Remove what turns out to be a useless __slots__ definition from Token…

…Matcher, add a useful one to Token, and confirm with a test that __slots__ does not behave in CPython according to the language docs.

parsimonious/expressions.py

@@ -233,8 +233,6 @@ class TokenMatcher(Literal):
     This is for use only with TokenGrammars.
 
     """
-    __slots__ = []  # so this doesn't grow a __dict__
-
     def _uncached_match(self, token_list, pos, cache, error):
         if token_list[pos].type == self.literal:
             return Node(self.name, token_list, pos, pos + 1)

parsimonious/tests/test_expressions.py

@@ -259,3 +259,30 @@ def test_unicode(self):
 
         """
         unicode(rule_grammar)
+
+
+class SlotsTests(TestCase):
+    """Tests to do with __slots__"""
+
+    def test_subclassing(self):
+        """Make sure a subclass of a __slots__-less class can introduce new
+        slots itself.
+
+        This isn't supposed to work, according to the language docs:
+
+            When inheriting from a class without __slots__, the __dict__
+            attribute of that class will always be accessible, so a __slots__
+            definition in the subclass is meaningless.
+
+        But it does.
+
+        """
+        class Smoo(Optional):
+            __slots__ = ['smoo']
+
+            def __init__(self):
+                self.smoo = 'smoo'
+
+        smoo = Smoo()
+        eq_(smoo.__dict__, {})  # has a __dict__ but with no smoo in it
+        eq_(smoo.smoo, 'smoo')  # The smoo attr ended up in a slot.

parsimonious/utils.py

@@ -37,6 +37,8 @@ class Token(StrAndRepr):
     must have a ``type`` attr.
 
     """
+    __slots__ = ['type']
+
     def __init__(self, type):
         self.type = type