On 1 December 2017 at 01:23, Ronald Oussoren <[email protected]> wrote:
> 1) Last time around Mark Shannon worried that this introduces infinite
> recursion in the language itself (in my crummy summary, please read this
> message to get the real concern
> <https://mail.python.org/pipermail/python-dev/2015-July/140938.html>). Is
> this truly a problem? I don’t think there is a problem, but I’m worried
> that I don’t fully understand Mark’s concerns.
>
> 2) PEP 487 introduced __init_subclass__ as a class method to avoid having to
> write a metaclass for a number of use cases. My PEP currently does require
> a metaclass, but it might be nicer to switch to a regular class method
> instead (like __init_subclass__).
I think the second point there may actually allow you to resolve the
first one, by way of making `__getdescriptor__` an optional override
of the typical lookup algorithm. That is:
def _PyType_Lookup(tp, name):
# New optional override for descriptor lookups
try:
# Ordinary attribute lookup, *NOT* a descriptor lookup
getdesc = tp.__getdescriptor__
except AttributeError:
pass
else:
return getdesc(name)
# Default algorithm used in the absence of an override
mro = tp.mro()
assert isinstance(mro, tuple)
for base in mro:
assert isinstance(base, type)
try:
return base.__dict__[name]
except KeyError:
pass
return None
If you did go this way, then we'd want to add a
"types.getdescriptor(cls, name)" API to expose _PyType_Lookup at the
Python layer, since "getattr(type(obj), name)" wouldn't be an accurate
emulation of the algorithm any more.
Cheers,
Nick.
--
Nick Coghlan | [email protected] | Brisbane, Australia
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