Python extension methods

You can just modify the class directly, sometimes known as monkey patching.

def MyMethod(self):
      return self + self

MyClass.MyMethod = MyMethod
del(MyMethod)#clean up namespace

I’m not 100% sure you can do this on a special class like str, but it’s fine for your user-defined classes.

Update

You confirm in a comment my suspicion that this is not possible for a builtin like str. In which case I believe there is no analogue to C# extension methods for such classes.

Finally, the convenience of these methods, in both C# and Python, comes with an associated risk. Using these techniques can make code more complex to understand and maintain.