Python 2.x - creating a static instance of a class in one class

Question

I am trying to write "enum class" in python. Some of the trouble I'm currently experiencing is the inability to define enum values ​​inside the enum class. That is, I can do this:

class Fruit: def __init__(self, name): self.name = name class Fruits(Enum): Apple = Fruit("apple") 

But I would like to do this or a similarly legible equivalent:

 class Fruit(Enum): def __init__(self, name): self.name = name Apple = Fruit("apple") 

Unfortunately, I get the following error: name 'Fruit' not defined

What are the visibility rules in this case? Are there any little-known Python tricks that could help me? I would prefer something that can be written in the Enum metaclass, as this will make use less cumbersome for the user.

Answer 1

You can create a metaclass that would do something simple:

 class MetaEnum(type): def __new__(cls, class_name, parents, attrs): def __init__(self, name=None): if name is not None: self.name = name attrs['__init__'] = __init__ Klass = type.__new__(cls, class_name, parents, attrs) if 'instances' in attrs: for name in attrs['instances']: setattr(Klass, name.capitalize(), Klass(name)) del Klass.instances # clean up return Klass class Fruit(object): __metaclass__ = MetaEnum instances = ('apple', 'banana', 'cranberry') for attr_name in dir(Fruit): if not attr_name.startswith('_'): attr = getattr(Fruit, attr_name) if type(attr) is Fruit: print('Fruit.{}, is a Fruit named {}'.format(attr_name, getattr(attr, 'name'))) else: print('Fruit.{}, is a {}'.format(attr, type(attr))) 

Output:

 Fruit.Apple, is a Fruit named apple Fruit.Banana, is a Fruit named banana Fruit.Cranberry, is a Fruit named cranberry