python enums with attributes

Question

Consider:

class Item:
   def __init__(self, a, b):
       self.a = a
       self.b = b

class Items:
    GREEN = Item('a', 'b')
    BLUE = Item('c', 'd')

Is there a way to adapt the ideas for simple enums to this case? (see this question) Ideally, as in Java, I would like to cram it all into one class.

Java model:

enum EnumWithAttrs {
    GREEN("a", "b"),
    BLUE("c", "d");

    EnumWithAttrs(String a, String b) {
      this.a = a;
      this.b = b;
    }

    private String a;
    private String b;

    /* accessors and other java noise */
}

Answer 1

Python 3.4 has a new Enum data type (which has been backported as enum34 and enhanced as aenum¹). Both enum34 and aenum² easily support your use case:

• aenum (Python 2/3)

    import aenum
    class EnumWithAttrs(aenum.AutoNumberEnum):
        _init_ = 'a b'
        GREEN = 'a', 'b'
        BLUE = 'c', 'd'
  

• enum34 (Python 2/3) or standard library enum (Python 3.4+)

    import enum
    class EnumWithAttrs(enum.Enum):
  
        def __new__(cls, *args, **kwds):
            value = len(cls.__members__) + 1
            obj = object.__new__(cls)
            obj._value_ = value
            return obj
        def __init__(self, a, b):
            self.a = a
            self.b = b
  
        GREEN = 'a', 'b'
        BLUE = 'c', 'd'
  

And in use:

>>> EnumWithAttrs.BLUE
<EnumWithAttrs.BLUE: 1>

>>> EnumWithAttrs.BLUE.a
'c'

---

¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

²aenum also supports NamedConstants and metaclass-based NamedTuples.

Answer 2

For Python 3:

class Status(Enum):
    READY = "ready", "I'm ready to do whatever is needed"
    ERROR = "error", "Something went wrong here"

    def __new__(cls, *args, **kwds):
        obj = object.__new__(cls)
        obj._value_ = args[0]
        return obj

    # ignore the first param since it's already set by __new__
    def __init__(self, _: str, description: str = None):
        self._description_ = description

    def __str__(self):
        return self.value

    # this makes sure that the description is read-only
    @property
    def description(self):
        return self._description_

And you can use it as a standard enum or factory by type:

print(Status.READY)
# ready
print(Status.READY.description)
# I'm ready to do whatever is needed
print(Status("ready")) # this does not create a new object
# ready