How to check if string exists in Enum of strings?

Question

I have created the following Enum:

from enum import Enum

class Action(str, Enum):
    NEW_CUSTOMER = "new_customer"
    LOGIN = "login"
    BLOCK = "block"

I have inherited from str, too, so that I can do things such as:

action = "new_customer"
...
if action == Action.NEW_CUSTOMER:
    ...

I would now like to be able to check if a string is in this Enum, such as:

if "new_customer" in Action:
    ....

I have tried adding the following method to the class:

def __contains__(self, item):
    return item in [i for i in self]

However, when I run this code:

print("new_customer" in [i for i in Action])
print("new_customer" in Action)

I get this exception:

True
Traceback (most recent call last):
  File "/Users/kevinobrien/Documents/Projects/crazywall/utils.py", line 24, in <module>
    print("new_customer" in Action)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/enum.py", line 310, in __contains__
    raise TypeError(
TypeError: unsupported operand type(s) for 'in': 'str' and 'EnumMeta'

No, I have shown that I can do something like `"new_customer" in [i for i in Action]`, but I want something cleaner such as `"new_customer" in Action`. — KOB, Aug 10 '20 at 07:44
I've reopened this question. It's about strings, and doesn't restrict try/catch — Mad Physicist, Aug 10 '20 at 13:36

Pierre D · Accepted Answer · 2022-06-02T17:26:35.240

11

I just bumped into this problem today; I had to change a number of subpackages for Python 3.8.

Perhaps an alternative to the other solutions here is the following, inspired by the excellent answer here to a similar question, as well as @MadPhysicist's answer on this page:

from enum import Enum, EnumMeta


class MetaEnum(EnumMeta):
    def __contains__(cls, item):
        try:
            cls(item)
        except ValueError:
            return False
        return True    


class BaseEnum(Enum, metaclass=MetaEnum):
    pass


class Stuff(BaseEnum):
    foo = 1
    bar = 5

Tests (either in py37 or 38):

>>> 1 in Stuff
True

>>> Stuff.foo in Stuff
True

>>> 2 in Stuff
False

>>> 2.3 in Stuff
False

>>> 'zero' in Stuff
False

edited Jun 02 '22 at 17:26

answered Dec 09 '20 at 22:19

Pierre D

19,195
6
50
84

Very clean solution and achieves exactly what I wanted by being able to use the `in` keyword. Thanks! – KOB Dec 10 '20 at 05:34
Brilliant solution. Thank you very much. – mutantkeyboard May 20 '22 at 07:45
`EnumMeta` is undefined? – alphabetasoup Jun 02 '22 at 02:31
@alphabetasoup: thx, I had implied the `import`. Edited the answer to make that explicit. – Pierre D Jun 02 '22 at 17:27

score 7 · Answer 2 · answered Aug 10 '20 at 07:52

You can check if the enum contains a value by calling it:

>>> Action('new_customer')
Action.NEW_CUSTOMER

If the object you pass in is not guarantee to be in the enum, you can use a try block to capture the resulting ValueError. E.g.,

def is_action(obj):
    try:
        Action(obj)
    except ValueError:
        return False
    return True

score 6 · Answer 3 · answered Mar 31 '21 at 15:53

Given an enum of languages

class Language(enum.Enum):
    en = 'en'
    zh = 'zh'

    @classmethod
    def has_member_key(cls, key):
        return key in cls.__members__

print(Language.has_member_key('tu')) => False
print(Language.has_member_key('en')) => True

DarrylG · Answer 4 · 2020-08-10T13:38:47.803

Since Action is a derived class of Enum, we can use the fact that Enum has a member called _value2member_map_.

value2member_map is a private attribute (i.e. Internally in CPython) tthat maps values to names(will only work for hashable values though). However, it's not a good idea to rely on private attributes as they can be changed anytime.

Reference

We get the following:

if "new_customer" in Action._value2member_map_:  # works

which is close to your desired:

if "new_customer" in Action:  # doesn't work (i.e. TypeError)

score 0 · Answer 5 · answered Nov 17 '21 at 09:34

You can also check contains in enum by brackets like in dict

class Action(Enum):
    NEW_CUSTOMER = 1
    LOGIN = 2
    BLOCK = 3

action = 'new_customer'
try:
    action = Action[action.upper()]
    print("action type exists")
except KeyError:
    print("action type doesn't exists")

How to check if string exists in Enum of strings?

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