On Sun, Dec 18, 2022 at 10:23:18PM -0500, David Mertz, Ph.D. wrote:
> 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__()`.
No, they aren't perfectly fine, because as soon as you apply any
operation to your string subclass, you get back a plain vanilla string
which bypasses your custom `__new__` and so does not perform the
validation or security check.
> But this much (say with a better validator) gets you static type checking,
> syntax highlighting, and inherent documentation of intent.
Any half-way decent static type-checker will immediately fail as soon as
you call a method on this html string, because it will know that the
method returns a vanilla string, not a html string. And that's exactly
what mypy does:
[steve ~]$ cat static_check_test.py
class html(str):
pass
def func(s:html) -> None:
pass
func(html('').lower())
[steve ~]$ mypy static_check_test.py
static_check_test.py:7: error: Argument 1 to "func" has incompatible
type "str"; expected "html"
Found 1 error in 1 file (checked 1 source file)
Same with auto-completion. Either auto-complete will correctly show you
that what you thought was a html object isn't, and fail to show any
additional methods you added; or worse, it will wrongly think it is a
html object when it isn't, and allow you to autocorrect methods that
don't exist.
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