On Tue, Dec 20, 2022 at 6:20 PM Lucas Wiman <lucas.wiman@gmail.com> wrote:
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).
I wonder how many of these issues would go away if userString subclassed for str. Maybe some? But at the C level, duck typing simply doesn't work -- you need access to an actual C string struct. Code that worked with strings *could* have a little bit of wrapper for subclasses that would dig into it to find the actual str underneath -- but if that code had to be written everywhere strings are used in C -- that could be a pretty big project -- probably what Guido meant by: "Fixing this will be a major project, probably for Python 3000k" I don't suppose it has been addressed at all? Note: at least for string paths, the builtins all use fspath() (or something) so that should be easy to make work. (and seems to with my prototype already) There is a related issue with json.dump etc, json.dump works with my prototype as well. -CHB -- Christopher Barker, PhD (Chris) Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython