In the Python implementation ofpickle setting some handlers to None causes a fallback to other methods.

• SettingPickler.reducer_override to None has the same effect as not setting this attribute -- falling back to other methods.
• Setting thedispatch_table to None leads to falling back to__reduce_ex__.
• Setting the__reduce_ex__ attribute to None leads to falling back to__reduce__.
• Setting the__setstate__ attribute to None leads to falling back to the default implementation of__setstate__ (setting__dict__ and slots).

In the C implementation all this leads toTypeError: 'NoneType' object is not callable.

Thecopy module follows the Python implementation ofpickle, except that it fails for any false value of__reduce__, not just None.

In most other code setting a dunder method to None leads to a TypeError (either raise explicitly or just generic "'NoneType' object is not callable").

We should decide what behavior should be in thepickle andcopy module. On one hand, the Python implementation was primary. On other hand, it was inconsistent between thepickle andcopy modules and was already changed in the past, the C implementation ofpickle is now used by most users, and it is more consistent with other code.

See also:
https://mail.python.org/archives/list/python-dev@python.org/thread/YGAK34DRWJFSIV2VZ4NC2J24XO37GCMM/
https://docs.python.org/3.10/reference/datamodel.html#id2
#70146
#55781
#60755

Linked PRs

• gh-93627: Align Python implementation of pickle with C implementation of pickle #103035
• gh-93627: Align copy module behaviour with pickle module #109498