Fix len() sometimes raising ValueError on empty node lists (fixes #174)

CHANGELOG

@@ -1,5 +1,6 @@
 v0.5 (unreleased):
 
+- Fixed len() sometimes raising ValueError on empty node lists.
 - Fixed release script after changes to PyPI.
 
 v0.4.4 (released December 30, 2016):

docs/changelog.rst

@@ -7,6 +7,7 @@ v0.5
 Unreleased
 (`changes <https://github.com/earwig/mwparserfromhell/compare/v0.4.4...develop>`__):
 
+- Fixed ``len()`` sometimes raising ``ValueError`` on empty node lists.
 - Fixed release script after changes to PyPI.
 
 v0.4.4

mwparserfromhell/smart_list.py

@@ -271,7 +271,7 @@ class _ListProxy(_SliceNormalizerMixIn, list):
             return bool(self._render())
 
     def __len__(self):
-        return (self._stop - self._start) // self._step
+        return max((self._stop - self._start) // self._step, 0)
 
     def __getitem__(self, key):
         if isinstance(key, slice):

tests/test_smart_list.py

@@ -398,6 +398,7 @@ class TestSmartList(unittest.TestCase):
         self.assertEqual([4, 3, 2, 1.9, 1.8, 5, 6], child1)
         self.assertEqual([4, 3, 2, 1.9, 1.8], child2)
         self.assertEqual([], child3)
+        self.assertEqual(0, len(child3))
 
         del child1
         self.assertEqual([1, 4, 3, 2, 1.9, 1.8, 5, 6], parent)