renumbering chapters >= 19

24-class-metaprog/sentinel/sentinel.py

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+"""
+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}")

24-class-metaprog/sentinel/sentinel_test.py

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+import pickle
+
+import pytest
+
+from sentinel import Sentinel
+
+
+class PlainSentinel(Sentinel):
+    pass
+
+
+class SentinelCustomRepr(Sentinel):
+    repr = '***SentinelRepr***'
+
+
+def test_repr():
+    assert repr(PlainSentinel) == 'PlainSentinel'
+
+
+def test_cannot_instantiate():
+    with pytest.raises(TypeError) as e:
+        PlainSentinel()
+    msg = "'PlainSentinel' is a sentinel and cannot be instantiated"
+    assert msg in str(e.value)
+
+
+def test_custom_repr():
+    assert repr(SentinelCustomRepr) == '***SentinelRepr***'
+
+
+def test_pickle():
+    s = pickle.dumps(SentinelCustomRepr)
+    ps = pickle.loads(s)
+    assert ps is SentinelCustomRepr
+
+
+def test_sentinel_comes_ready_to_use():
+    assert repr(Sentinel) == 'Sentinel'
+    s = pickle.dumps(Sentinel)
+    ps = pickle.loads(s)
+    assert ps is Sentinel