Fixes to docs and InteractiveShell.instance()

* Updated messaging docs to say that the prompt informtion always
  is returned in execute_reply.
* Started to fix InteractiveShell.instance() to properly handle
  subclasses.

IPython/core/interactiveshell.py

@@ -126,6 +126,9 b' class SeparateStr(Str):'
         value = value.replace('\\n','\n')
         return super(SeparateStr, self).validate(obj, value)
 
+class MultipleInstanceError(Exception):
+    pass
+
 
 #-----------------------------------------------------------------------------
 # Main IPython class
@@ -268,14 +271,17 b' class InteractiveShell(Configurable, Magic):'
         """Returns a global InteractiveShell instance."""
         if not hasattr(cls, "_instance"):
             cls._instance = cls(*args, **kwargs)
-            print cls
-            print cls._instance
             # Now make sure that the instance will also be returned by
             # the subclasses instance attribute.
-            for subclass in cls.mro():
+            for subclass in cls.mro()[1:]:
                 if issubclass(subclass, InteractiveShell):
-                    print subclass
-                    subclass._instance = cls._instance
+                    if not hasattr(subclass, '_instance'):
+                        subclass._instance = cls._instance
+                    else:
+                        raise MultipleInstanceError(
+                            'Multiple conflicting subclass of '
+                            'InteractiveShell have been created.'
+                        )
         return cls._instance
 
     @classmethod

docs/source/development/messaging.txt

@@ -164,12 +164,6 b' Message type: ``execute_reply``::'
       # One of: 'ok' OR 'error' OR 'abort'
       'status' : str,
 
-      # Any additional data depends on status value
-    }
-
-When status is 'ok', the following extra fields are present::
-
-    {
       # This has the same structure as the output of a prompt request, but is
       # for the client to set up the *next* prompt (with identical limitations
       # to a prompt request)
@@ -186,7 +180,11 b" When status is 'ok', the following extra fields are present::"
       # to correct the previously written number in-place, terminal ones may
       # re-print a corrected one if desired.
       'prompt_number' : int,
+    }
+
+When status is 'ok', the following extra fields are present::
 
+    {
       # The kernel will often transform the input provided to it.  This
       # contains the transformed code, which is what was actually executed.
       'transformed_code' : str,