Josh Rosenberg <[email protected]> added the comment:
First off, the OP's original case seems like a use case for
functools.singledispatch. Not really related to the problem, just thought I'd
mention it.
Secondly, are we sure we want to make such a guarantee? That restricts the
underlying storage to ordered types (list/dict; possibly tuple at the cost of
making modifications slightly more expensive), or an unordered type with
additional ordering layered on it (like old-school OrderedDict).
That does tie our hands in the future. For example, it seems like it would be a
perfectly reasonable approach for the internal collection of subclasses to be
implemented as a weakref.WeakSet (some future version of it implemented in C,
rather than the current Python layer version) so as to reduce code duplication
and improve handling when a subclass disappears. Right now, tp_subclasses is a
dict keyed by the raw memory address of the subclass (understandable, but eww),
with a value of a weakref to the subclass itself. There is tons of custom code
involved in handling this (e.g. the dict only self-cleans because the dealloc
for classes explicitly removes the subclass from the parent classes, but every
use of the dict still has to assume weakrefs have gone dead anyway, because of
reentrancy issues; these are solved problems in WeakSet which hides all the
complexity from the user). Being able to use WeakSets would mean a huge amount
of special purpose code in typeobject.c could go away,
but guaranteeing ordering would make that more difficult (it would require
providing an ordering guarantee for WeakSet, which, being built on set, would
likely require ordering guarantees for sets in general, or changing WeakSet to
be built on dicts).
There is also (at least) one edge case that would need to be fixed (based on a
brief skim of the code). type_set_bases (which handles assignment to __bases__
AFAICT, admittedly a niche use case) simplified its own implementation by
making the process of changing __bases__ be to remove itself as a subclass of
all of its original bases, then add itself as a subclass of the new bases. This
is done even if there are overlaps in the bases, and even if the new bases are
the same.
Minimal repro:
>>> class A: pass
>>> class B(A): pass
>>> class C(A): pass
>>> A.__subclasses__() # Appear in definition order
[__main__.B, __main__.C]
>>> B.__bases__ = B.__bases__ # Should be no-op...
>>> A.__subclasses__() # But oops, order changed
[__main__.C, __main__.B]
I'm not going to claim this is common or useful (I've done something like this
exactly once, interactively, while making an OrderedCounter from OrderedDict
and Counter back before dicts were ordered; I got the inheritance order wrong
and reversed it after the fact), but making the guarantee would be more than
just stating it; we'd either have to complicate the code to back it up, or
qualify the guarantee with some weird, possibly CPython-specific details.
----------
nosy: +josh.r
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<https://bugs.python.org/issue34805>
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