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Stack OverflowHow can I represent an 'Enum' in Python?
[+947] [46] sectrean
[2008-08-31 15:55:47]
[ python enums ]
[ http://stackoverflow.com/questions/36932/how-can-i-represent-an-enum-in-python ]

I'm mainly a C# developer, but I'm currently working on a project in Python.

How can I represent the equivalent of an Enum in Python?

(12) I would be interested in a use case for enums in python, I didn't encounter such a need so far. - Paweł Prażak
I'm doing a python project on polygon triangulation, and I need to classify the vertices of a polygon into one of five types before a plane sweep, and during the plane sweep the usual CCW sorted order of the vertices is destroyed in favor of a lexicographic ordering. Precomputing the types as enums and looking up the type in a hash table later is the natural choice. - JeremyKun
(49) My most common use cases for enums are: 0) tokens to control operation of some function or class, and 1) states for a state machine. As an example of (0), all the constants in the Python stat module should IMHO be enums rather than just bare int constants. As an example of (1), imagine a state machine for controlling (e.g.) a plastic casting machine, with states like: cold, heating, ready, close_mould, injecting, open_mould, eject_casting You could usefully write code like if state < ready: which would be true for both cold and heating. - steveha
(19) @PawełPrażak , I think a more constructive comment would have been to explain how you would in Python solve a problem similar to one which you would use enums to solve in C. Or explain why such problems don't exist in Python. - Brennan Vincent
(1) @BrennanVincent I'm afraid it could be rather difficult because there isn't enough context provided in the question about the underlying problem to be solved. So IMHO the only way to answer question How do I do enums in Python is to say You just don't ;) - Paweł Prażak
Please consider accepting Danilo Bargen's answer… - Neil G
This tutorial shows a decent example of how to: dreamsyssoft.com/python-scripting-tutorial/enum-tutorial.php - Triton Man
Now there is a standard Enum type in Python 3.4. Read this post: stackoverflow.com/questions/16653129/… - Javier
[+989] [2009-11-08 03:15:28] Alec Thomas [ACCEPTED]

Enums have been added to Python 3.4 as described in PEP 435 [1]. It has also been backported to 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4 [2] on pypi.

from enum import Enum
Animal = Enum('Animal', 'ant bee cat dog')

or equivalently:

class Animals(Enum):
    ant = 1
    bee = 2
    cat = 3
    dog = 4

In earlier versions, one way of accomplishing enums is:

def enum(**enums):
    return type('Enum', (), enums)

which is used like so:

>>> Numbers = enum(ONE=1, TWO=2, THREE='three')
>>> Numbers.ONE
1
>>> Numbers.TWO
2
>>> Numbers.THREE
'three'

You can also easily support automatic enumeration with something like this:

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    return type('Enum', (), enums)

and used like so:

>>> Numbers = enum('ZERO', 'ONE', 'TWO')
>>> Numbers.ZERO
0
>>> Numbers.ONE
1

Support for converting the values back to names can be added this way:

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    reverse = dict((value, key) for key, value in enums.iteritems())
    enums['reverse_mapping'] = reverse
    return type('Enum', (), enums)

This overwrites anything with that name, but it is useful for rendering your enums in output. It will throw KeyError if the reverse mapping doesn't exist. With the first example:

>>> Numbers.reverse_mapping['three']
'THREE'
[1] http://www.python.org/dev/peps/pep-0435/
[2] https://pypi.python.org/pypi/enum34

(13) This just went into my snippet database - bravo for Python type creation! - new123456
(13) +1 for a none-to-novel approach to using type() docs.python.org/library/functions.html#type - Zachary Young
(5) Ingenious solution! I wonder if/how it could be made to print the name of the identifiers when debugging? For example, having print "The value is %r" % Numbers.ONE output The value is ONE instead of The value is 1... - Pascal Bourque
(2) Note that this isn't really that similar to a C enum, in that one cannot meaningfully have variables/objects of a type returned by enum. - Marcin
neat solution. @Pascal Bourque, this is not exactly what you want, but may give you some idea: stackoverflow.com/a/11147900/545299 - Oxdeadbeef
(3) Although the values themselves aren't typed, you can type the enumeration class so they're distinct from each other: return type(named.get('enum_type', 'Enum'), (), enums) then calling with enum('ONE', 'TWO', THREE='three', enum_type='Numbers') - Jake Biesinger
This is very similar to my enum snippet: def enum(*args): return type('Enum', (object,), dict(zip(args, iter(object, None)))). - DRayX
@PascalBourque: Just added a reverse mapping to address your question. See the bottom - acjay
I said I'd never want enums in Python. But here I am, writing several little state machines, and suddenly I really do. This is an excellent solution! - Tom Swirly
This edit should be a separate answer. - Tyler Crompton
(1) Oh, I noticed a slight improvement that could be made. zip(x, range(len(x)) is a code smell - it can be replaced by enumerate to get enums = dict((x, i) for i, x in enumerate(sequential)), **named) which doesn't construct all those lists that zip/range do. - Tom Swirly
@Alec Thomas I think the reverse_mapping is broken since Numbers.THREE == Numbers.reverse_mapping['three'] is false. Is there a simple fix for this? - ArtB
(1) @ArtB That is by design: Numbers.THREE == 'three' and Numbers.reverse_mapping['three'] == 'THREE'. - Alec Thomas
Sorry, I was expecting Java's Enum.valueOf semantics where the reverse gives you the enum. - ArtB
Is that slower than the named tuple? - Nikolay Golub
(1) It may be of value to compare your solutions with PEP 435. - Thijs van Dien
(1) Thanks for the enum implementation. But is it possible to find the length of the class? - Winston
I am a newbie to Python. I have used the above version in my code which is on Python2.7 and it works fine.. Now I want to iterate over the above enumeration (python2.7) version but can't seem to do it..I presume it has to implement something like iterable like in Java.. any ideas please? - quirkystack
For Python 2.7 I prefer the simpler answer below, because my IDE (PyCharm) can auto-complete members. - Toby Champion
Very useful, but one caveat compared to C enums: In the solution (for older pythons) the names are not constants: Numbers.ONE = 2 could be typed later, altering the value. See @hans_meine 's named_constants for a safer solution: github.com/hmeine/named_constants - ToolmakerSteve
Here, building on others' work, I now have developed a simple solution to enum-like type.attribute named constants; unlike the above answer, the values are constants (read-only). stackoverflow.com/a/20508128/199364 Thanks for the inspiration to find an even better way! - ToolmakerSteve
The problem with enum package is that its enum types don't survive pickling-unpickling. Create an enumaration Foo = Enum("foo", "bar"), then compare pickle.loads(pickle.dumps(Foo.foo)) == Foo.foo; the result is False. It hurts when you have a multiprocessing program, because seemingly equal values stop passing equality check. - sastanin
1
[+432] [2008-08-31 16:06:14] Alexandru Nedelcu

Before PEP-435, Python didn't have an equivalent but you could implement your own.

Myself, I like keeping it simple (I've seen some horribly complex examples on the net), something like this ...

class Animal:
    DOG = 1
    CAT = 2

x = Animal.DOG

In Python 3.4 ( PEP 435 [1]), you can make Enum the base class. This gets you a little bit of extra functionality, described in the PEP. For example, enum values are distinct from integers.

class Animal(Enum):
    DOG = 1
    CAT = 2

print(Animal.DOG)
<Animal.DOG: 1>

If you don't want to type the values, use the following shortcut:

class Animal(Enum):
    DOG, CAT = range(2)
[1] http://www.python.org/dev/peps/pep-0435/

