On Tue, Dec 20, 2022 at 5:38 PM Christopher Barker <pythonchb@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-no...), 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