proper __bool__ on complement set and tests

boltons/setutils.py

@@ -907,11 +907,18 @@ class _ComplementSet(object):
         return hash(self._included) ^ hash(self._excluded)
 
     def __len__(self):
-        if self._included:
+        if self._included is not None:
             return len(self._included)
         raise NotImplementedError('complemented sets have undefined length')
 
     def __iter__(self):
-        if self._included:
+        if self._included is not None:
             return iter(self._included)
         raise NotImplementedError('complemented sets have undefined contents')
+
+    def __bool__(self):
+        if self._included is not None:
+            return bool(self._included)
+        return True
+
+    __nonzero__ = __bool__  # py2 compat

tests/test_setutils.py

@@ -117,6 +117,8 @@ def test_complement_set():
     assert (cab ^ cbc | set('b')) == (sab | sbc)
     everything = complement(frozenset())
     assert everything in everything  # https://en.wikipedia.org/wiki/Russell%27s_paradox
+    assert bool(cab)
+    assert not complement(u)
     # destructive testing
     cab ^= sab
     cab ^= sab