Python3 - Convert enum to different datatypes

Question

I'm currently working with an enum of the following type:

class System(Enum):
  FIRST = 1
  SECOND = 2

Now I'd like to be able to do the following:

a = System.FIRST
url = a.getSystemURL()

where the url for the enumeration members FIRST and SECOND are different of course.

I could create a dictionary with the enumeration members as keys and the urls as values, but this won't assure that if I later add an enumeration member I'll remember to add corresponding the dictionary entry.

Is there a clean way to have an enumeration with multiple values for the enumeration members? And to name these different values?

Something like this:

class System(Enum):
  Values = (Value, url, something)
  FIRST = 1, 'https://www.example.com', 42
  SECOND = 2, 'https://www.test.com', 13

Aran-Fey · Accepted Answer · 2018-03-25 23:13:24Z

5

There is an example like this in the documentation. If the class defines an __init__ method, the enum values will be passed to it as arguments. This means you can define your enum like so:

class System(Enum):
    FIRST = 1, 'https://www.example.com', 42
    SECOND = 2, 'https://www.test.com', 13

    def __init__(self, value, url, something):
        self.value_ = value
        self.url = url
        self.something = something

(Note that value is a special attribute reserved by enums, so I named the attribute value_ to avoid a name clash.)

You can now access these attributes on each enum member:

>>> System.FIRST.url
'https://www.example.com'

answered Mar 25, 2018 at 23:13

Aran-Fey

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2 Comments

user9115052 Over a year ago

Thank you. This is exactly what I was looking for! If I only use one property. Should I still add the ints to the definition? For the enum would you prefer just to put the url so enum_mem.value and enum_mem.url will give the same result?

Aran-Fey Over a year ago

@user9115052 I'd say it depends on your use case. Generally speaking, there's no need to have an integer associated with each enum value. But if you have a use for that integer in your program, go ahead and add it.

Ethan Furman · Accepted Answer · 2018-03-27 17:08:13Z

Both the stdlib Enum and aenum¹ easily support your use case (although more work is required for the stdlib version).

stdlib enum 3.4+ (See the docs for __new__ and __init__ explanations.)

import enum
class System(enum.Enum):

    def __new__(cls, *args, **kwds):
        value = len(cls.__members__) + 1
        obj = object.__new__(cls)
        obj._value_ = value
        return obj

    def __init__(self, url, something):
        self.url = url
        self.something = something

    FIRST = 'https://www.example.com', 42
    SECOND = 'https://www.test.com', 13

aenum -- Usable for both Python 2 and 3 code. aenum also makes a few things easier -- the above code would be:

import aenum
class System(aenum.AutoNumberEnum):
    _init_ = 'url something'
    FIRST = 'https://www.example.com', 42
    SECOND = 'https://www.test.com', 13

And in use:

--> System.FIRST
<System.First: 1>

--> System.FIRST.url
'https://www.example.com'

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

Patrick Haugh · Accepted Answer · 2018-03-25 23:05:10Z

0

Make the values a namedtuple.

from collections import namedtuple
from enum import Enum

Entry = namedtuple('Entry', ['value', 'url', 'something'])

class System(Enum):
  FIRST = Entry(1, 'https://www.example.com', 42)
  SECOND = Entry(2, 'https://www.test.com', 13)

print(System.FIRST.value.url)
# https://www.example.com

answered Mar 25, 2018 at 23:05

Patrick Haugh

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Comments

abarnert · Accepted Answer · 2018-03-25 23:12:56Z

0

Have you tried the code you wrote? It already just works! Of course you don't really have multiple values, you have a single tuple value—but that isn't a problem:

>>> class System(Enum):
...     FIRST = 1, 'https://www.example.com', 42
...     SECOND = 2, 'https://www.test.com', 13
>>> _, url, _ = System.SECOND.value
>>> url
'https://www.example.com'
>>> a = System.FIRST
>>> url = a.value[1]
>>> url
'https://www.example.com'

There are things you can do to make this a little nicer, like using a namedtuple instead of a plain tuple so you can write a.value.url instead of a.value[1], but you've already written something that works.

answered Mar 25, 2018 at 23:12

abarnert

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