On Tue, Dec 20, 2022 at 5:38 PM Christopher Barker <python...@gmail.com>
wrote:
> But collections.UserString does exist -- so if you want to subclass, and
> performance isn't critical, then use that. Steven A pointed out that
> UserStrings are not instances of str though. I think THAT is a bug. And
> it's probably that way because with the magic of duck typing, no one cared
> -- but with all the static type hinting going on now, that is a bigger
> liability than it used to be. Also basue when it was written, you couldn't
> subclass str.
>
> Though I will note that run-time type checking of string is relatively
> common compared to other types, due to the whole a-str-is-a-sequence-of-str
> issue making the distinction between a sequence of strings and a string
> itself is sometimes needed. And str is rarely duck typed.
>
Note that UserString does break some built-in functionality, like you can't
apply regular expressions to a UserString:
>>> class FooString(UserString):
... pass
...
>>> re.compile(r"asdf").match(FooString("asdf"))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: expected string or bytes-like object, got 'FooString'
There is more discussion in this thread (
https://stackoverflow.com/questions/59756050/python3-when-userstring-does-not-behave-as-a-string),
including a link to a very old bug (https://bugs.python.org/issue232493).
There is a related issue with json.dump etc, though it can be worked around
since there is a python-only json implementation.
I have run into this in practice at a previous job, with a runtime "taint"
tracker for logging access to certain database fields in a Django
application. Many views would select all fields from a table, then not
actually use the fields I needed to log access to, which generated false
positives. (Obviously the "correct" design is to only select data that is
relevant for the given code, but I was instrumenting a legacy codebase with
updated compliance requirements.) So I think there is some legitimate use
for this, though object proxies can be made to work around most of the
issues.
- Lucas
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/DZQNWKTSUTKRMA4WFXVERW46AMPYDEAX/
Code of Conduct: http://python.org/psf/codeofconduct/