(1) What's the point of defining numerical values (1 and 2)? They seem useless, and that's why I prefer zacherates' solution. - bortzmeyer
(2) It is clear from the example that constants are being defined. There is an issue with type safety, however, which may cause problems. - strager
You should have first created an instance of Animal class.. X = Animal() z = X.DOG - aatifh
(27) No, it's a class variable. - Georg Schölly
(8) -1, there are several reasons that the Type-Safe Enum pattern was created. See java.sun.com/developer/Books/shiftintojava/page1.html. Yes I know that this is a python question, but a good pattern is a good pattern. - WolfmanDragon
(170) Python is dynamic by default. There's no valid reason to enforce compile-time safety in a language like Python, especially when there is none. And another thing ... a good pattern is only good in the context in which it was created. A good pattern can also be superseded or completely useless, depending on the tools you're using. - Alexandru Nedelcu
(2) Using range is generally better - Casebash
what if we inherit set and use getattr method to use Enum, see stackoverflow.com/questions/36932/… - shahjapan
What if I have 100 values, I have to write the numbers for each of them? Yuk! - L̲̳o̲̳̳n̲̳̳g̲̳̳p̲̳o̲̳̳k̲̳̳e̲̳̳
(11) @Longpoke if you have 100 values, then you're definitely doing something wrong ;) I like numbers associated with my enums ... they are easy to write (vs strings), can be easily persisted in a database, and are compatible with the C/C++ enum, which makes for easier marshaling. - Alexandru Nedelcu
(37) I use this, with the numbers replaced by object(). - Tobu
(2) X = object() is inconvenient because it doesn't know what it is (you can only compare to namespace.X), and risky because copy.deepcopy() or serialization/deserialiaztion creates a new one, which isn't equal to any of those you defined! Numbers are at least safe, but strings are usually better. - Beni Cherniavsky-Paskin
Would this be better implemented with a dictionary, so that it would be easier to match the animal type with its ordinal value? - kdazzle
Can someone explain to me what is going on here? - g33kz0r
(4) The original PEP354 is no longer merely rejected, but is now marked superseded. PEP435 adds a standard Enum for Python 3.4. See python.org/dev/peps/pep-0435 - Peter Hansen
(1) This answer should also mention the flufl.enum package and enables backcomp for python<3.4. - Voo
2
[+187] [2010-02-02 07:21:46] shahjapan

Here is what I use:

class Enum(set):
    def __getattr__(self, name):
        if name in self:
            return name
        raise AttributeError

Here is its implementation:

Animals = Enum(["DOG", "CAT", "HORSE"])

print(Animals.DOG)

(24) I'd say this is the most Pythonic, and correct, way. Implementing it now! - Humphrey Bogart
(24) Excellent. This can be further improved by overriding __setattr__(self, name, value) and maybe __delattr__(self, name) so that if you accidentally write Animals.DOG = CAT, it won't silently succeed. - Joonas Pulakka
(8) @shahjapan: Interesting, but relatively slow: a test is done for each access like Animals.DOG; also, the values of the constats are strings, so that comparisons with these constants are slower than if, say, integers were allowed as values. - EOL
@EOL yes that might be slower, but comparing to other solutions this solution found more convenient, user friendly, more readable then others and hence code maintenance will be much easier & faster... - shahjapan
(2) @shahjapan: I would argue that this solution is not as legible as the shorter solutions of Alexandru or Mark, for instance. It's an interesting solution, though. :) - EOL
I tried using setattr() function inside __init__() method instead of overidding __getattr__() method. I am assume this is supposed work the same way: class Enum(object): def __init__(self, enum_string_list): if type(enum_string_list) == list: for enum_string in enum_string_list: setattr(self, enum_string, enum_string) else: raise AttributeError - Harshith J.V.
(3) The faster, really pythonic solution would instead use try/except rather than that if clause. It's better to beg for forgiveness than to ask for permission. - André Terra
I had upvoted this post and copied and pasted the code without testing it. Now I see that the enum value is not an integer, but a string. Animals.DOG prints "DOG", not 0. - user2233706
3
[+111] [2008-08-31 20:31:22] Mark Harrison

If you need the numeric values, here's the quickest way:

dog, cat, rabbit = range(3)

(24) You can make it even shorter: dog, cat, rabbit = range(3) You don't really need the parens around the tuple unpacking, and of course range() goes from 0 by default. - steveha
(32) But then you have to count the number of enumerated things, which can be very annoying, it should be done automatically. - L̲̳o̲̳̳n̲̳̳g̲̳̳p̲̳o̲̳̳k̲̳̳e̲̳̳
4
[+62] [2009-10-07 02:47:33] Ashwin

The best solution for you would depend on what you require from your fake enum.

Simple enum:

If you need the enum as only a list of names identifying different items, the solution by Mark Harrison (above) is great:

Pen, Pencil, Eraser = range(0, 3)

Using a range also allows you to set any starting value:

Pen, Pencil, Eraser = range(9, 12)

In addition to the above, if you also require that the items belong to a container of some sort, then embed them in a class:

class Stationary:
    Pen, Pencil, Eraser = range(0, 3)

To use the enum item, you would now need to use the container name and the item name:

stype = Stationary.Pen

Complex enum:

For long lists of enum or more complicated uses of enum, these solutions will not suffice. You could look to the recipe by Will Ware for Simulating Enumerations in Python published in the Python Cookbook. An online version of that is available here [1].

More info:

PEP 354: Enumerations in Python [2] has the interesting details of a proposal for enum in Python and why it was rejected.

[1] http://code.activestate.com/recipes/67107/
[2] http://www.python.org/dev/peps/pep-0354/

I like this solution; short, simple; just like the C enum... - Malkocoglu
This is exactly what I was looking for, great compromise between usefulness and complexity. - Sergio Acosta
5
[+61] [2008-09-01 16:05:25] Aaron Maenpaa

The typesafe enum pattern which was used in Java pre-JDK 5 has a number of advantages. Much like in Alexandru's answer, you create a class and class level fields are the enum values; however, the enum values are instances of the class rather than small integers. This has the advantage that your enum values don't inadvertently compare equal to small integers, you can control how they're printed, add arbitrary methods if that's useful and make assertions using isinstance:

class Animal:
   def __init__(self, name):
       self.name = name

   def __str__(self):
       return self.name

   def __repr__(self):
       return "<Animal: %s>" % self

Animal.DOG = Animal("dog")
Animal.CAT = Animal("cat")

>>> x = Animal.DOG
>>> x
<Animal: dog>
>>> x == 1
False

A recent thread on python-dev [1] pointed out there are a couple of enum libraries in the wild, including:

[1] http://mail.python.org/pipermail/python-dev/2010-November/105873.html
[2] http://packages.python.org/flufl.enum/docs/using.html
[3] http://pypi.python.org/pypi/lazr.enum
[4] http://pypi.python.org/pypi/enum/

(13) I think this is a very bad approach. Animal.DOG = Animal("dog") Animal.DOG2 = Animal("dog") assert Animal.DOG == Animal.DOG2 fails... - Confusion
(8) @Confusion The user isn't supposed to call the constructor, the fact that there's even a constructor is an implementation detail and you have to communicate to who ever is using your code that making new enumeration values makes no sense and that exiting code will not "do the right thing". Of course that doesn't stop you from implementing Animal.from_name("dog") --> Animal.DOG. - Aaron Maenpaa
(9) "the advantage that your enum values don't inadvertently compare equal to small integers" What's the advantage in this? What's wrong with comparing your enum to integers? Especially if you store the enum in the database, you usually want it to be stored as integers, so you'll have to compare it to integers at some point. - ibz
@AaronMcSmooth Incorrect: dir(Animal) --> ['CAT', 'DOG', 'doc', 'init', 'module', 'repr', 'str'] - Aaron Maenpaa
(3) @Aaaron Maenpaa. correct. It's still a broken and overly complicated way to do it. - aaronasterling
(4) @AaronMcSmooth That really depends on whether you're coming in from the C perspective of "Enums are just names for a couple of ints" or the more object oriented approach where enum values are actual objects and have methods (which is how enums in Java 1.5 are, and which the type safe enum pattern was going for). Personally, I don't like switch statements so I lean towards enum values that are actual objects. - Aaron Maenpaa
How does this implementation of the type-safe enum pattern allow each enum value to provide its own implementation of a method? - Nathan
@Nathan You can declare a (unbound) function, bind it with get to the DOG object, for example, and then add it as a member of DOG. Or you can create a subclass of Animal for each constant that needs a specific method. - Thiago Chaves
Now try to do Flags.FLAG1 | Flags.FLAG2, oh, it doesn't work! - LtWorf
6
[+35] [2008-08-31 18:10:50] dF.

Python doesn't have a built-in equivalent to enum, and other answers have ideas for implementing your own (you may also be interested in the over the top version [1] in the Python cookbook).

