renumbering chapters >= 19

24-class-metaprog/tinyenums/microenum.py

@@ -0,0 +1,65 @@
+# This is an implementation of an idea by João S. O. Bueno (@gwidion)
+# shared privately with me, with permission to use in Fluent Python 2e.
+
+"""
+Testing ``WilyDict``::
+
+    >>> adict = WilyDict()
+    >>> len(adict)
+    0
+    >>> adict['first']
+    0
+    >>> adict
+    {'first': 0}
+    >>> adict['second']
+    1
+    >>> adict['third']
+    2
+    >>> len(adict)
+    3
+    >>> adict
+    {'first': 0, 'second': 1, 'third': 2}
+    >>> adict['__magic__']
+    Traceback (most recent call last):
+      ...
+    KeyError: '__magic__'
+
+Testing ``MicroEnum``::
+
+    >>> class Flavor(MicroEnum):
+    ...     cocoa
+    ...     coconut
+    ...     vanilla
+    >>> Flavor.cocoa, Flavor.vanilla
+    (0, 2)
+    >>> Flavor[1]
+    'coconut'
+"""
+
+
+class WilyDict(dict):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.__next_value = 0
+
+    def __missing__(self, key):
+        if key.startswith('__') and key.endswith('__'):
+            raise KeyError(key)
+        self[key] = value = self.__next_value
+        self.__next_value += 1
+        return value
+
+
+class MicroEnumMeta(type):
+    def __prepare__(name, bases, **kwargs):
+        return WilyDict()
+
+    def __getitem__(cls, key):
+        for k, v in cls.__dict__.items():
+            if v == key:
+                return k
+        raise KeyError(key)
+
+
+class MicroEnum(metaclass=MicroEnumMeta):
+    pass