PEP 661: Add bool value to reference impl and examples (#4051)

peps/pep-0661.rst

@@ -154,7 +154,10 @@ boolean value, and the name of its module::
     >>> MISSING = Sentinel('MISSING', repr='mymodule.MISSING')
     >>> MISSING
     mymodule.MISSING
-    >>> MEGA = Sentinel('MEGA', repr='<MEGA>', module_name='mymodule')
+    >>> MEGA = Sentinel('MEGA',
+                        repr='<MEGA>',
+                        bool_value=False,
+                        module_name='mymodule')
     <MEGA>
 
 Checking if a value is such a sentinel *should* be done using the ``is``
@@ -235,9 +238,10 @@ simplified version follows::
     class Sentinel:
         """Unique sentinel values."""
 
-        def __new__(cls, name, repr=None, module_name=None):
+        def __new__(cls, name, repr=None, bool_value=True, module_name=None):
             name = str(name)
             repr = str(repr) if repr else f'<{name.split(".")[-1]}>'
+            bool_value = bool(bool_value)
             if module_name is None:
                 try:
                     module_name = \
@@ -254,6 +258,7 @@ simplified version follows::
             sentinel = super().__new__(cls)
             sentinel._name = name
             sentinel._repr = repr
+            sentinel._bool_value = bool_value
             sentinel._module_name = module_name
 
             return _registry.setdefault(registry_key, sentinel)
@@ -261,6 +266,9 @@ simplified version follows::
         def __repr__(self):
             return self._repr
 
+        def __bool__(self):
+            return self._bool_value
+
         def __reduce__(self):
             return (
                 self.__class__,