However, in situations where an enum would be called for in C, I usually end up just using simple strings: because of the way objects/attributes are implemented, (C)Python is optimized to work very fast with short strings anyway, so there wouldn't really be any performance benefit to using integers. To guard against typos / invalid values you can insert checks in selected places.

ANIMALS = ['cat', 'dog', 'python']

def take_for_a_walk(animal):
    assert animal in ANIMALS
    ...

(One disadvantage compared to using a class is that you lose the benefit of autocomplete)

[1] http://code.activestate.com/recipes/67107/

I prefer this solution. I like to use built-in types where possible. - Seun Osewa
That version isn't really over the top. It just has a lot of supplied testing code - Casebash
(1) Actually, the "correct" version is in the comments and is much more complex - the main version has a minor bug. - Casebash
7
[+29] [2010-11-03 23:02:54] royal

So, I agree. Let's not enforce type safety in Python, but I would like to protect myself from silly mistakes. So what do we think about this?

class Animal(object):
    values = ['Horse','Dog','Cat']

    class __metaclass__(type):
        def __getattr__(self, name):
            return self.values.index(name)

It keeps me from value-collision in defining my enums.

>>> Animal.Cat
2

There's another handy advantage: really fast reverse lookups:

def nameOf(self, i):
    return Animal.values[i]

(1) Thanks! Ended up using it last-night and realized it also has the advantage of very fast reverse lookups... (above) - royal
(5) You can use self instead of Animal in metaclass, btw. name_of is more pythonic IMHO than nameOf - Paweł Prażak
I like this, but you might as well lock values in for efficiency with a tuple? I played around with it and came up with a version that sets self.values from args in init. It is nice to be able to declare Animal = Enum('horse', 'dog', 'cat'). I also catch the ValueError in getattr in the event of a missing item in self.values -- it seems better to raise an AttributeError with the supplied name string instead. I couldn't get the metaclass to work in Python 2.7 based on my limited knowledge in that area, but my custom Enum class works fine with straight instance methods. - trojjer
8
[+24] [2008-09-20 11:49:38] user18695
def M_add_class_attribs(attribs):
    def foo(name, bases, dict_):
        for v, k in attribs:
            dict_[k] = v
        return type(name, bases, dict_)
    return foo

def enum(*names):
    class Foo(object):
        __metaclass__ = M_add_class_attribs(enumerate(names))
        def __setattr__(self, name, value):  # this makes it read-only
            raise NotImplementedError
    return Foo()

Use it like this:

Animal = enum('DOG', 'CAT')
Animal.DOG # returns 0
Animal.CAT # returns 1
Animal.DOG = 2 # raises NotImplementedError

if you just want unique symbols and don't care about the values, replace this line:

__metaclass__ = M_add_class_attribs(enumerate(names))

with this:

__metaclass__ = M_add_class_attribs((object(), name) for name in names)

(11) IMHO it would be cleaner if you changed enum(names) to enum(*names) - then you could drop the extra parenthesis when calling it. - Chris Lutz
I like this approach. I actually changed it to set the attribute value to the same string as the name, which has the nice property that Animal.DOG == 'DOG', so they stringify themselves for you automatically. (Helps immensely for printing out debug output.) - Ted Mielczarek
9
[+22] [2012-02-08 20:59:58] Zoetic

An Enum class can be a one-liner.

class Enum(tuple): __getattr__ = tuple.index

How to use it (forward and reverse lookup, keys, values, items, etc.)

>>> State = Enum(['Unclaimed', 'Claimed'])
>>> State.Claimed
1
>>> State[1]
'Claimed'
>>> State
('Unclaimed', 'Claimed')
>>> range(len(State))
[0, 1]
>>> [(k, State[k]) for k in range(len(State))]
[(0, 'Unclaimed'), (1, 'Claimed')]
>>> [(k, getattr(State, k)) for k in State]
[('Unclaimed', 0), ('Claimed', 1)]

(1) That looks pretty clever! - K. Brafford
Probably my favourite so far! - MiJyn
10
[+14] [2010-11-29 02:05:55] mbac32768

I prefer to define enums in Python like so:

class Animal:
  class Dog: pass
  class Cat: pass

x = Animal.Dog

It's more bug-proof than using integers since you don't have to worry about ensuring that the integers are unique (e.g. if you said Dog = 1 and Cat = 1 you'd be screwed).

It's more bug-proof than using strings since you don't have to worry about typos (e.g. x == "catt" fails silently, but x == Animal.Catt is a runtime exception).


11
[+13] [2013-05-10 16:09:02] Danilo Bargen

On 2013-05-10, Guido agreed to accept PEP 435 [1] into the Python 3.4 standard library. This means that Python finally has builtin support for enumerations!

There is a backport available for Python 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4. It's on Pypi as enum34 [2].

Declaration:

>>> from enum import Enum
>>> class Color(Enum):
...     red = 1
...     green = 2
...     blue = 3

Representation:

>>> print(Color.red)
Color.red
>>> print(repr(Color.red))
<Color.red: 1>

Iteration:

>>> for color in Color:
...   print(color)
...
Color.red
Color.green
Color.blue

Programmatic access:

>>> Color(1)
Color.red
>>> Color['blue']
Color.blue

For more information, refer to the proposal [3]. Official documentation will probably follow soon.

[1] http://www.python.org/dev/peps/pep-0435/
[2] https://pypi.python.org/pypi/enum34
[3] http://www.python.org/dev/peps/pep-0435/

12
[+12] [2009-02-02 23:39:53] Luciano Ramalho

What I use:

class Enum(object):
    def __init__(self, names, separator=None):
        self.names = names.split(separator)
        for value, name in enumerate(self.names):
            setattr(self, name.upper(), value)
    def tuples(self):
        return tuple(enumerate(self.names))

How to use:

>>> state = Enum('draft published retracted')
>>> state.DRAFT
0
>>> state.RETRACTED
2
>>> state.FOO
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'Enum' object has no attribute 'FOO'
>>> state.tuples()
((0, 'draft'), (1, 'published'), (2, 'retracted'))

So this gives you integer constants like state.PUBLISHED and the two-tuples to use as choices in Django models.


13
[+11] [2008-08-31 16:09:53] dguaraglia

Hmmm... I suppose the closest thing to an enum would be a dictionary, defined either like this:

months = {
    'January': 1,
    'February': 2,
    ...
}

or

months = dict(
    January=1,
    February=2,
    ...
)

Then, you can use the symbolic name for the constants like this:

mymonth = months['January']

There are other options, like a list of tuples, or a tuple of tuples, but the dictionary is the only one that provides you with a "symbolic" (constant string) way to access the value.

Edit: I like Alexandru's answer too!


And most of all you can easily iterate on a dictionary if you need to access its values like you need its string values to appear as combo box items. So use a dictionary as a replacement for enumerations instead. - Lemuel Adane
14
[+11] [2011-08-07 05:51:01] agf

Another, very simple, implementation of an enum in Python, using namedtuple:

from collections import namedtuple

def enum(*keys):
    return namedtuple('Enum', keys)(*keys)

MyEnum = enum('FOO', 'BAR', 'BAZ')

or, alternatively,

# With sequential number values
def enum(*keys):
    return namedtuple('Enum', keys)(*range(len(keys)))

# From a dict / keyword args
def enum(**kwargs):
    return namedtuple('Enum', kwargs.keys())(*kwargs.values())

Like the method above that subclasses set, this allows:

'FOO' in MyEnum
other = MyEnum.FOO
assert other == MyEnum.FOO

But has more flexibility as it can have different keys and values. This allows

MyEnum.FOO < MyEnum.BAR

to act as is expected if you use the version that fills in sequential number values.


You lack two *'s: should be *range(len(keys) and *kwargs.values() . . . I don't have edit privileges or I'd fix it up. - Ben Roberts
Fixed it, thanks. - agf
15
[+9] [2008-09-02 03:20:30] tuxedo

davidg recommends using dicts. I'd go one step further and use sets:

months = set('January', 'February', ..., 'December')

Now you can test whether a value matches one of the values in the set like this:

if m in months:

like dF, though, I usually just use string constants in place of enums.


yep!, much better if u inherit set and provide getattr method ! - shahjapan
16
[+9] [2009-11-18 02:51:40] steveha

This is the best one I have seen: "First Class Enums in Python"

http://code.activestate.com/recipes/413486/

It gives you a class, and the class contains all the enums. The enums can be compared to each other, but don't have any particular value; you can't use them as an integer value. (I resisted this at first because I am used to C enums, which are integer values. But if you can't use it as an integer, you can't use it as an integer by mistake so overall I think it is a win.) Each enum is a unique value. You can print enums, you can iterate over them, you can test that an enum value is "in" the enum. It's pretty complete and slick.

