compoundElements now behave boolean-ly correct under py2

ferenda/elements/elements.py

@@ -204,6 +204,14 @@ def __new__(cls, arg=[], *args, **kwargs):
     def __str__(self):
         return self.as_plaintext()
 
+    def __nonzero__(self):
+        # on py2, we've had problems with empty elements (x =
+        # CompoundElement()) being truthy. Avoid this by defining a
+        # proper bool criteria using the magic __nonzero__
+        # method. These problems don't show up on py3, so we don't
+        # bother to define __bool__
+        return len(self) != 0
+
     def _cleanstring(self, s):
         # valid chars according to the XML spec
         def _valid(i):

test/testElements.py

@@ -40,7 +40,7 @@ def test_serialize_roundtrip(self):
                              ordinal=2,
                              title="Native types")
                  ])
-        # roundtrip using the default XML format 
+        # roundtrip using the default XML format
         serialized = serialize(tree)
         self.assertIsInstance(serialized, str)
         newtree = deserialize(serialized, caller_globals=globals())
@@ -120,6 +120,20 @@ def test_compound(self):
                                         Paragraph(["World"])])]).as_plaintext(),
                          "Hello World")
 
+
+    def test_empty_compound(self):
+        # empty elements should be false
+        x = CompoundElement()
+        self.assertFalse(bool(x))
+
+        # even elements with metadata should be false, if they do not have any actual items
+        x = CompoundElement(id="42", foo="bar")
+        self.assertFalse(bool(x))
+
+        # but an element with some data should be true
+        x = CompoundElement(["actual", "data"], id="42", foo="bar")
+        self.assertTrue(bool(x))
+
     def test_unicode(self):
         x = UnicodeElement("Hello world", id="42")
         self.assertEqual(b'<unicodeelement xmlns="http://www.w3.org/1999/xhtml" id="42">Hello world</unicodeelement>',