sync with Atlas repo

interfaces/subclassing_builtins.rst

@@ -0,0 +1,31 @@
+====================================
+Subclassing built-in caveats
+====================================
+
+::
+
+    >>> class D1(dict):
+    ...     def __getitem__(self, key):
+    ...         return 42
+    ...
+    >>> d1 = D1(a='foo')
+    >>> d1
+    {'a': 'foo'}
+    >>> d1['a']
+    42
+    >>> d1.get('a')
+    'foo'
+
+::
+
+    >>> class D2(dict):
+    ...     def get(self, key):
+    ...         return 42
+    ...
+    >>> d2 = D2(a='foo')
+    >>> d2
+    {'a': 'foo'}
+    >>> d2['a']
+    'foo'
+    >>> d2.get('a')
+    42