On Sun, Dec 18, 2022 at 8:29 PM Steven D'Aprano <st...@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.
>
I'd agree to "limited", but not "hostile." Look at the suggestions I
mentioned: validate, canoncialize, security check. All of those are
perfectly fine in `.__new__()`. E.g.:
In [1]: class html(str):
...: def __new__(cls, s):
...: if not "<" in s:
...: raise ValueError("That doesn't look like HTML")
...: return str.__new__(cls, s)
In [2]: html("<h1>Hello</h1>")
In [3]: html("Hello")
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-3-71d16160c9ad> in <module>
----> 1 html("Hello")
<ipython-input-1-e9d5da1202f3> in __new__(cls, s)
2 def __new__(cls, s):
3 if not "<" in s:
----> 4 raise ValueError("That doesn't look like HTML")
5
ValueError: That doesn't look like HTML
I readily acknowledge that's not a very thorough validator :-).
But this much (say with a better validator) gets you static type checking,
syntax highlighting, and inherent documentation of intent.
I know that lots of things one can do with a str subclass wind up producing
a str instead. But if the thing you do is just "make sure it is created as
the right kind of thing for static checking and editor assistance, I don't
care about any of that falling back.
--
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons. Intellectual property is
to the 21st century what the slave trade was to the 16th.
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