Edit (cfi): The above link is not Python 3 compatible. Here's my port of enum.py to Python 3:

def cmp(a,b):
   if a < b: return -1
   if b < a: return 1
   return 0


def Enum(*names):
   ##assert names, "Empty enums are not supported" # <- Don't like empty enums? Uncomment!

   class EnumClass(object):
      __slots__ = names
      def __iter__(self):        return iter(constants)
      def __len__(self):         return len(constants)
      def __getitem__(self, i):  return constants[i]
      def __repr__(self):        return 'Enum' + str(names)
      def __str__(self):         return 'enum ' + str(constants)

   class EnumValue(object):
      __slots__ = ('__value')
      def __init__(self, value): self.__value = value
      Value = property(lambda self: self.__value)
      EnumType = property(lambda self: EnumType)
      def __hash__(self):        return hash(self.__value)
      def __cmp__(self, other):
         # C fans might want to remove the following assertion
         # to make all enums comparable by ordinal value {;))
         assert self.EnumType is other.EnumType, "Only values from the same enum are comparable"
         return cmp(self.__value, other.__value)
      def __lt__(self, other):   return self.__cmp__(other) < 0
      def __eq__(self, other):   return self.__cmp__(other) == 0
      def __invert__(self):      return constants[maximum - self.__value]
      def __nonzero__(self):     return bool(self.__value)
      def __repr__(self):        return str(names[self.__value])

   maximum = len(names) - 1
   constants = [None] * len(names)
   for i, each in enumerate(names):
      val = EnumValue(i)
      setattr(EnumClass, each, val)
      constants[i] = val
   constants = tuple(constants)
   EnumType = EnumClass()
   return EnumType


if __name__ == '__main__':
   print( '\n*** Enum Demo ***')
   print( '--- Days of week ---')
   Days = Enum('Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su')
   print( Days)
   print( Days.Mo)
   print( Days.Fr)
   print( Days.Mo < Days.Fr)
   print( list(Days))
   for each in Days:
      print( 'Day:', each)
   print( '--- Yes/No ---')
   Confirmation = Enum('No', 'Yes')
   answer = Confirmation.No
   print( 'Your answer is not', ~answer)

This is an excellent, robust solution. It met all the criteria I had that several of the more popular suggestions lacked. - Gregyski
This recipe was used as the basis for a PEP, which was rejected. python.org/dev/peps/pep-0354 One extension that I like: enum values should have a member variable that lets you get the internal integer value out. It shouldn't be possible to cast an enum to an integer by mistake, so the .__int__() method should raise an exception for an enum; but there should be a way to get the value out. And it should be possible to set specific integer values in at class definition time, so you could use an enum for things like the constants in the stat module. - steveha
17
[+6] [2011-12-22 02:16:04] IfLoop

I have had occasion to need of an Enum class, for the purpose of decoding a binary file format. The features I happened to want is concise enum definition, the ability to freely create instances of the enum by either integer value or string, and a useful representation. Here's what I ended up with:

>>> class Enum(int):
...     def __new__(cls, value):
...         if isinstance(value, str):
...             return getattr(cls, value)
...         elif isinstance(value, int):
...             return cls.__index[value]
...     def __str__(self): return self.__name
...     def __repr__(self): return "%s.%s" % (type(self).__name__, self.__name)
...     class __metaclass__(type):
...         def __new__(mcls, name, bases, attrs):
...             attrs['__slots__'] = ['_Enum__name']
...             cls = type.__new__(mcls, name, bases, attrs)
...             cls._Enum__index = _index = {}
...             for base in reversed(bases):
...                 if hasattr(base, '_Enum__index'):
...                     _index.update(base._Enum__index)
...             # create all of the instances of the new class
...             for attr in attrs.keys():
...                 value = attrs[attr]
...                 if isinstance(value, int):
...                     evalue = int.__new__(cls, value)
...                     evalue._Enum__name = attr
...                     _index[value] = evalue
...                     setattr(cls, attr, evalue)
...             return cls
... 

A whimsical example of using it:

>>> class Citrus(Enum):
...     Lemon = 1
...     Lime = 2
... 
>>> Citrus.Lemon
Citrus.Lemon
>>> 
>>> Citrus(1)
Citrus.Lemon
>>> Citrus(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __new__
KeyError: 5
>>> class Fruit(Citrus):
...     Apple = 3
...     Banana = 4
... 
>>> Fruit.Apple
Fruit.Apple
>>> Fruit.Lemon
Citrus.Lemon
>>> Fruit(1)
Citrus.Lemon
>>> Fruit(3)
Fruit.Apple
>>> "%d %s %r" % ((Fruit.Apple,)*3)
'3 Apple Fruit.Apple'
>>> Fruit(1) is Citrus.Lemon
True

Key features:

  • str(), int() and repr() all produce the most useful output possible, respectively the name of the enumartion, its integer value, and a Python expression that evaluates back to the enumeration.
  • Enumerated values returned by the constructor are limited strictly to the predefined values, no accidental enum values.
  • Enumerated values are singletons; they can be strictly compared with is

I really like the use of a superclass with its own metaclass, to make it easy to define enums. What's missing here is a __contains__ method. I'd like to be able to check that a given variable is part of the enum - mostly because I want the enums for allowable values of a function parameter. - xorsyst
This is actually a slightly trimmed version of the one I originally created (which you can find here: enum_strict.py )v which defines a __instancecheck__ method. Classes are not collections of instances, so 1 in Fruit is absurd. However, the linked version supports isinstance(1, Fruit) which would be more correct in terms of the notion of classes and instances. - IfLoop
But forgetting classes and thinking in terms of enums, then it makes sense to think of them as a collection. For example, I might have an enum of file opening modes (MODE.OPEN, MODE.WRITE, etc). I want to verify the parameters to my function: if mode in MODE: reads a lot better than isintance(mode, Mode) - xorsyst
18
[+5] [2013-05-10 16:22:42] Riaz Rizvi

The new standard in Python is PEP 435 [1], so an Enum class will be available in future versions of Python:

>>> from enum import Enum

However to begin using it now you can install the original library [2] that motivated the PEP:

#sudo pip install flufl.enum   //or #sudo easy_install flufl.enum

Then you can use it as per its online guide [3]:

>>> from flufl.enum import Enum
>>> class Colors(Enum):
...     red = 1
...     green = 2
...     blue = 3
>>> for color in Colors: print color
Colors.red
Colors.green
Colors.blue
[1] http://www.python.org/dev/peps/pep-0435/
[2] http://bazaar.launchpad.net/~barry/flufl.enum/trunk/view/head:/flufl/enum/README.rst
[3] http://pythonhosted.org/flufl.enum/docs/using.html

19
[+3] [2008-09-19 03:37:43] Rick Harris

Alexandru's suggestion of using class constants for enums works quite well.

I also like to add a dictionary for each set of constants to lookup a human-readable string representation.

This serves two purposes: a) it provides a simple way to pretty-print your enum and b) the dictionary logically groups the constants so that you can test for membership.

class Animal:    
  TYPE_DOG = 1
  TYPE_CAT = 2

  type2str = {
    TYPE_DOG: "dog",
    TYPE_CAT: "cat"
  }

  def __init__(self, type_):
    assert type_ in self.type2str.keys()
    self._type = type_

  def __repr__(self):
    return "<%s type=%s>" % (
        self.__class__.__name__, self.type2str[self._type].upper())

20
[+2] [2010-03-16 22:36:42] pythonic metaphor

The enum package from PyPI [1] provides a robust implementation of enums. An earlier answer mentioned PEP 354; this was rejected but the proposal was implemented http://pypi.python.org/pypi/enum.

