Enum 中实现的哪一部分塑造了 `isinstance` 的行为？

Question

isinstance(x, E) , when applied on a Enum's child class E , checks for whether the first parameter x is a member of e . For example, with the following definition:

from enum import Enum

class E(Enum):
    X = 1
    Y = 2
    Z = 3

isinstance(EX, E) returns True , but isinstance(1, E) returns False . I'm puzzled how theEnum implementation makes this work: I don't even see __instancecheck__ being overridden. How does the Enum implementation make isinstance calls work in this way?

Answer 1

There is nothing special done at runtime to handle isinstance . The special behaviour for enums is all done during class construction when you derive from Enum .

Eg.

>>> from enum import Enum
>>> class E(Enum):
...     X = 1
...     Y = 2
... 
>>> 
>>> type(E.X)
<enum 'E'>
>>> E.X == 1
False

Enum has EnumMeta as its metaclass, and the special handling is in EnumMeta.__new__ which builds the E enum class. What it does, among other things, is to replace all the attributes you define as ints with instances of E with the given value.