On Tue, Dec 20, 2022 at 6:20 PM Lucas Wiman <lucas.wi...@gmail.com> wrote:
> 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).
>
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
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