Usage is easy and elegant:

>>> from enum import Enum
>>> Colors = Enum('red', 'blue', 'green')
>>> shirt_color = Colors.green
>>> shirt_color = Colors[2]
>>> shirt_color > Colors.red
True
>>> shirt_color.index
2
>>> str(shirt_color)
'green'
[1] http://en.wikipedia.org/wiki/Python_Package_Index

21
[+2] [2011-06-04 11:24:06] Cameron Hayne

I needed the possibility of having the enum values be floats (not just integers) for use in defining API's where the enum classes are part of the API. My requirements (and implementation) are in the blog post Enums in Python: a more flexible and powerful implementation [1].

[1] http://franktheblue.blogspot.com/2011/05/enums-in-python-more-flexible-and.html

22
[+2] [2011-09-18 01:26:54] Michael Truog

This solution is a simple way of getting a class for the enumeration defined as a list (no more annoying integer assignments):

enumeration.py:

import new

def create(class_name, names):
    return new.classobj(
        class_name, (object,), dict((y, x) for x, y in enumerate(names))
    )

example.py:

import enumeration

Colors = enumeration.create('Colors', (
    'red',
    'orange',
    'yellow',
    'green',
    'blue',
    'violet',
))

(1) This is a really old-timey way to create classes. Why not simply use type(class_name, (object,), dict(...)) instead? - terminus
23
[+2] [2012-01-18 06:09:18] bj0

I really like Alec Thomas' solution (http://stackoverflow.com/a/1695250):

def enum(**enums):
    '''simple constant "enums"'''
    return type('Enum', (object,), enums)

It's elegant and clean looking, but it's just a function that creates a class with the specified attributes.

With a little modification to the function, we can get it to act a little more 'enumy':

NOTE: I created the following examples by trying to reproduce the behavior of pygtk's new style 'enums' (like Gtk.MessageType.WARNING)

def enum_base(t, **enums):
    '''enums with a base class'''
    T = type('Enum', (t,), {})
    for key,val in enums.items():
        setattr(T, key, T(val))

    return T

This creates an enum based off a specified type. In addition to giving attribute access like the previous function, it behaves as you would expect an Enum to with respect to types. It also inherits the base class.

For example, integer enums:

>>> Numbers = enum_base(int, ONE=1, TWO=2, THREE=3)
>>> Numbers.ONE
1
>>> x = Numbers.TWO
>>> 10 + x
12
>>> type(Numbers)
<type 'type'>
>>> type(Numbers.ONE)
<class 'Enum'>
>>> isinstance(x, Numbers)
True

Another interesting thing that can be done with this method is customize specific behavior by overriding built-in methods:

def enum_repr(t, **enums):
    '''enums with a base class and repr() output'''
    class Enum(t):
        def __repr__(self):
            return '<enum {0} of type Enum({1})>'.format(self._name, t.__name__)

    for key,val in enums.items():
        i = Enum(val)
        i._name = key
        setattr(Enum, key, i)

    return Enum



>>> Numbers = enum_repr(int, ONE=1, TWO=2, THREE=3)
>>> repr(Numbers.ONE)
'<enum ONE of type Enum(int)>'
>>> str(Numbers.ONE)
'1'

this "base" type idea is neat :) - MestreLion
yea, note that you can also do this with the new Python 3.4 Enum: python.org/dev/peps/pep-0435/#other-derived-enumerations - bj0
24
[+2] [2013-04-19 01:16:20] abarnert

While the original enum proposal, PEP 354 [1], was rejected years ago, it keeps coming back up. Some kind of enum was intended to be added to 3.2, but it got pushed back to 3.3 and then forgotten. And now there's a PEP 435 [2] intended for inclusion in Python 3.4. The reference implementation of PEP 435 is flufl.enum [3].

As of April 2013, there seems to be a general consensus that something should be added to the standard library in 3.4—as long as people can agree on what that "something" should be. That's the hard part. See the threads starting here [4] and here [5], and a half dozen other threads in the early months of 2013.

Meanwhile, every time this comes up, a slew of new designs and implementations appear on PyPI, ActiveState, etc., so if you don't like the FLUFL design, try a PyPI search [6].

[1] http://www.python.org/dev/peps/pep-0354/
[2] http://www.python.org/dev/peps/pep-0435/
[3] http://pythonhosted.org/flufl.enum/docs/using.html
[4] http://mail.python.org/pipermail/python-ideas/2013-January/019003.html
[5] http://mail.python.org/pipermail/python-ideas/2013-February/019332.html
[6] https://pypi.python.org/pypi?%3aaction=search&term=enum&submit=search

25
[+1] [2008-09-19 12:45:27] nikow

You can take a look at the traits [1] package. This gives you something like type safety and many other useful features.

But it really depends on what you want to use such an enum for.

[1] http://code.enthought.com/projects/traits/

26
[+1] [2008-10-21 02:08:29] Jake

It's funny, I just had a need for this the other day and I couldnt find an implementation worth using... so I wrote my own:

import functools

class EnumValue(object):
    def __init__(self,name,value,type):
        self.__value=value
        self.__name=name
        self.Type=type
    def __str__(self):
        return self.__name
    def __repr__(self):#2.6 only... so change to what ever you need...
        return '{cls}({0!r},{1!r},{2})'.format(self.__name,self.__value,self.Type.__name__,cls=type(self).__name__)

    def __hash__(self):
        return hash(self.__value)
    def __nonzero__(self):
        return bool(self.__value)
    def __cmp__(self,other):
        if isinstance(other,EnumValue):
            return cmp(self.__value,other.__value)
        else:
            return cmp(self.__value,other)#hopefully their the same type... but who cares?
    def __or__(self,other):
        if other is None:
            return self
        elif type(self) is not type(other):
            raise TypeError()
        return EnumValue('{0.Name} | {1.Name}'.format(self,other),self.Value|other.Value,self.Type)
    def __and__(self,other):
        if other is None:
            return self
        elif type(self) is not type(other):
            raise TypeError()
        return EnumValue('{0.Name} & {1.Name}'.format(self,other),self.Value&other.Value,self.Type)
    def __contains__(self,other):
        if self.Value==other.Value:
            return True
        return bool(self&other)
    def __invert__(self):
        enumerables=self.Type.__enumerables__
        return functools.reduce(EnumValue.__or__,(enum for enum in enumerables.itervalues() if enum not in self))

    @property
    def Name(self):
        return self.__name

    @property
    def Value(self):
        return self.__value

class EnumMeta(type):
    @staticmethod
    def __addToReverseLookup(rev,value,newKeys,nextIter,force=True):
        if value in rev:
            forced,items=rev.get(value,(force,()) )
            if forced and force: #value was forced, so just append
                rev[value]=(True,items+newKeys)
            elif not forced:#move it to a new spot
                next=nextIter.next()
                EnumMeta.__addToReverseLookup(rev,next,items,nextIter,False)
                rev[value]=(force,newKeys)
            else: #not forcing this value
                next = nextIter.next()
                EnumMeta.__addToReverseLookup(rev,next,newKeys,nextIter,False)
                rev[value]=(force,newKeys)
        else:#set it and forget it
            rev[value]=(force,newKeys)
        return value

    def __init__(cls,name,bases,atts):
        classVars=vars(cls)
        enums = classVars.get('__enumerables__',None)
        nextIter = getattr(cls,'__nextitr__',itertools.count)()
        reverseLookup={}
        values={}

        if enums is not None:
            #build reverse lookup
            for item in enums:
                if isinstance(item,(tuple,list)):
                    items=list(item)
                    value=items.pop()
                    EnumMeta.__addToReverseLookup(reverseLookup,value,tuple(map(str,items)),nextIter)
                else:
                    value=nextIter.next()
                    value=EnumMeta.__addToReverseLookup(reverseLookup,value,(str(item),),nextIter,False)#add it to the reverse lookup, but don't force it to that value

            #build values and clean up reverse lookup
            for value,fkeys in reverseLookup.iteritems():
                f,keys=fkeys
                for key in keys:
                    enum=EnumValue(key,value,cls)
                    setattr(cls,key,enum)
                    values[key]=enum
                reverseLookup[value]=tuple(val for val in values.itervalues() if val.Value == value)
        setattr(cls,'__reverseLookup__',reverseLookup)
        setattr(cls,'__enumerables__',values)
        setattr(cls,'_Max',max([key for key in reverseLookup] or [0]))
        return super(EnumMeta,cls).__init__(name,bases,atts)

    def __iter__(cls):
        for enum in cls.__enumerables__.itervalues():
            yield enum
    def GetEnumByName(cls,name):
        return cls.__enumerables__.get(name,None)
    def GetEnumByValue(cls,value):
        return cls.__reverseLookup__.get(value,(None,))[0]

class Enum(object):
    __metaclass__=EnumMeta
    __enumerables__=None

class FlagEnum(Enum):
    @staticmethod
    def __nextitr__():
        yield 0
        for val in itertools.count():
            yield 2**val

def enum(name,*args):
    return EnumMeta(name,(Enum,),dict(__enumerables__=args))

