The following code causes ElementTree.Element.__deepcopy__() to raise a
SystemError:

class BadMemo:
    def get(*args):
        return None

import xml.etree.ElementTree
xml.etree.ElementTree.Element('foo').__deepcopy__(BadMemo())


this is because _elementtree_Element___deepcopy__() (in Modules/_elementtree.c)
assumes that memo is a dictionary, and passes it to PyDict_SetItem(), which
raises the SystemError.

I suppose the Python implementation supports arbitrary mapping, not just dict. We could use general mapping API in the C implementation, but I think it isn't worth. __deepcopy__ is used internally by copy.deepcopy(), which always passes a dict. I think it isn't worth to backport a fix, and it doesn't need tests. The behavior of calling __deepcopy__ with non-dict is implementation detail. But it shouldn't crash or raise SystemError.

New changeset d056818ed2ade6d28190a375d7183f4aef9caa55 by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31428: Prevent raising a SystemError in case the memo arg of ElementTree.Element.__deepcopy__() isn't a dictionary. (#3512)
https://github.com/python/cpython/commit/d056818ed2ade6d28190a375d7183f4aef9caa55