How can I represent an 'Enum' in Python?

Question

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?

Pattypatulous

Answer 1

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

For more advanced Enum techniques try the aenum library (2.7, 3.3+, same author as enum34. Code is not perfectly compatible between py2 and py3, e.g. you'll need __order__ in python 2).

• To use enum34, do $ pip install enum34
• To use aenum, do $ pip install aenum

Installing enum (no numbers) will install a completely different and incompatible version.

---

from enum import Enum     # for enum34, or the stdlib version
# from aenum import Enum  # for the aenum version
Animal = Enum('Animal', 'ant bee cat dog')

Animal.ant  # returns <Animal.ant: 1>
Animal['ant']  # returns <Animal.ant: 1> (string lookup)
Animal.ant.name  # returns 'ant' (inverse lookup)

or equivalently:

class Animal(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 a KeyError if the reverse mapping doesn't exist. With the first example:

>>> Numbers.reverse_mapping['three']
'THREE'

---

If you are using MyPy another way to express "enums" is with typing.Literal.

For example:

from typing import Literal #python >=3.8
from typing_extensions import Literal #python 2.7, 3.4-3.7


Animal = Literal['ant', 'bee', 'cat', 'dog']

def hello_animal(animal: Animal):
    print(f"hello {animal}")

hello_animal('rock') # error
hello_animal('bee') # passes

Snelling

I was not been able to understand, why did they pass kwargs(**named) in the method enum(*sequential, **named) ? Please explains. Without kwargs also it will works. I checked it. – Adapter

It would be nice to update the Python 2 function to be compatible with Python 3's functional API of Enum(name, values) – Cleodell

The var kwargs (**named) in the enum function for older versions is to support custom values: enum("blue", "red", "green", black=0) – Calandracalandria

cool but such enums are not recognizable in Intellisense – Epochal

What about default values? – Thaddeus

One should mention auto() here. – Bedder

The last example does not throw an error for me and I'm on python 10. Did you mean mypy type checking would catch it or it could be caught at runtime too? – Deeplaid

hello_animal('rock') does not throw an exception for me either – Stone

Answer 2

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), you can make Enum the base class. This gets you a little bit of extra functionality, described in the PEP. For example, enum members are distinct from integers, and they are composed of a name and a value.

from enum import Enum

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

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

print(Animal.DOG.value)
# 1

print(Animal.DOG.name)
# "DOG"

---

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

class Animal(Enum):
    DOG, CAT = range(2)

---

Enum implementations can be converted to lists and are iterable. The order of its members is the declaration order and has nothing to do with their values. For example:

class Animal(Enum):
    DOG = 1
    CAT = 2
    COW = 0

list(Animal)
# [<Animal.DOG: 1>, <Animal.CAT: 2>, <Animal.COW: 0>]

[animal.value for animal in Animal]
# [1, 2, 0]

Animal.CAT in Animal
# True

Agra

What's the point of defining numerical values (1 and 2)? They seem useless, and that's why I prefer zacherates' solution. – Curacy

It is clear from the example that constants are being defined. There is an issue with type safety, however, which may cause problems. – Pleurodynia

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. – Madonia

Using range is generally better – Siphonostele

what if we inherit set and use getattr method to use Enum, see #37432 – Cleavers

What if I have 100 values, I have to write the numbers for each of them? Yuk! – Tonsillectomy

@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. – Madonia

I use this, with the numbers replaced by object(). – Knightly

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. – Pali

Would this be better implemented with a dictionary, so that it would be easier to match the animal type with its ordinal value? – Seeto

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 – Clerc

This answer should also mention the flufl.enum package and enables backcomp for python<3.4. – Inimitable

@AlexandruNedelcu, Everyone that has come to this page looking for an answer to this question probably has a good reason to enforce valid values with an enum. How about enforcing an API, improving readability, improving an IDE interface, or limiting the effects when the values of constants change between revisions? – Tiger

I use this style of enumeration because: a) removing an item doesn't renumber following items (important if values are saved to disk); b) static type checkers (eg: mypy) will know that Animal.CAT is an int and show type errors if used incorrectly; c) ancillary data can be stored alongside the values (eg: MAMMALS = {CAT, DOG} as another class attribute). – Beberg

Answer 3

Here is one implementation:

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

Here is its usage:

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

print(Animals.DOG)

Cleavers

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. – Rudelson

@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. – Culosio

@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. :) – Culosio

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 – Imprescriptible

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. – Blastocyst

@AndréTerra: how do you check for set membership in a try-except block? – Tchad

@AndreTerra: Uh? What would you put in your try-catch? – Demibastion

This technique works great for string constants. You can do "My pet is a "+Animals.DOG. The python3 Enum class would require a str method to remove the class name and would still require a str() cast...unless I'm missing something. – Diminution

If you're on Python 3.4+: Does this have any advantage over enum.Enum? – Incorrigible

Answer 4

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

dog, cat, rabbit = range(3)

In Python 3.x you can also add a starred placeholder at the end, which will soak up all the remaining values of the range in case you don't mind wasting memory and cannot count:

dog, cat, rabbit, horse, *_ = range(100)

Pampas

But this might take more memory! – Restate

I do not see the point of the starred placeholder given that Python will check the number of values to unpack (so it will do the counting for you). – Cannady

@GabrielDevillers, I think Python will raise an exception if there is a mismatch on number of elements in the tuple to assign. – Pampas

Indeed, it does in my test (Python2,3) but that means that any counting mistake from the programmer will be catched on first test (with a message giving the correct count). – Cannady

Answer 5

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 Stationery:
    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 = Stationery.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.

