枚举多个 inheritance 中的 str 相等性如何在 python 中工作

Question

what is the mechanism by which the following str to Enum equality comparison works?

from enum import Enum

class SE(str, Enum):
    a = 'this'
    b = 'works'

assert SE.a == 'this', "i can't believe that worked"  # passes

whereas the following fails:

class E(Enum):
    a = 'this'
    b = 'does not'

assert E.a == 'this', "i'm not surprised that didn't"  # raises

the weird thing is that str(SE.a) == 'SE.a' , so i can't figure out how SE.a == 'this' works under the hood.

thanks.

to be clear: this is python 3.6

Answer 1

In an Enum , there is no __eq__ method; two instances are equal only if they are the same object, and the EnumMeta metaclass ensures that only one object per value is created.

class E(Enum):
    a = "foo"
    b = "foo"

assert E.a is E.b

Ea == 'this' is false because Ea is 'this' is false.

An instance of SE , though, is first and foremost a str , as can be seen in its method resolution order (MRO):

>>> SE.__mro__
(<enum 'SE'>, <class 'str'>, <enum 'Enum'>, <class 'object'>)

That means SE.a == 'this' , which is equivalent to SE.__eq__(SE.a, 'this') tries str.__eq__(SE.a, 'this') first, which evaluates to True .

Answer 2

SE subclasses both str and Enum and str is higher on the Method Resolution Order .

class SE(str, Enum):
    a = 'this'
    b = 'works'

print(SE.mro())
# [<enum 'SE'>, <class 'str'>, <enum 'Enum'>, <class 'object'>]

This means that Python first searches a method in str and only then in Enum , which means that str.__eq__ is used for the comparison.