How to check if string exists in Enum of strings?

问题

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'

回答1:

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:

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

回答2:

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

回答3:

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)

来源：https://stackoverflow.com/questions/63335753/how-to-check-if-string-exists-in-enum-of-strings

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