python - __annotations__ 类中条目的顺序是否与其定义一致？

Question

假设我们有一个类定义:

class A:
    z: 1
    b: 2
    Z: 3
    g: 4
    C: 5
    A: 6
    d: 7

标准 CPython 为 A.__annotations__ 提供了这个值:

{
    'z': 1,
    'b': 2,
    'Z': 3,
    'g': 4,
    'C': 5,
    'A': 6,
    'd': 7
}

总是这样吗？规范对此有何规定？

Answer 1

PEP 526 中给出的变量注释语法规范陈述如下:

at the module or class level, if the item being annotated is a simple name, then it and the annotation will be stored in the __annotations__ attribute of that module or class (mangled if private) as an ordered mapping from names to evaluated annotations.

因此默认似乎保证了有序映射。但是，PEP 还声明了以下内容:

__annotations__ is writable ... But attempting to update __annotations__ to something other than an ordered mapping may result in a TypeError ... (Note that the assignment to __annotations__ ... is accepted by the Python interpreter without questioning it - but the subsequent type annotation expects it to be a MutableMapping ...)

因此，由于 MutableMapping 本身并不是有序的，因此第三方类的 __annotations__ 至少有可能是一个映射，它定义了一个任意的顺序:

from collections.abc import MutableMapping

class D(MutableMapping):
    def __init__(self, *args):
        self._d = {}

    def __getitem__(self, key):
        return self._d[key]

    def __setitem__(self, key, value):
        self._d[key] = value

    def __delitem__(self, key):
        del self._d[key]

    def __iter__(self):
        return reversed(self._d)

    def __len__(self):
        return len(self._d)


class A:
    __annotations__ = D()
    z: 1
    b: 2
    Z: 3
    g: 4
    C: 5
    A: 6
    d: 7

for item in A.__annotations__.items():
    print(item)

('d', 7)
('A', 6)
('C', 5)
('g', 4)
('Z', 3)
('b', 2)
('z', 1)