adding sentinel metaclass example

25-class-metaprog/sentinel/sentinel.py

@@ -0,0 +1,24 @@
+"""
+
+    >>> class Missing(Sentinel): pass
+    >>> Missing
+    <Missing>
+    >>> class CustomRepr(Sentinel):
+    ...     repr = '*** sentinel ***'
+    ...
+    >>> CustomRepr
+    *** sentinel ***
+
+"""
+
+class SentinelMeta(type):
+    def __repr__(cls):
+        try:
+            return cls.repr
+        except AttributeError:
+            return f'<{cls.__name__}>'
+
+class Sentinel(metaclass=SentinelMeta):
+    def __new__(cls):
+        return cls
+

25-class-metaprog/sentinel/sentinel_test.py

@@ -0,0 +1,22 @@
+from sentinel import Sentinel
+
+class PlainSentinel(Sentinel): pass
+
+
+class SentinelCustomRepr(Sentinel):
+    repr = '***SentinelRepr***'
+
+
+def test_repr():
+    assert repr(PlainSentinel) == '<PlainSentinel>'
+
+
+def test_pickle():
+    from pickle import dumps, loads
+    s = dumps(PlainSentinel)
+    ps = loads(s)
+    assert ps is PlainSentinel
+
+def test_custom_repr():
+    assert repr(SentinelCustomRepr) == '***SentinelRepr***'
+