I recently was confused whether to raise a `PicklingError` or `TypeError` in `__getstate__` if objects of my class can't and shouldn't be pickled. [1]

Terry Reedy advised I should use `TypeError`. [2]

I wonder if the `pickle` module documention should explicitly recommend using `TypeError` if a class wants to say that its objects can't be pickled. What do you think?

[1] https://mail.python.org/pipermail/python-list/2014-April/670987.html
[2] https://mail.python.org/pipermail/python-list/2014-April/671002.html

When pickling an object fails on line 70 of copy_reg.py, a "TypeError" is raised. However, according to section 11.1.3 of the Standard Library documentation, when an unpicklable object is passed to the dump() method the "PicklingError" exception will be raised.

PicklingError is raised in following cases:

1. Programmical Pickler error (Pickler.__init__() was not called by subclass' __init__()).

2. Due to protocol or implementation limitations: memoizing more than 2**32 objects, saving non-ASCII globals with protocol < 3, etc. Exception type is implementation depended. Sometimes OverflowError or struct.error can be raised instead.

3. When __reduce__/__reduce_ex__ returns invalid value: neither str or tuple, a tuple has wrong size, first tuple element is not a callable, first argument for __newobj__/__newobj_ex__ has no __new__ or doesn't match a type of pickled object, global lookup is failed or doesn't match pickled object, etc.

4. When extension code is not an integer or out of range.

2.7 docs are no longer revised