Take it or leave it, it did what I needed it to do :)

Use it like:

class Air(FlagEnum):
    __enumerables__=('None','Oxygen','Nitrogen','Hydrogen')

class Mammals(Enum):
    __enumerables__=('Bat','Whale',('Dog','Puppy',1),'Cat')
Bool = enum('Bool','Yes',('No',0))

27
[+1] [2009-11-17 20:54:34] Paul McGuire

I had need of some symbolic constants in pyparsing to represent left and right associativity of binary operators. I used class constants like this:

# an internal class, not intended to be seen by client code
class _Constants(object):
    pass


# an enumeration of constants for operator associativity
opAssoc = _Constants()
opAssoc.LEFT = object()
opAssoc.RIGHT = object()

Now when client code wants to use these constants, they can import the entire enum using:

import opAssoc from pyparsing

The enumerations are unique, they can be tested with 'is' instead of '==', they don't take up a big footprint in my code for a minor concept, and they are easily imported into the client code. They don't support any fancy str() behavior, but so far that is in the YAGNI [1] category.

[1] http://c2.com/xp/YouArentGonnaNeedIt.html

(1) Downvote, quoi? - Paul McGuire
28
[+1] [2009-11-18 02:49:33] steveha

Here is another one. It seems somewhat similar to the general approach used by @Cipher. The author called it yapenum, "yet another Python enum".

http://blog.bstpierre.org/yet-another-python-enum-module


29
[+1] [2010-05-07 02:05:03] L̲̳o̲̳̳n̲̳̳g̲̳̳p̲̳o̲̳̳k̲̳̳e̲̳̳

Why must enumerations be ints? Unfortunately, I can't think of any good looking construct to produce this without changing the Python language, so I'll use strings:

class Enumerator(object):
    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        if self.name == other:
            return True
        return self is other

    def __ne__(self, other):
        if self.name != other:
            return False
        return self is other

    def __repr__(self):
        return 'Enumerator({0})'.format(self.name)

    def __str__(self):
        return self.name

class Enum(object):
    def __init__(self, *enumerators):
        for e in enumerators:
            setattr(self, e, Enumerator(e))
    def __getitem__(self, key):
        return getattr(self, key)

Then again maybe it's even better now that we can naturally test against strings, for the sake of configuration files or other remote input.

Example:

class Cow(object):
    State = Enum(
        'standing',
        'walking',
        'eating',
        'mooing',
        'sleeping',
        'dead',
        'dying'
    )
    state = State.standing

In [1]: from enum import Enum

In [2]: c = Cow()

In [3]: c2 = Cow()

In [4]: c.state, c2.state
Out[4]: (Enumerator(standing), Enumerator(standing))

In [5]: c.state == c2.state
Out[5]: True

In [6]: c.State.mooing
Out[6]: Enumerator(mooing)

In [7]: c.State['mooing']
Out[7]: Enumerator(mooing)

In [8]: c.state = Cow.State.dead

In [9]: c.state == c2.state
Out[9]: False

In [10]: c.state == Cow.State.dead
Out[10]: True

In [11]: c.state == 'dead'
Out[11]: True

In [12]: c.state == Cow.State['dead']
Out[11]: True

If you have integer field in a mysql db you often want int enums - wim
30
[+1] [2011-06-14 17:29:49] Roy Hyunjin Han

Here is a variant on Alec Thomas's solution [1]:

def enum(*args, **kwargs):
    return type('Enum', (), dict((y, x) for x, y in enumerate(args), **kwargs)) 

x = enum('POOH', 'TIGGER', 'EEYORE', 'ROO', 'PIGLET', 'RABBIT', 'OWL')
assert x.POOH == 0
assert x.TIGGER == 1
[1] http://stackoverflow.com/questions/36932/whats-the-best-way-to-implement-an-enum-in-python/1695250#1695250

31
[+1] [2012-06-21 22:43:55] Oxdeadbeef

A variant (with support to get an enum value's name) to Alec Thomas's neat answer [1]:

class EnumBase(type):
    def __init__(self, name, base, fields):
        super(EnumBase, self).__init__(name, base, fields)
        self.__mapping = dict((v, k) for k, v in fields.iteritems())
    def __getitem__(self, val):
        return self.__mapping[val]

def enum(*seq, **named):
    enums = dict(zip(seq, range(len(seq))), **named)
    return EnumBase('Enum', (), enums)

Numbers = enum(ONE=1, TWO=2, THREE='three')
print Numbers.TWO
print Numbers[Numbers.ONE]
print Numbers[2]
print Numbers['three']
[1] http://stackoverflow.com/questions/36932/whats-the-best-way-to-implement-an-enum-in-python/1695250#1695250

32
[+1] [2013-01-31 14:27:43] Noctis Skytower

The solution that I usually use is this simple function to get an instance of a dynamically created class.

def enum(names):
    "Create a simple enumeration having similarities to C."
    return type('enum', (), dict(map(reversed, enumerate(
        names.replace(',', ' ').split())), __slots__=()))()

Using it is as simple as calling the function with a string having the names that you want to reference.

grade = enum('A B C D F')
state = enum('awake, sleeping, dead')

The values are just integers, so you can take advantage of that if desired (just like in the C language).

>>> grade.A
0
>>> grade.B
1
>>> grade.F == 4
True
>>> state.dead == 2
True

33
[+1] [2013-12-11 13:49:01] Saša Šijak

From python 3.4 there will be official support for enums. You can find documentation and examples here on python 3.4 docs page [1].

Enumerations are created using the class syntax, which makes them easy to read and write. An alternative creation method is described in Functional API. To define an enumeration, subclass Enum as follows:

from enum import Enum
class Color(Enum):
     red = 1
     green = 2
     blue = 3
[1] http://docs.python.org/3.4/library/enum.html

34
[0] [2009-10-19 10:21:39] Natim

Use the following.

TYPE = {'EAN13':   u'EAN-13',
        'CODE39':  u'Code 39',
        'CODE128': u'Code 128',
        'i25':     u'Interleaved 2 of 5',}

>>> TYPE.items()
[('EAN13', u'EAN-13'), ('i25', u'Interleaved 2 of 5'), ('CODE39', u'Code 39'), ('CODE128', u'Code 128')]
>>> TYPE.keys()
['EAN13', 'i25', 'CODE39', 'CODE128']
>>> TYPE.values()
[u'EAN-13', u'Interleaved 2 of 5', u'Code 39', u'Code 128']

I used that for Django [1] model choices, and it looks very pythonic. It is not really an Enum, but it does the job.

[1] http://en.wikipedia.org/wiki/Django_%28web_framework%29

35
[0] [2010-03-05 20:24:21] iobaixas

Following the Java like enum implementation proposed by Aaron Maenpaa, I came out with the following. The idea was to make it generic and parseable.

class Enum:
    #'''
    #Java like implementation for enums.
    #
    #Usage:
    #class Tool(Enum): name = 'Tool'
    #Tool.DRILL = Tool.register('drill')
    #Tool.HAMMER = Tool.register('hammer')
    #Tool.WRENCH = Tool.register('wrench')
    #'''

    name = 'Enum'    # Enum name
    _reg = dict([])   # Enum registered values

    @classmethod
    def register(cls, value):
        #'''
        #Registers a new value in this enum.
        #
        #@param value: New enum value.
        #
        #@return: New value wrapper instance.
        #'''
        inst = cls(value)
        cls._reg[value] = inst
        return inst

    @classmethod
    def parse(cls, value):
        #'''
        #Parses a value, returning the enum instance.
        #
        #@param value: Enum value.
        #
        #@return: Value corresp instance.        
        #'''
        return cls._reg.get(value)    

    def __init__(self, value):
        #'''
        #Constructor (only for internal use).
        #'''
        self.value = value

    def __str__(self):
        #'''
        #str() overload.
        #'''
        return self.value

    def __repr__(self):
        #'''
        #repr() overload.
        #'''
        return "<" + self.name + ": " + self.value + ">"

