Add sentinel.py.

voussoirkit/sentinel.py

@@ -0,0 +1,26 @@
+class Sentinel:
+    '''
+    Sentinel objects are used when you need to have some kind of default,
+    placeholder, or special value that can't be confused with any other value
+    in your program. For example, if you are waiting for a function to return a
+    value, but you use `None` as a placeholder, someone might get confused and
+    think that the function actually returned None.
+
+    You can get cheap sentinels by just creating plain Python `object()`s, but
+    they are bad for printing because you'll just see an ID and not know what
+    the sentinel represents. This simple Sentinel class lets you give them a
+    name and adjust their truthyness which can be useful.
+
+    Some implementations of sentinels also make them singletons. These ones are
+    not singletons! Separate sentinels will never == each other even if you use
+    the same name!
+    '''
+    def __init__(self, name, truthyness=True):
+        self.name = name
+        self.truthyness = truthyness
+
+    def __bool__(self):
+        return bool(self.truthyness)
+
+    def __repr__(self):
+        return f'<Sentinel {repr(self.name)} id={id(self)} like {bool(self.truthyness)}>'