Several issues noticed while skimming the code...

[njsmith]

• Multiset.__delitem__ is untested and appears to be broken: it doesn't update self._total. Maybe it should be implemented as self[element] = 0 to reuse the __setitem__ code?

• The docstring for BaseMultiset.union says "For a variant of the operation which modifies the multiset in place see :meth:`union`.". But... that's what I'm looking at :-)

• Your package metadata says you support both py2 and py3, but the wheel on pypi is marked as being py3-only. Maybe you need to set the universal=1 option in setup.cfg?

• Not really a bug, but I found the semantics of remove and discard surprising: I would have expected them to be the inverse of add, rather than removing/discarding all copies by default. Maybe a less surprising design would be to have remove remove 1 item, discard discard 0 or 1 items, and then have remove_all and discard_all methods for when you want to do that? I don't know if it's worth breaking backwards compatibility over though.

[wheerd]

Thanks a lot for your feeback! I am happy that someone took the time to look through the code :-)
I fixed the issues you mentioned and will release a new version soon.

I am not sure about your last point. While I can see how this might be confusing, I do not want to make breaking changes to the API here. I don't think that the impact is that grave, especially since those methods can also be used to explicitly only remove/discard one element, e.g. with remove(1).

[wheerd]

I released it as version 2.0.1.