renumbering chapters >= 19

24-class-metaprog/sentinel/sentinel.py

@@ -0,0 +1,41 @@
+"""
+This module provides a ``Sentinel`` class that can be used directly as a
+sentinel singleton, or subclassed if a distinct sentinel singleton is needed.
+
+The ``repr`` of a ``Sentinel`` class is its name::
+
+    >>> class Missing(Sentinel): pass
+    >>> Missing
+    Missing
+
+If a different ``repr`` is required,
+you can define it as a class attribute::
+
+    >>> class CustomRepr(Sentinel):
+    ...     repr = '<CustomRepr>'
+    ...
+    >>> CustomRepr
+    <CustomRepr>
+
+``Sentinel`` classes cannot be instantiated::
+
+    >>> Missing()
+    Traceback (most recent call last):
+      ...
+    TypeError: 'Missing' is a sentinel and cannot be instantiated
+
+"""
+
+
+class _SentinelMeta(type):
+    def __repr__(cls):
+        try:
+            return cls.repr
+        except AttributeError:
+            return cls.__name__
+
+
+class Sentinel(metaclass=_SentinelMeta):
+    def __new__(cls):
+        msg = 'is a sentinel and cannot be instantiated'
+        raise TypeError(f"'{cls!r}' {msg}")