Made InteractiveShell a SingletonConfigurable.

I have moved the instance method to a new subclass of Configurable
so we can use it on other configurable classes, like Application.
InteractiveShell now just inherits from this class.

IPython/config/configurable.py

@@ -232,3 +232,9 b' class SingletonConfigurable(Configurable):'
                 'Multiple incompatible subclass instances of '
                 '%s are being created.' % cls.__name__
             )
+
+    @classmethod
+    def initialized(cls):
+        """Has an instance been created?"""
+        return hasattr(cls, "_instance") and cls._instance is not None
+

IPython/config/tests/test_configurable.py

@@ -152,17 +152,21 b' class TestSingletonConfigurable(TestCase):'
     def test_instance(self):
         from IPython.config.configurable import SingletonConfigurable
         class Foo(SingletonConfigurable): pass
+        self.assertEquals(Foo.initialized(), False)
         foo = Foo.instance()
+        self.assertEquals(Foo.initialized(), True)
         self.assertEquals(foo, Foo.instance())
         self.assertEquals(SingletonConfigurable._instance, None)
 
     def test_inheritance(self):
-
         class Bar(SingletonConfigurable): pass
         class Bam(Bar): pass
+        self.assertEquals(Bar.initialized(), False)
+        self.assertEquals(Bam.initialized(), False)
         bam = Bam.instance()
         bam == Bar.instance()
+        self.assertEquals(Bar.initialized(), True)
+        self.assertEquals(Bam.initialized(), True)
         self.assertEquals(bam, Bam._instance)
         self.assertEquals(bam, Bar._instance)
         self.assertEquals(SingletonConfigurable._instance, None)
-        
  No newline at end of file

IPython/core/interactiveshell.py

@@ -31,7 +31,7 b' import tempfile'
 import types
 from contextlib import nested
 
-from IPython.config.configurable import Configurable
+from IPython.config.configurable import SingletonConfigurable
 from IPython.core import debugger, oinspect
 from IPython.core import history as ipcorehist
 from IPython.core import page
@@ -132,9 +132,7 b' class SeparateStr(Str):'
         value = value.replace('\\n','\n')
         return super(SeparateStr, self).validate(obj, value)
 
-class MultipleInstanceError(Exception):
-    pass
-    
+
 class ReadlineNoRecord(object):
     """Context manager to execute some code, then reload readline history
     so that interactive input to the code doesn't appear when pressing up."""
@@ -194,7 +192,7 b" called (even if no arguments are present). The default is '1'."
 # Main IPython class
 #-----------------------------------------------------------------------------
 
-class InteractiveShell(Configurable, Magic):
+class InteractiveShell(SingletonConfigurable, Magic):
     """An enhanced, interactive shell for Python."""
 
     _instance = None
@@ -427,31 +425,6 b' class InteractiveShell(Configurable, Magic):'
         self.hooks.late_startup_hook()
         atexit.register(self.atexit_operations)
 
-    @classmethod
-    def instance(cls, *args, **kwargs):
-        """Returns a global InteractiveShell instance."""
-        if cls._instance is None:
-            inst = cls(*args, **kwargs)
-            # Now make sure that the instance will also be returned by
-            # the subclasses instance attribute.
-            for subclass in cls.mro():
-                if issubclass(cls, subclass) and \
-                       issubclass(subclass, InteractiveShell):
-                    subclass._instance = inst
-                else:
-                    break
-        if isinstance(cls._instance, cls):
-            return cls._instance
-        else:
-            raise MultipleInstanceError(
-                'Multiple incompatible subclass instances of '
-                'InteractiveShell are being created.'
-            )
-
-    @classmethod
-    def initialized(cls):
-        return hasattr(cls, "_instance")
-
     def get_ipython(self):
         """Return the currently running IPython instance."""
         return self