Per the 3.6.0 docs:
CPython implementation detail: In CPython 3.6 and later, the
__class__
cell is passed to the metaclass as a__classcell__
entry in the class namespace. If present, this must be propagated up to thetype.__new__
call in order for the class to be initialized correctly. Failing to do so will result in aDeprecationWarning
in Python 3.6, and aRuntimeWarning
in the future.
Can someone provide an example of doing this correctly?
An example where it's actually needed?
The warning is raised if you use super that relies on
__class__
being available or reference__class__
inside the class body.What the text essentially says is that, this is needed if you define a custom meta-class and tamper with the namespace you get before passing it up to
type.__new__
. You'll need to be careful and always make sure you pass__classcell__
totype.__new__
in yourmetaclass.__new__
.That is, if you create a new fancy namespace to pass up, always check if
__classcell__
is defined in the original namespace created and add it:The file you linked in the comment is actually the first of many attempted patches,
issue23722_classcell_reference_validation_v2.diff
is the final patch that made it in, from Issue 23722.An example of doing this correctly can be seen in a pull request made to Django that uses this to fix an issue that was introduced in Python 3.6:
The
__classcell__
is simply added to the new namespace before being passed totype.__new__
.