I can not understand how this repr could be used to reproduce the object. - Tony Veijalainen
@Tony it is not required to do so, though it is a good thing. - Jeremy Banks
36
[0] [2010-05-26 13:14:14] Denis Ryzhkov
def enum( *names ):

    '''
    Makes enum.
    Usage:
        E = enum( 'YOUR', 'KEYS', 'HERE' )
        print( E.HERE )
    '''

    class Enum():
        pass
    for index, name in enumerate( names ):
        setattr( Enum, name, index )
    return Enum

37
[0] [2010-06-04 16:33:27] daegga

I like the Java [1] enum, that's how I do it in Python:

def enum(clsdef):
    class Enum(object):
        __slots__=tuple([var for var in clsdef.__dict__ if isinstance((getattr(clsdef, var)), tuple) and not var.startswith('__')])

        def __new__(cls, *args, **kwargs):
            if not '_the_instance' in cls.__dict__:
                cls._the_instance = object.__new__(cls, *args, **kwargs)
            return cls._the_instance

        def __init__(self):
            clsdef.values=lambda cls, e=Enum: e.values()
            clsdef.valueOf=lambda cls, n, e=self: e.valueOf(n)
            for ordinal, key in enumerate(self.__class__.__slots__):
                args=getattr(clsdef, key)
                instance=clsdef(*args)
                instance._name=key
                instance._ordinal=ordinal
                setattr(self, key, instance)

        @classmethod
        def values(cls):
            if not hasattr(cls, '_values'):
                cls._values=[getattr(cls, name) for name in cls.__slots__]
            return cls._values

        def valueOf(self, name):
            return getattr(self, name)

        def __repr__(self):
            return ''.join(['<class Enum (', clsdef.__name__, ') at ', str(hex(id(self))), '>'])

    return Enum()

Sample use:

i=2
@enum
class Test(object):
    A=("a",1)
    B=("b",)
    C=("c",2)
    D=tuple()
    E=("e",3)

    while True:
        try:
            F, G, H, I, J, K, L, M, N, O=[tuple() for _ in range(i)]
            break;
        except ValueError:
            i+=1

    def __init__(self, name="default", aparam=0):
        self.name=name
        self.avalue=aparam

All class variables are defined as a tuple, just like the constructor. So far, you can't use named arguments.

[1] http://en.wikipedia.org/wiki/Java_%28programming_language%29

python3k btw, don't know if it runs on 2.x - daegga
38
[0] [2012-04-04 02:43:36] jianpx

I use a metaclass to implement an enumeration (in my thought, it is a const). Here is the code:

class ConstMeta(type):
    '''
    Metaclass for some class that store constants
    '''
    def __init__(cls, name, bases, dct):
        '''
        init class instance
        '''
        def static_attrs():
            '''
            @rtype: (static_attrs, static_val_set)
            @return: Static attributes in dict format and static value set
            '''
            import types
            attrs = {}
            val_set = set()
            #Maybe more
            filter_names = set(['__doc__', '__init__', '__metaclass__', '__module__', '__main__'])
            for key, value in dct.iteritems():
                if type(value) != types.FunctionType and key not in filter_names:
                    if len(value) != 2:
                        raise NotImplementedError('not support for values that is not 2 elements!')
                    #Check value[0] duplication.
                    if value[0] not in val_set:
                        val_set.add(value[0])
                    else:
                        raise KeyError("%s 's key: %s is duplicated!" % (dict([(key, value)]), value[0]))
                    attrs[key] = value
            return attrs, val_set

        attrs, val_set = static_attrs()
        #Set STATIC_ATTRS to class instance so that can reuse
        setattr(cls, 'STATIC_ATTRS', attrs)
        setattr(cls, 'static_val_set', val_set)
        super(ConstMeta, cls).__init__(name, bases, dct)

    def __getattribute__(cls, name):
        '''
        Rewrite the special function so as to get correct attribute value
        '''
        static_attrs = object.__getattribute__(cls, 'STATIC_ATTRS')
        if name in static_attrs:
            return static_attrs[name][0]
        return object.__getattribute__(cls, name)

    def static_values(cls):
        '''
        Put values in static attribute into a list, use the function to validate value.
        @return: Set of values
        '''
        return cls.static_val_set

    def __getitem__(cls, key):
        '''
        Rewrite to make syntax SomeConstClass[key] works, and return desc string of related static value.
        @return: Desc string of related static value
        '''
        for k, v in cls.STATIC_ATTRS.iteritems():
            if v[0] == key:
                return v[1]
        raise KeyError('Key: %s does not exists in %s !' % (str(key), repr(cls)))


class Const(object):
    '''
    Base class for constant class.

    @usage:

    Definition: (must inherit from Const class!
        >>> class SomeConst(Const):
        >>>   STATUS_NAME_1 = (1, 'desc for the status1')
        >>>   STATUS_NAME_2 = (2, 'desc for the status2')

    Invoke(base upper SomeConst class):
    1) SomeConst.STATUS_NAME_1 returns 1
    2) SomeConst[1] returns 'desc for the status1'
    3) SomeConst.STATIC_ATTRS returns {'STATUS_NAME_1': (1, 'desc for the status1'), 'STATUS_NAME_2': (2, 'desc for the status2')}
    4) SomeConst.static_values() returns set([1, 2])

    Attention:
    SomeCosnt's value 1, 2 can not be duplicated!
    If WrongConst is like this, it will raise KeyError:
    class WrongConst(Const):
        STATUS_NAME_1 = (1, 'desc for the status1')
        STATUS_NAME_2 = (1, 'desc for the status2')
    '''
    __metaclass__ = ConstMeta
##################################################################
#Const Base Class ends
##################################################################


def main():
    class STATUS(Const):
        ERROR = (-3, '??')
        OK = (0, '??')

    print STATUS.ERROR
    print STATUS.static_values()
    print STATUS.STATIC_ATTRS

    #Usage sample:
    user_input = 1
    #Validate input:
    print user_input in STATUS.static_values()
    #Template render like:
    print '<select>'
    for key, value in STATUS.STATIC_ATTRS.items():
        print '<option value="%s">%s</option>' % (value[0], value[1])
    print '</select>'


if __name__ == '__main__':
    main()

39
[0] [2012-11-20 13:18:46] Jah

I like to use lists or sets as enumerations. For example:

>>> packet_types = ['INIT', 'FINI', 'RECV', 'SEND']
>>> packet_types.index('INIT')
0
>>> packet_types.index('FINI')
1
>>>

40
[0] [2013-04-08 18:58:52] FDS

Python 2.7 and find_name()

Here is an easy-to-read implementation of the chosen idea with some helper methods, which perhaps are more Pythonic and cleaner to use than "reverse_mapping". Requires Python >= 2.7.

To address some comments below, Enums are quite useful to prevent spelling mistakes in code, e.g. for state machines, error classifiers, etc.

def Enum(*sequential, **named):
  """Generate a new enum type. Usage example:

  ErrorClass = Enum('STOP','GO')
  print ErrorClass.find_name(ErrorClass.STOP)
    = "STOP"
  print ErrorClass.find_val("STOP")
    = 0
  ErrorClass.FOO     # Raises AttributeError
  """
  enums = { v:k for k,v in enumerate(sequential) } if not named else named

  @classmethod
  def find_name(cls, val):
    result = [ k for k,v in cls.__dict__.iteritems() if v == val ]
    if not len(result):
        raise ValueError("Value %s not found in Enum" % val)
    return result[0]

  @classmethod
  def find_val(cls, n):
    return getattr(cls, n)

  enums['find_val'] = find_val
  enums['find_name'] = find_name
  return type('Enum', (), enums)

Thank you for posting this as a new answer instead of an edit. - Tyler Crompton
41
[0] [2013-06-19 21:30:47] Chris Johnson

Here's an approach with some different characteristics I find valuable:

  • allows > and < comparison based on order in enum, not lexical order
  • can address item by name, property or index: x.a, x['a'] or x[0]
  • supports slicing operations like [:] or [-1]

and most importantly prevents comparisons between enums of different types!

