a class. It can be set with a @classmethod on a method
  4. Now because we also want the string to be appended to BleepedString object, to also be bleeped, so we need to override __add__, so when string appended via + operator, will also be bleeped. Interestingly __iadd__ is not inherited, so I need to add __iadd__ method too, so that I can do +=
  5. Notice that inside __add__ I use bleep method like a normal method. That is the reason is set a @classmethod decorator in bleep method, I need the method to be available without need to be instantiated, while able to use it like a normal method in a object.
  6. Actually just override __add__ and __iadd__ is not enough to override the str behavior. Because the parent class str uses __repr__ to provide value to be used in a + and += operation, which is why I override __repr__ method, by replacing the return value to bleeped string.
  7. Then we need to override __str__ so that it return a bleeped string.

This is just a modified example what I used in my work, and have a very different and useful function. But it capture the same idea, overriding the default string behavior and return value.

Cheers!

  • The program wouldn’t run properly until I changed the definition of bleep() to:

    def bleep(cls, data):

    Thanks for the example, though! It helped me to override the str class.

     

  • Also, the apostrophe (‘) is showing up on this page as &039; Each &039; should be replaced with just one ‘.