Complete fix for __del__ errors with .reset(), add unit test.

IPython/core/interactiveshell.py

@@ -983,11 +983,12 b' class InteractiveShell(Configurable, Magic):'
             ns.clear()
             
         # The main execution namespaces must be cleared very carefully,
-        # skipping the deletion of the __builtin__ key, because doing so would
-        # cause errors in many object's __del__ methods.
+        # skipping the deletion of the builtin-related keys, because doing so
+        # would cause errors in many object's __del__ methods.
         for ns in [self.user_ns, self.user_global_ns]:
             drop_keys = set(ns.keys())
             drop_keys.discard('__builtin__')
+            drop_keys.discard('__builtins__')
             for k in drop_keys:
                 del ns[k]
 

IPython/core/tests/test_run.py

@@ -83,6 +83,22 b' def doctest_run_builtins():'
        ....: 
     """
 
+def doctest_reset_del():
+    """Test that resetting doesn't cause errors in __del__ methods.
+
+    In [2]: class A(object):
+       ...:     def __del__(self):
+       ...:         str("Hi")
+       ...: 
+
+    In [3]: a = A()
+
+    In [4]: get_ipython().reset()
+
+    In [5]: 1+1
+    Out[5]: 2
+    """
+
 # For some tests, it will be handy to organize them in a class with a common
 # setup that makes a temp file