Python Enum：如何获取具有多个属性的枚举值

Question

I have defined two enums with attributes. When I want to access an enum element by specifying its attribute it works for enum A (one attribute) but not for enum B (two attributes):

from enum import Enum

class A(Enum):
    ValOne = ('One')
    ValTwo = ('Two')

    def __init__(self, num):
        self.num = num

class B(Enum):
    ValOne = ('Val', 'One')
    ValTwo = ('Val', 'Two')

    def __init__(self, val, num):
        self.val = val
        self.num = num

print(A('One'))
print(B('Val', 'One'))

I get the following output:

A.ValOne
Traceback (most recent call last):
  File "test.py", line 19, in <module>
    print(B('Val', 'One'))
  File "/usr/lib/python3.8/enum.py", line 341, in __call__
    return cls._create_(
  File "/usr/lib/python3.8/enum.py", line 444, in _create_
    _, first_enum = cls._get_mixins_(cls, bases)
  File "/usr/lib/python3.8/enum.py", line 576, in _get_mixins_
    raise TypeError("Cannot extend enumerations")
TypeError: Cannot extend enumerations

What am I missing here?

Answer 1

The correct syntax is B(('Val', 'One')) , passing the value of the enum directly (thus in this case a tuple), or simply naming the enum variant by name: B.ValOne .

I must admit this is confusing, with __init__ automagically destructuring the tuple into two arguments. The error isn't helpful either.

Answer 2

The biggest thing you missing is that ('One') is not a tuple -- you need a comma ( , ), which would look like ('One', ) .

So A is made of single, non-tuple, values 'One' and 'Two' .

B , however, is made of tuples, but

 B('Val', 'One')

is not passing a tuple to B , it is passing two arguments: 'Val' and 'One' . As @orlp mentioned, passing a tuple using function syntax looks like:

B(('Val', 'One'))

Finally, in the definitions of A 's and B 's members, you do not need the parentheses:

class A(Enum):
    ValOne = 'One'
    ValTwo = 'Two'

    def __init__(self, num):
        self.num = num


class B(Enum):
    ValOne = 'Val', 'One'
    ValTwo = 'Val', 'Two'

    def __init__(self, val, num):
        self.val = val
        self.num = num