It actually is documented as being supported here:
https://docs.python.org/3.10/reference/datamodel.html#id2, and as mentioned
there __iter__(), __reversed__() and __contains__() also have special support
so they avoid trying to fallback to __getitem__ etc. Also some discussion at
https://github.com/python/cpython/issues/70146. For most methods the conclusion
back then makes sense, a specific exception message being raised would just
potentially slow the interpreter.
- Spencer
On 1 May 2022, at 3:22 pm, Serhiy Storchaka <storch...@gmail.com> wrote:
There is a special handling of `__hash__` set to None in the interpreter core.
This is because every class inherited the `__hash__` attribute from "object",
and setting `__hash__ = None` is a simple way to make it unhashable. It makes
hash() raising the correct type of exception (TypeError), but with unhelpful
error message "'NoneType' object is not callable". The special case was added
to make the error message more relevant: "unhashable type: '{typename}'".
There is similar situation with other special methods defined in "object" or
other common classes. Sometimes we want to cancel the default inherited
behavior.
>>> dir(object)
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__',
'__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__',
'__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__',
'__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__']
I propose to support officially the idiom "__dunder__ = None" and add special
cases to raise more specialized exception instead of "TypeError: 'NoneType'
object is not callable" for most of special method where cancelling the default
behavior makes sense (for example I do not think that we need better error
message for `__repr__ = None`).
The question is how to interpret value None:
* Always raise TypeError (with changed message)? This is what happen currently
when you set the method to None, this is the most compatible option.
* Always raise an error, but allow to change it to more appropriate type (for
example AttributeError for __setattr__)?
* Interpret value None the same way as an absent attribute?
For `__hash__` or `__class_getitem__` all three options mean the same. But
absent `__mro_entries__` and `__mro_entries__ = None` currently give different
results. It is even more complicated for pickling: absent `__reduce_ex__` and
`__reduce_ex__ = None` mean the same in the Python implementation, but give
different results in the C implementation.
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