Response to comments, added one more example, corrected a typo

python · gvanrossum · Jul 5, 2016 · Jul 4, 2016 · Jul 5, 2016 · Jul 5, 2016
commit 28f9095199da4e9d2db955d5e6bcf756302afadf
diff --git a/pep-0484.txt b/pep-0484.txt
@@ -851,8 +851,8 @@ Support for singleton types in unions
 -------------------------------------
 
 A singleton instance is frequently used to mark some special condition,
-in particular in situations when ``None`` is a valid value for a variable.
-Example::
+in particular in situations where ``None`` is also a valid value
+for a variable. Example::
 
   _empty = object()
 
@@ -866,7 +866,7 @@ Example::
 
 To allow precise typing in such situations, the user should use
 the ``Union`` type in conjuction with the ``enum.Enum`` class provided
-by the standard library, so that type errors could be caught statically::
+by the standard library, so that type errors can be caught statically::
 
   from typing import Union
   from enum import Enum
@@ -886,9 +886,24 @@ by the standard library, so that type errors could be caught statically::
       else:  # At this point typechecker knows that x can only have type int
           return x * 2
 
-Since the subclasses of ``Enum`` could not be further subclassed,
+Since the subclasses of ``Enum`` cannot be further subclassed,
 the type of variable ``x`` can be statically inferred in all branches
-of the above example.
+of the above example. The same approach is applicable if more than one
+singleton object is needed: one can use an enumeration that has more than
+one value::
+
+  class Reason(Enum):
+      timeout = 1
+      error = 2
+
+  def process(response: Union[str, Reason]='') -> str:
+      if response is Reason.timeout:
+          return 'TIMEOUT'
+      elif response is Reason.error:
+          return 'ERROR'
+      else:
+          # response can be only str, all other possible values exhausted
+          return 'PROCESSED: ' + response
 
 
 The ``Any`` type