英文:

How does python3.11's StrEnum's MRO work differently for str and repr?

问题 {#heading}

Python 3.11引入了StrEnum和IntEnum，它们分别继承自str或int，并且还继承自ReprEnum，而ReprEnum则继承自Enum。

ReprEnum的实现实际上是空的。

>>> print(inspect.getsource(ReprEnum))
class ReprEnum(Enum):
    """
    Only changes the repr(), leaving str() and format() to the mixed-in type.
    """

如果我创建一个StrEnum并检查其MRO，我可以看到str排在第一位。

class Strings(StrEnum):
    A = "a"
>>> Strings.mro
(<enum 'Strings'>, <enum 'StrEnum'>, <class 'str'>, <enum 'ReprEnum'>, <enum 'Enum'>, <class 'object'>)

str和Enum都定义了__str__和__repr__方法。

>>> str.__repr__
<slot wrapper '__repr__' of 'str' objects>
>>> str.__str__
<slot wrapper '__str__' of 'str' objects>
>>> Enum.__repr__
<function Enum.__repr__ at 0x7ffff69f72e0>
>>> Enum.__str__
<function Enum.__str__ at 0x7ffff69f7380>

那么，__repr__如何从Enum继承，而__str__如何从str继承呢？ 英文:

Python3.11 introduced StrEnum and IntEnum which inherit str or int respectively, and also inherit ReprEnum, which in turn inherits Enum.

ReprEnum's implementation is actually empty.

>>> print(inspect.getsource(ReprEnum))
class ReprEnum(Enum):
    """
    Only changes the repr(), leaving str() and format() to the mixed-in type.
    """

If I create a StrEnum and check the MRO, I can see that str comes first.

class Strings(StrEnum):
    A = "a"
>>> Strings.mro
(<enum 'Strings'>, <enum 'StrEnum'>, <class 'str'>, <enum 'ReprEnum'>, <enum 'Enum'>, <class 'object'>)

Both str and Enum define a __str__ and a __repr__

>>> str.__repr__
<slot wrapper '__repr__' of 'str' objects>
>>> str.__str__
<slot wrapper '__str__' of 'str' objects>
>>> Enum.__repr__
<function Enum.__repr__ at 0x7ffff69f72e0>
>>> Enum.__str__
<function Enum.__str__ at 0x7ffff69f7380>

How then does __repr__ get inherited from Enum and __str__ get inherited from str?

答案1 {#1}

得分: 3

`__repr__` 方法以正常方式继承自 `Enum`（通过 `StrEnum`）
>>> Strings.__repr__ is StrEnum.__repr__ is Enum.__repr__
True
</code></pre>
<p>对于 <code>__str__</code> 方法，元类 <code>EnumType</code> 检查了 <code>ReprEnum</code> 的存在，并在类定义时将混入数据类型的 <code>str</code> 和 <code>format</code> 处理&quot;提升&quot;到类命名空间中 <a href="https://github.com/python/cpython/blob/v3.11.4/Lib/enum.py#L575-L592">这里</a>：</p>
<pre><code>class EnumType(type):

    ...

    def __new__(metacls, cls, bases, classdict, *, boundary=None, _simple=False, **kwds):

        ...

        # 同样，对于 ReprEnum 的特殊处理
        if ReprEnum is not None and ReprEnum in bases:
            if member_type is object:
                raise TypeError(
                        'ReprEnum 子类必须与数据类型（即 int、str、float 等）混合使用'
                        )
            if '__format__' not in classdict:
                enum_class.__format__ = member_type.__format__
                classdict['__format__'] = enum_class.__format__
            if '__str__' not in classdict:
                method = member_type.__str__
                if method is object.__str__:
                    # 如果 member_type 没有定义 __str__，object.__str__ 将使用其 __repr__，
                    # 所以我们也将使用其 __repr__
                    method = member_type.__repr__
                enum_class.__str__ = method
                classdict['__str__'] = enum_class.__str__

        ...
</code></pre>
<p>现在 <code>Strings.__str__</code> 方法可以直接在类命名空间中找到，不需要遍历 MRO。</p>
<pre><code>&lt;details&gt;
&lt;summary&gt;英文:&lt;/summary&gt;

The `__repr__` method comes the normal way, inherited from `Enum` (via `StrEnum`)
</code></pre>
<br />
<p>&gt;&gt;&gt; Strings.<strong>repr</strong> is StrEnum.<strong>repr</strong> is Enum.<strong>repr</strong><br />
True</p>
<pre><code>For the `__str__` method, the metaclass `EnumType` checks for the presence of `ReprEnum` and &amp;quot;hoists up&amp;quot; the `str` and `format` handling of the mixed-in data type into the class namespace at class definition time [here](https://github.com/python/cpython/blob/v3.11.4/Lib/enum.py#L575-L592):
</code></pre>
<br />
<p>class EnumType(type):</p>
<pre><code>...

def new(metacls, cls, bases, classdict, \*, boundary=None, _simple=False, \*\*kwds):


    ...

    # Also, special handling for ReprEnum
    if ReprEnum is not None and ReprEnum in bases:
        if member_type is object:
            raise TypeError(
                    &amp;amp;#39;ReprEnum subclasses must be mixed with a data type (i.e.&amp;amp;#39;
                    &amp;amp;#39; int, str, float, etc.)&amp;amp;#39;
                    )
        if &amp;amp;#39;__format__&amp;amp;#39; not in classdict:
            enum_class.__format__ = member_type.__format__
            classdict[&amp;amp;#39;__format__&amp;amp;#39;] = enum_class.__format__
        if &amp;amp;#39;__str__&amp;amp;#39; not in classdict:
            method = member_type.__str__
            if method is object.__str__:
                # if member_type does not define __str__, object.__str__ will use
                # its __repr__ instead, so we&amp;amp;#39;ll also use its __repr__
                method = member_type.__repr__
            enum_class.__str__ = method
            classdict[&amp;amp;#39;__str__&amp;amp;#39;] = enum_class.__str__

    ...
</code></pre>
<br />
<pre><code>Now that a `Strings.__str__` method may be found directly in the class namespace, the MRO needn&amp;#39;t be traversed.

&lt;/details&gt;
</code></pre>
<br />