[Python-Dev] RE: interning string subclasses

[Tim Peters]

[Tim]
> Question:  Should we complain if someone tries to intern an instance of
> a string subclass?  I hate to slow any code on those paths.

[Guido]
> I think in this case intern(s) should return intern(str(s)).  The fast
> path checks ob_sinterned first, and that should always point to a real
> string for a string subclass.

I think I'm wrestling with more than one problem here.  The bogus one
confusing everything else appears to be this:

>>> class S(str):
...     pass
...
[7098 refs]
>>> hash(S('abc'))
0
[7100 refs]
>>> hash(S('cdefg'))
0
[7102 refs]
>>>

That is, PyObject_Hash(s) is always 0 for any instance s of a subclass of
str.  This makes dict-based operations "almost always" believe that an s
like this used as a key doesn't match a real string with the same value.

>>> d = {'a': 1}
[7106 refs]
>>> d[S('a')]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
KeyError: a
[7143 refs]
>>>

This applies too to the intern dict.  So first I think str_subtype_new has
to finish initializing the subclass string object.  Then I can get confused
about interning for the right reasons <wink>.