More info:

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

Huh

with range you can omit the first argument if it's 0 – Hath

Another fake enum that suits some purposes is my_enum = dict(map(reversed, enumerate(str.split('Item0 Item1 Item2')))). Then my_enum can be used in look-up, e.g., my_enum['Item0'] can be an index into a sequence. You might want to wrap the result of str.split in a function that throws an exception if there are any duplicates. – Gove

Nice! For Flags you can Flag1, Flag2, Flag3 = [2**i for i in range(3)] – Mcgrath

Answer 6

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 pointed out there are a couple of enum libraries in the wild, including:

• flufl.enum
• lazr.enum
• ... and the imaginatively named enum

Dialogize

I think this is a very bad approach. Animal.DOG = Animal("dog") Animal.DOG2 = Animal("dog") assert Animal.DOG == Animal.DOG2 fails... – Ectoenzyme

@Ectoenzyme 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. – Dialogize

"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. – Blunge

@AaronMcSmooth Incorrect: dir(Animal) --> ['CAT', 'DOG', 'doc', 'init', 'module', 'repr', 'str'] – Dialogize

@Aaaron Maenpaa. correct. It's still a broken and overly complicated way to do it. – Wickedness

@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. – Dialogize

How does this implementation of the type-safe enum pattern allow each enum value to provide its own implementation of a method? – Diacritic

@Diacritic 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. – Camacho

Now try to do Flags.FLAG1 | Flags.FLAG2, oh, it doesn't work! – Bewilder

I prefer this approach. a symbol in an enum is not an integer, we just use integers in C, because, well, it's C. comparing object is fast, the "or" can be easily implemented, – Stopple

Answer 7

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)]

Hoofer

I think it is the simplest end most elegant solution. In python 2.4 ( yes, old legacy server ) tuples haven't index. I solved replacing with list. – Bramble

I tried this in a Jupyter notebook and discovered that it wouldn't work as a one-line definition, but that putting the getattr definition on a second (indented) line would be accepted. – Thayne

This solution let's me use the in keyword to search for members which is neat. Example usage: 'Claimed' in Enum(['Unclaimed', 'Claimed']) – Impure

I don't like such string-initialization, because such type will not be handled by IntelliSense. So builtin type and custom of it is better – Epochal

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, now that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 8

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 name_of(self, i):
    return self.values[i]

Seleneselenious

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. – Picasso

I do not have the faintest idea about how you use this class !?? Could you explain what you do with it ? – Hydrogenate

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 9

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 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)

Substation

I prefer this solution. I like to use built-in types where possible. – Sims

That version isn't really over the top. It just has a lot of supplied testing code – Siphonostele

Actually, the "correct" version is in the comments and is much more complex - the main version has a minor bug. – Siphonostele

Answer 10

On 2013-05-10, Guido agreed to accept PEP 435 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.

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. Official documentation will probably follow soon.

Selfpropulsion

Answer 11

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).

---

ADDENDUM : You can even enhance this solution by having Dog and Cat inherit from a symbol class with the right metaclass :

class SymbolClass(type):
    def __repr__(self): return self.__qualname__
    def __str__(self): return self.__name__

class Symbol(metaclass=SymbolClass): pass


class Animal:
    class Dog(Symbol): pass
    class Cat(Symbol): pass

Then, if you use those values to e.g. index a dictionary, Requesting it's representation will make them appear nicely:

>>> mydict = {Animal.Dog: 'Wan Wan', Animal.Cat: 'Nyaa'}
>>> mydict
{Animal.Dog: 'Wan Wan', Animal.Cat: 'Nyaa'}

Sunshade

Answer 12

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)

Bloodstained

IMHO it would be cleaner if you changed enum(names) to enum(*names) - then you could drop the extra parenthesis when calling it. – Schuck

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.) – Ewold

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, now that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 13

Keep it simple, using old Python 2.x (see below for Python 3!):

class Enum(object): 
    def __init__(self, tupleList):
            self.tupleList = tupleList
    
    def __getattr__(self, name):
            return self.tupleList.index(name)

Then:

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

---

Keep it simple when using Python 3:

from enum import Enum
class MyEnum(Enum):
    UP = 1
    DOWN = 2
    LEFT = 3
    RIGHT = 4

Then:

MyEnum.DOWN

See: https://docs.python.org/3/library/enum.html

Shadoof

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

@MartinThoma Yes, this is from years ago. When you use Python 3. I definitely advice you to use the built-in Enum: docs.python.org/3/library/enum.html – Shadoof

Answer 14

From Python 3.4 there is official support for enums. You can find documentation and examples here on Python 3.4 documentation page.