Based closely on http://code.activestate.com/recipes/413486-first-class-enums-in-python.

Many doctests included here to illustrate what's different about this approach.

def enum(*names):
    """
SYNOPSIS
    Well-behaved enumerated type, easier than creating custom classes

DESCRIPTION
    Create a custom type that implements an enumeration.  Similar in concept
    to a C enum but with some additional capabilities and protections.  See
    http://code.activestate.com/recipes/413486-first-class-enums-in-python/.

PARAMETERS
    names       Ordered list of names.  The order in which names are given
                will be the sort order in the enum type.  Duplicate names
                are not allowed.  Unicode names are mapped to ASCII.

RETURNS
    Object of type enum, with the input names and the enumerated values.

EXAMPLES
    >>> letters = enum('a','e','i','o','u','b','c','y','z')
    >>> letters.a < letters.e
    True

    ## index by property
    >>> letters.a
    a

    ## index by position
    >>> letters[0]
    a

    ## index by name, helpful for bridging string inputs to enum
    >>> letters['a']
    a

    ## sorting by order in the enum() create, not character value
    >>> letters.u < letters.b
    True

    ## normal slicing operations available
    >>> letters[-1]
    z

    ## error since there are not 100 items in enum
    >>> letters[99]
    Traceback (most recent call last):
        ...
    IndexError: tuple index out of range

    ## error since name does not exist in enum
    >>> letters['ggg']
    Traceback (most recent call last):
        ...
    ValueError: tuple.index(x): x not in tuple

    ## enums must be named using valid Python identifiers
    >>> numbers = enum(1,2,3,4)
    Traceback (most recent call last):
        ...
    AssertionError: Enum values must be string or unicode

    >>> a = enum('-a','-b')
    Traceback (most recent call last):
        ...
    TypeError: Error when calling the metaclass bases
        __slots__ must be identifiers

    ## create another enum
    >>> tags = enum('a','b','c')
    >>> tags.a
    a
    >>> letters.a
    a

    ## can't compare values from different enums
    >>> letters.a == tags.a
    Traceback (most recent call last):
        ...
    AssertionError: Only values from the same enum are comparable

    >>> letters.a < tags.a
    Traceback (most recent call last):
        ...
    AssertionError: Only values from the same enum are comparable

    ## can't update enum after create
    >>> letters.a = 'x'
    Traceback (most recent call last):
        ...
    AttributeError: 'EnumClass' object attribute 'a' is read-only

    ## can't update enum after create
    >>> del letters.u
    Traceback (most recent call last):
        ...
    AttributeError: 'EnumClass' object attribute 'u' is read-only

    ## can't have non-unique enum values
    >>> x = enum('a','b','c','a')
    Traceback (most recent call last):
        ...
    AssertionError: Enums must not repeat values

    ## can't have zero enum values
    >>> x = enum()
    Traceback (most recent call last):
        ...
    AssertionError: Empty enums are not supported

    ## can't have enum values that look like special function names
    ## since these could collide and lead to non-obvious errors
    >>> x = enum('a','b','c','__cmp__')
    Traceback (most recent call last):
        ...
    AssertionError: Enum values beginning with __ are not supported

LIMITATIONS
    Enum values of unicode type are not preserved, mapped to ASCII instead.

    """
    ## must have at least one enum value
    assert names, 'Empty enums are not supported'
    ## enum values must be strings
    assert len([i for i in names if not isinstance(i, types.StringTypes) and not \
        isinstance(i, unicode)]) == 0, 'Enum values must be string or unicode'
    ## enum values must not collide with special function names
    assert len([i for i in names if i.startswith("__")]) == 0,\
        'Enum values beginning with __ are not supported'
    ## each enum value must be unique from all others
    assert names == uniquify(names), 'Enums must not repeat values'

    class EnumClass(object):
        """ See parent function for explanation """

        __slots__ = names

        def __iter__(self):
            return iter(constants)

        def __len__(self):
            return len(constants)

        def __getitem__(self, i):
            ## this makes xx['name'] possible
            if isinstance(i, types.StringTypes):
                i = names.index(i)
            ## handles the more normal xx[0]
            return constants[i]

        def __repr__(self):
            return 'enum' + str(names)

        def __str__(self):
            return 'enum ' + str(constants)

        def index(self, i):
            return names.index(i)

    class EnumValue(object):
        """ See parent function for explanation """

        __slots__ = ('__value')

        def __init__(self, value):
            self.__value = value

        value = property(lambda self: self.__value)

        enumtype = property(lambda self: enumtype)

        def __hash__(self):
            return hash(self.__value)

        def __cmp__(self, other):
            assert self.enumtype is other.enumtype, 'Only values from the same enum are comparable'
            return cmp(self.value, other.value)

        def __invert__(self):
            return constants[maximum - self.value]

        def __nonzero__(self):
            ## return bool(self.value)
            ## Original code led to bool(x[0])==False, not correct
            return True

        def __repr__(self):
            return str(names[self.value])

    maximum = len(names) - 1
    constants = [None] * len(names)
    for i, each in enumerate(names):
        val = EnumValue(i)
        setattr(EnumClass, each, val)
        constants[i] = val
    constants = tuple(constants)
    enumtype = EnumClass()
    return enumtype

42
[0] [2013-09-05 04:12:03] David Rebbe

Didn't see this one in the list of answers, here is the one I whipped up. It allows the use of 'in' keyword and len() method:

class EnumTypeError(TypeError):
    pass

class Enum(object):
    """
    Minics enum type from different languages
    Usage:
    Letters = Enum(list('abc'))
    a = Letters.a
    print(a in Letters) # True
    print(54 in Letters) # False
    """
    def __init__(self, enums):
        if isinstance(enums, dict):
            self.__dict__.update(enums)
        elif isinstance(enums, list) or isinstance(enums, tuple):
            self.__dict__.update(**dict((v,k) for k,v in enumerate(enums)))
        else:
            raise EnumTypeError

    def __contains__(self, key):
        return key in self.__dict__.values()

    def __len__(self):
        return len(self.__dict__.values())


if __name__ == '__main__':
    print('Using a dictionary to create Enum:')
    Letters = Enum(dict((v,k) for k,v in enumerate(list('abcde'))))
    a = Letters.a
    print('\tIs a in e?', a in Letters)
    print('\tIs 54 in e?', 54 in Letters)
    print('\tLength of Letters enum:', len(Letters))

    print('\nUsing a list to create Enum:')
    Letters = Enum(list('abcde'))
    a = Letters.a
    print('\tIs a in e?', a in Letters)
    print('\tIs 54 in e?', 54 in Letters)
    print('\tLength of Letters enum:', len(Letters))

    try:
        # make sure we raise an exception if we pass an invalid arg
        Failure = Enum('This is a Failure')
        print('Failure')
    except EnumTypeError:
        print('Success!')

Output:

Using a dictionary to create Enum:
        Is a in e? True
        Is 54 in e? False
        Length of Letters enum: 5

Using a list to create Enum:
        Is a in e? True
        Is 54 in e? False
        Length of Letters enum: 5
Success!

43
[0] [2013-10-10 08:50:40] Ofer

See here [1] a Python Enum class that supplies some .NET Enum properties. Enjoy!

[1] http://ofers-python-articles.pythonblogs.com/227_ofers_python_articles/archive/1298_python_enum_class.html

44
[0] [2014-03-17 17:34:55] Mohammad Rafay Aleem

Here is a nice Python recipe that I found here: http://code.activestate.com/recipes/577024-yet-another-enum-for-python/

def enum(typename, field_names):
    "Create a new enumeration type"

    if isinstance(field_names, str):
        field_names = field_names.replace(',', ' ').split()
    d = dict((reversed(nv) for nv in enumerate(field_names)), __slots__ = ())
    return type(typename, (object,), d)()

Example Usage:

STATE = enum('STATE', 'GET_QUIZ, GET_VERSE, TEACH')

More details can be found on the recipe page.


45
[0] [2014-03-28 21:44:30] danger89

Keep it simple:

class Enum(object): 
    def __init__(self, tupleList):
            self.tupleList = tupleList

    def __getattr__(self, name):
            return self.tupleList.index(name)

Than:

DIRECTION = Enum(('UP', 'DOWN', 'LEFT', 'RIGHT'))
DIRECTION.DOWN
1

46