Add explanation of singletons in unions to PEP484

By Ivan Levkivskyi (https://github.com/python/typing/pull/240).

pep-0484.txt

@@ -846,6 +846,66 @@ This is equivalent to::
 
   def handle_employee(e: Optional[Employee] = None) -> None: ...
 
+
+Support for singleton types in unions
+-------------------------------------
+
+A singleton instance is frequently used to mark some special condition,
+in particular in situations where ``None`` is also a valid value
+for a variable. Example::
+
+  _empty = object()
+
+  def func(x=_empty):
+      if x is _empty:  # default argument value
+          return 0
+      elif x is None:  # argument was provided and it's None
+          return 1
+      else:
+          return x * 2
+
+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 can be caught statically::
+
+  from typing import Union
+  from enum import Enum
+
+  class Empty(Enum):
+      token = 0
+  _empty = Empty.token
+
+  def func(x: Union[int, None, Empty] = _empty) -> int:
+
+      boom = x * 42  # This fails type check
+
+      if x is _empty:
+          return 0
+      elif x is None:
+          return 1
+      else:  # At this point typechecker knows that x can only have type int
+          return x * 2
+
+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. 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
 ----------------