ENH: Add a repr to np._NoValue

This change _NoValue from a class to an instance, which is more inline with the builtin None.

Fixes gh-8991, closes gh-9592

1 files changed, 19 insertions, 5 deletions

diff --git a/numpy/_globals.py b/numpy/_globals.py
index 2d7b69bc4..9a7b458f1 100644
--- a/numpy/_globals.py
+++ b/numpy/_globals.py
@@ -52,11 +52,25 @@ class VisibleDeprecationWarning(UserWarning):
     """
     pass
 
-
-class _NoValue(object):
+class _NoValueType(object):
     """Special keyword value.
 
-    This class may be used as the default value assigned to a deprecated
-    keyword in order to check if it has been given a user defined value.
+    The instance of this class may be used as the default value assigned to a
+    deprecated keyword in order to check if it has been given a user defined
+    value.
     """
-    pass
+    __instance = None
+    def __new__(cls):
+        # ensure that only one instance exists
+        if not cls.__instance:
+            cls.__instance = super(_NoValueType, cls).__new__(cls)
+        return cls.__instance
+
+    # needed for python 2 to preserve identity through a pickle
+    def __reduce__(self):
+        return (self.__class__, ())
+
+    def __repr__(self):
+        return "<no value>"
+
+_NoValue = _NoValueType()