On 19/12/2022 03:23, David Mertz, Ph.D. wrote:

On Sun, Dec 18, 2022 at 8:29 PM Steven D'Aprano <steve@pearwood.info> wrote:

> However, if you want to allow these types to possibly *do* something with > the strings inside (validate them, canonicalize them, do a security check, > etc), I think I like the other way: > class html(str): pass > class css(str): pass

The problem with this is that the builtins are positively hostile to subclassing. The issue is demonstrated with this toy example:

class mystr(str):     def method(self):         return 1234

s = mystr("hello") print(s.method())  # This is fine. print(s.upper().method())  # This is not.

Yes, you have to do some more work with the methods you need to use: class mystr(str):     def method(self):         return 1234     def upper(self):         return mystr(str(self).upper()) s = mystr("hello") print(s.method())          # prints 1234 print(s.upper())           # prints HELLO print(s.upper().method())  # prints 1234 Best wishes Rob Cliffe