PR 11172 makes deleting characters_written to be as for other non-special attributes. Deleting it will be successful only if it was set before, and will raise an AttributeError otherwise.

>>> e = OSError()
>>> e.characters_written = 1
>>> del e.characters_written
>>> del e.characters_written
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: characters_written