I think setting a patch mock's side_effect to raise an exception is a valid use case.

In this example, we're simulating a blockchain error in an API that we're coding, and testing that instead of a 500 Internal Server Error, our API returns a 400 with a nice error message:

with mock.patch('yourapi.w3.eth') as thing:

    eth.sendRawTransaction.side_effect = lambda: raise SomeException('some message')

    result = testclient.post('/yourapi')

    assert result.status_code == 400

    assert result.message = 'some message'