Add drop_by_id method to shell, to remove variables added by extensions.

IPython/core/interactiveshell.py

@@ -1265,6 +1265,24 b' class InteractiveShell(SingletonConfigurable, Magic):'
             for name,val in vdict.iteritems():
                 config_ns[name] = val
 
+    def drop_by_id(self, variables):
+        """Remove a dict of variables from the user namespace, if they are the
+        same as the values in the dictionary.
+        
+        This is intended for use by extensions: variables that they've added can
+        be taken back out if they are unloaded, without removing any that the
+        user has overwritten.
+        
+        Parameters
+        ----------
+        variables : dict
+          A dictionary mapping object names (as strings) to the objects.
+        """
+        for name, obj in variables.iteritems():
+            if name in self.user_ns and self.user_ns[name] is obj:
+                del self.user_ns[name]
+                self.user_ns_hidden.pop(name, None)
+
     #-------------------------------------------------------------------------
     # Things related to object introspection
     #-------------------------------------------------------------------------

IPython/core/tests/test_interactiveshell.py

@@ -179,4 +179,17 b' class InteractiveShellTestCase(unittest.TestCase):'
         finally:
             io.stderr = save_stderr
 
-
+    def test_drop_by_id(self):
+        ip = get_ipython()
+        myvars = {"a":object(), "b":object(), "c": object()}
+        ip.push(myvars, interactive=False)
+        for name in myvars:
+            assert name in ip.user_ns, name
+            assert name in ip.user_ns_hidden, name
+        ip.user_ns['b'] = 12
+        ip.drop_by_id(myvars)
+        for name in ["a", "c"]:
+            assert name not in ip.user_ns, name
+            assert name not in ip.user_ns_hidden, name
+        assert ip.user_ns['b'] == 12
+        ip.reset()