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

Judaic

Back porting is now supported as well. This is the way to go. – Gassing

Answer 15

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())




# Example for dictionary param:
values = {"Salad": 20, "Carrot": 99, "Tomato": "No i'm not"} 
Vegetables= enum(**values)

# >>> print(Vegetables.Tomato)        'No i'm not'


# Example for keyworded params: 
Fruits = enum(Apple="Steve Jobs", Peach=1, Banana=2)

# >>> print(Fruits.Apple)             'Steve Jobs'

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.

Chicory

I liked this approach the most because is easy and clear. One note for who is willing to use it.. The collections module is going to move into collections.abc in Python 3.10. This module does not seem to include namedtuple, so it could be possible that namedtuplestays inside the collections module – Civilly

I have no idea how is the named tuple "enum" is supposed to work... It's a tuple, not an enum, it has as many fields as the "enum" has values, and those fields all have string values... how does one even use it?? – Worse

@Kubahasn'tforgottenMonica I don't really understand what you're asking. What is it you don't see how to do? I cover the basics in the answer. – Chicory

Answer 16

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!

Genie

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. – Sanction

Answer 17

The standard in Python is PEP 435, so an Enum class is available in Python 3.4+:

>>> from enum import Enum
>>> class Colors(Enum):
...     red = 1
...     green = 2
...     blue = 3
>>> for color in Colors: print color
Colors.red
Colors.green
Colors.blue

Moreno

Answer 18

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.

Howells

Answer 19

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.

Claudineclaudio

yep!, much better if u inherit set and provide getattr method ! – Cleavers

Answer 20

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)

Garda

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. – Selfmastery

Answer 21

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

Metropolis

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. – Surculose

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. – Pornocracy

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) – Surculose

I have put up something very similar, which supports more than just ints, and is documented and tested, on GitHub: github.com/hmeine/named_constants – Italianism

Answer 22

For old Python 2.x

def enum(*sequential, **named):
    enums = dict(zip(sequential, [object() for _ in range(len(sequential))]), **named)
    return type('Enum', (), enums)

If you name it, is your problem, but if not creating objects instead of values allows you to do this:

>>> DOG = enum('BARK', 'WALK', 'SIT')
>>> CAT = enum('MEOW', 'WALK', 'SIT')
>>> DOG.WALK == CAT.WALK
False

When using other implementations sited here (also when using named instances in my example) you must be sure you never try to compare objects from different enums. For here's a possible pitfall:

>>> DOG = enum('BARK'=1, 'WALK'=2, 'SIT'=3)
>>> CAT = enum('WALK'=1, 'SIT'=2)
>>> pet1_state = DOG.BARK
>>> pet2_state = CAT.WALK
>>> pet1_state == pet2_state
True

Yikes!

Benjamin

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

@MartinThoma I've updated the info, since this was meant for python 2.x – Benjamin

Answer 23

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'

Ephrem

this "base" type idea is neat :) – Wendish

yea, note that you can also do this with the new Python 3.4 Enum: python.org/dev/peps/pep-0435/#other-derived-enumerations – Ephrem

Answer 24

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

Weissmann

Answer 25

The enum package from PyPI 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'

Huh

Answer 26

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())

Refugio

Answer 27

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.

Spencer

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 28

While the original enum proposal, PEP 354, 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 intended for inclusion in Python 3.4. The reference implementation of PEP 435 is flufl.enum.

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 and here, 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.

Sartorius

Answer 29

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!

Raspings

Answer 30

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',
))

Platinotype

This is a really old-timey way to create classes. Why not simply use type(class_name, (object,), dict(...)) instead? – Annual

Answer 31

Here is a variant on Alec Thomas's solution:

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

Carlock

Any way that does not need to use ""? – Canara

Answer 32

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 model choices, and it looks very pythonic. It is not really an Enum, but it does the job.

Bond

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 33

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))

Sp

Answer 34

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
>>>

Cowart

Answer 35

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

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']

Instauration

do we have to specigy the =1 =2 etc? – Canara

Answer 36

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

Knout

If you have integer field in a mysql db you often want int enums – Coltish

Answer 37

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 category.

Sublieutenant

Answer 38

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)

Rhombus

Answer 39

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

Justajustemilieu

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, now that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

@MartinThoma If you want a very lightweight enumeration implementation in Python, this solution has sufficient merit for use. Would you rather use a > 1,000 line solution as provided in Python's standard library or something that could easily be shortened to a single line of code? – Justajustemilieu

I would rather use the builtin solution which is well tested than something custom. If you want something really lightweight, just write constants in a custom module. – Incorrigible

Answer 40

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()

Neoimpressionism

Answer 41

I like the Java 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.

Customable

python3k btw, don't know if it runs on 2.x – Customable

Answer 42

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 + ">"

Murcia

I can not understand how this repr could be used to reproduce the object. – Polynesia

@Tony it is not required to do so, though it is a good thing. – Wallacewallach

Does this have an advantage over enum.Enum? Or is this just an answer that should be deleted, not that Python 3.6 is already deprecated and Python 3.4 introduced the built-in enum package? – Incorrigible

Answer 43

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

Vara