ThreadSafeForwardingResult checks for empty tags incorrectly

[Benji York]

The ThreadSafeForwardingResult class in testtools has a helper method that checks whether or not a (new, gone) tuple of tags represents a non-empty set of tags. It is defined thusly:

    def _not_empty_tags(self, tags):
        return bool(tags[0] and tags[1])

As far as Gary and I can see, the above should be defined as

    def _not_empty_tags(self, tags):
        return bool(tags[0] or tags[1])

Or, a slight optimization:

    def _not_empty_tags(self, tags):
        return bool(tags[0] - tags[1])

[Jonathan Lange (jml)]

What's the actual problem caused by this incorrectness?

[Robert Collins (lifeless)]

That is indeed wrong, but set differencing wouldn't be right either -
if you assume that the sets are disjoint (they always should be) then
a logical or is correct. A set difference would return empty if for
some reason the tags are not disjoint.
>>> set([1]) - set([1])
set([])