Added SingletonConfigurable with instance method.

I have added a singleton configurable class for objects that
should have only a single instance, such as InteractiveShell and
the main application objects. For these classes the .instance()
method should be used to create/retrieve the instance.

IPython/config/configurable.py

@@ -37,6 +37,9 b' class ConfigurableError(Exception):'
     pass
 
 
+class MultipleInstanceError(ConfigurableError):
+    pass
+
 #-----------------------------------------------------------------------------
 # Configurable implementation
 #-----------------------------------------------------------------------------
@@ -170,4 +173,62 b' class Configurable(HasTraits):'
     def class_print_help(cls):
         print cls.class_get_help()
 
-            
  No newline at end of file
+
+class SingletonConfigurable(Configurable):
+    """A configurable that only allows one instance.
+
+    This class is for classes that should only have one instance of itself
+    or *any* subclass. To create and retrieve such a class use the
+    :meth:`SingletonConfigurable.instance` method.
+    """
+
+    _instance = None
+
+    @classmethod
+    def instance(cls, *args, **kwargs):
+        """Returns a global instance of this class.
+
+        This method create a new instance if none have previously been created
+        and returns a previously created instance is one already exists.
+
+        The arguments and keyword arguments passed to this method are passed
+        on to the :meth:`__init__` method of the class upon instantiation.
+
+        Examples
+        --------
+
+        Create a singleton class using instance, and retrieve it::
+
+            >>> from IPython.config.configurable import SingletonConfigurable
+            >>> class Foo(SingletonConfigurable): pass
+            >>> foo = Foo.instance()
+            >>> foo == Foo.instance()
+            True
+
+        Create a subclass that is retrived using the base class instance::
+
+            >>> class Bar(SingletonConfigurable): pass
+            >>> class Bam(Bar): pass
+            >>> bam = Bam.instance()
+            >>> bam == Bar.instance()
+            True
+        """
+        # Create and save the 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, SingletonConfigurable) and \
+                subclass != SingletonConfigurable:
+                    subclass._instance = inst
+                else:
+                    break
+        if isinstance(cls._instance, cls):
+            return cls._instance
+        else:
+            raise MultipleInstanceError(
+                'Multiple incompatible subclass instances of '
+                '%s are being created.' % cls.__name__
+            )

IPython/config/tests/test_configurable.py

@@ -22,10 +22,15 b' Authors:'
 
 from unittest import TestCase
 
-from IPython.config.configurable import Configurable, ConfigurableError
+from IPython.config.configurable import (
+    Configurable, 
+    SingletonConfigurable
+)
+
 from IPython.utils.traitlets import (
-    TraitError, Int, Float, Str
+    Int, Float, Str
 )
+
 from IPython.config.loader import Config
 
 
@@ -57,7 +62,7 b' class Bar(Foo):'
     c = Float(config=True, shortname="c", help="The string c.")
 
 
-class TestConfigurableConfig(TestCase):
+class TestConfigurable(TestCase):
 
     def test_default(self):
         c1 = Configurable()
@@ -141,3 +146,23 b' class TestConfigurableConfig(TestCase):'
     def test_help(self):
         self.assertEquals(MyConfigurable.class_get_help(), mc_help)
 
+
+class TestSingletonConfigurable(TestCase):
+
+    def test_instance(self):
+        from IPython.config.configurable import SingletonConfigurable
+        class Foo(SingletonConfigurable): pass
+        foo = Foo.instance()
+        self.assertEquals(foo, Foo.instance())
+        self.assertEquals(SingletonConfigurable._instance, None)
+
+    def test_inheritance(self):
+
+        class Bar(SingletonConfigurable): pass
+        class Bam(Bar): pass
+        bam = Bam.instance()
+        bam == Bar.instance()
+        self.assertEquals(bam, Bam._instance)
+        self.assertEquals(bam, Bar._instance)
+        self.assertEquals(SingletonConfigurable._instance, None)
+        
  No newline at end of file

IPython/config/tests/test_loader.py

@@ -117,7 +117,6 b' class TestKeyValueCL(TestCase):'
     def test_basic(self):
         cl = KeyValueConfigLoader()
         argv = [s.strip('c.') for s in pyfile.split('\n')[2:-1]]
-        print argv
         config = cl.load_config(argv)
         self.assertEquals(config.a, 10)
         self.assertEquals(config.b, 20)