ENH: pull sentinel construction into a function

zipline/pipeline/term.py

@@ -15,29 +15,13 @@ from zipline.errors import (
     WindowLengthNotSpecified,
 )
 from zipline.utils.memoize import lazyval
+from zipline.utils.sentinel import sentinel
 
 
-@object.__new__   # bind a single instance to the name 'NotSpecified'
-class NotSpecified(object):
-    """
-    Singleton sentinel value used for Term defaults.
-    """
-    __slots__ = ('__weakref__',)
-
-    def __new__(cls):
-        raise TypeError("Can't construct new instances of NotSpecified")
-
-    def __repr__(self):
-        return type(self).__name__
-
-    def __reduce__(self):
-        return type(self).__name__
-
-    def __deepcopy__(self, _memo):
-        return self
-
-    def __copy__(self):
-        return self
+NotSpecified = sentinel(
+    'NotSpecified',
+    'Singleton sentinel value used for Term defaults.',
+)
 
 
 class Term(with_metaclass(ABCMeta, object)):

zipline/utils/sentinel.py

@@ -0,0 +1,40 @@
+"""
+Construction of sentinel objects.
+
+Sentinel objects are used when you only care to check for object identity.
+"""
+import sys
+
+
+def sentinel(name, doc=None):
+    @object.__new__   # bind a single instance to the name 'NotSpecified'
+    class result(object):
+        __doc__ = doc
+        __slots__ = ('__weakref__',)
+
+        def __new__(cls):
+            raise TypeError("Can't construct new instances of %s" % name)
+
+        def __repr__(self):
+            return name
+
+        def __reduce__(self):
+            return name
+
+        def __deepcopy__(self, _memo):
+            return self
+
+        def __copy__(self):
+            return self
+
+    cls = type(result)
+    cls.__name__ = name
+    try:
+        # traverse up one frame to find the module where this is defined
+        cls.__module__ = sys._getframe(1).f_globals.get(
+            '__name__',
+            '__main__',
+        )
+    except (AttributeError, ValueError):
+        pass
+    return result