Make NOTHING falsey (#732)

* Make NOTHING falsey

Fixes #720

* Add newsfragment

* Python 2

changelog.d/732.change.rst

@@ -0,0 +1 @@
+``bool(attr.NOTHING)`` is now ``False``.

src/attr/_make.py

@@ -69,6 +69,12 @@ class _Nothing(object):
     def __repr__(self):
         return "NOTHING"
 
+    def __bool__(self):
+        return False
+
+    def __len__(self):
+        return 0  # __bool__ for Python 2
+
 
 NOTHING = _Nothing()
 """

tests/test_dunders.py

@@ -808,6 +808,13 @@ class TestNothing(object):
         assert not (_Nothing() != _Nothing())
         assert 1 != _Nothing()
 
+    def test_false(self):
+        """
+        NOTHING evaluates as falsey.
+        """
+        assert not NOTHING
+        assert False is bool(NOTHING)
+
 
 @attr.s(hash=True, order=True)
 class C(object):