> There's also documentation and tests that depend on this actual error message:
> Doc/library/doctest.rst:   ValueError: list.remove(x): x not in list
> Lib/test>/test_xml_etree.py:    ValueError: list.remove(x): x not in list
That’s a well-known doctest problem.  Just update the doc.  Writing robust doctests is an art:

>>> str(someobject)
'output that can change'
>>> 'something I want' in str(someobject)  # more robust, but less useful if it fails
True

>>> something.index(spam)
traceback blah:
ValueError: output that can change
>>> try: something.index(spam)
... except ValueError: print('spam not in something')  # more robust, but ugly
spam not in something