make sentinel class for some kwargs.

The signature is a little more sane with
`fun(..., default=module.value)`
instead of
`fun(..., default=<object object at 0x12345678>)`

IPython/core/formatters.py

@@ -21,6 +21,7 b' from decorator import decorator'
 
 from IPython.config.configurable import Configurable
 from IPython.core.getipython import get_ipython
+from IPython.utils.signatures import Sentinel
 from IPython.lib import pretty
 from IPython.utils.traitlets import (
     Bool, Dict, Integer, Unicode, CUnicode, ObjectName, List,
@@ -270,7 +271,13 b' def _get_type(obj):'
     """Return the type of an instance (old and new-style)"""
     return getattr(obj, '__class__', None) or type(obj)
 
-_raise_key_error = object()
+
+_raise_key_error = Sentinel('_raise_key_error', __name__, 
+"""
+Special value to raise a KeyError
+
+Raise KeyError in `BaseFormatter.pop` if passed as the default value to `pop`
+""")
 
 
 class BaseFormatter(Configurable):

IPython/utils/signatures.py

@@ -814,3 +814,19 b' class Signature(object):'
             rendered += ' -> {0}'.format(anno)
 
         return rendered
+
+## Fake unique value as KWargs, in some places.
+# do not put docstrings here or they will appear 
+# on created fake values.
+class Sentinel(object):
+
+    def __init__(self, name, module, docstring=None):
+        self.name = name
+        self.module = module
+        if docstring:
+            self.__doc__ = docstring
+
+
+    def __repr__(self):
+        return str(self.module)+'.'+self.name
+

jupyter_nbformat/__init__.py

@@ -14,6 +14,7 b' from . import v1'
 from . import v2
 from . import v3
 from . import v4
+from IPython.utils.signatures import Sentinel
 
 __all__ = ['versions', 'validate', 'ValidationError', 'convert', 'from_dict',
            'NotebookNode', 'current_nbformat', 'current_nbformat_minor',
@@ -40,18 +41,6 b' class NBFormatError(ValueError):'
     pass
 
 # no-conversion singleton
-class Sentinel(object):
-
-    def __init__(self, name, module, docstring=None):
-        self.name = name
-        self.module = module
-        if docstring:
-            self.__doc__ = docstring
-
-
-    def __repr__(self):
-        return str(self.module)+'.'+self.name
-
 NO_CONVERT = Sentinel('NO_CONVERT', __name__,
     """Value to prevent nbformat to convert notebooks to most recent version.
     """)

traitlets/traitlets.py

@@ -56,6 +56,7 b' from warnings import warn'
 from IPython.utils import py3compat
 from IPython.utils import eventful
 from IPython.utils.getargspec import getargspec
+from IPython.utils.signatures import Sentinel
 from IPython.utils.importstring import import_item
 from IPython.utils.py3compat import iteritems, string_types
 from IPython.testing.skipdoctest import skip_doctest
@@ -67,8 +68,11 b' SequenceTypes = (list, tuple, set, frozenset)'
 #-----------------------------------------------------------------------------
 
 
-class NoDefaultSpecified ( object ): pass
-NoDefaultSpecified = NoDefaultSpecified()
+NoDefaultSpecified = Sentinel('NoDefaultSpecified', __name__, 
+'''
+Used in Traitlets to specify that no defaults are set in kwargs
+'''
+)
 
 
 class Undefined ( object ): pass