ch25: simple enum metaclasses

25-class-metaprog/tinyenums/nanoenum.py

@@ -0,0 +1,39 @@
+"""
+Testing ``KeyIsValueDict``::
+
+    >>> adict = KeyIsValueDict()
+    >>> len(adict)
+    0
+    >>> adict['first']
+    'first'
+    >>> adict
+    {'first': 'first'}
+    >>> adict['second']
+    'second'
+    >>> len(adict)
+    2
+    >>> adict
+    {'first': 'first', 'second': 'second'}
+    >>> adict['__magic__']
+    Traceback (most recent call last):
+      ...
+    KeyError: '__magic__'
+"""
+
+
+class KeyIsValueDict(dict):
+
+    def __missing__(self, key):
+        if key.startswith('__') and key.endswith('__'):
+            raise KeyError(key)
+        self[key] = key
+        return key
+
+
+class NanoEnumMeta(type):
+    def __prepare__(name, bases, **kwargs):
+        return KeyIsValueDict()
+
+
+class NanoEnum(metaclass=NanoEnumMeta):
+    pass