@class 方法在 Python 中的 class 之外做了什么？

Question

在下面的代码中，如果存在 @classmethod 注释，则允许内部 def new() 代表目标的 __new__() -- 但 class 被传递了两次。如果 @classmethod 被删除，那么我们会收到类似“”的错误。 @classmethod 在这里做什么，没有它有什么办法吗？（我的动机是清晰：我不理解的代码似乎是等待发生的事故。）

"""Namedtuple annotation.

Creates a namedtuple out of a class, based on the signature of that class's
__init__ function. Defaults are respected. After namedtuple's initializer is
run, the original __init__ is run as well, allowing one to assign synthetic
parameters and internal book-keeping variables.

The class must not have varargs or keyword args.
"""     
import collections
import inspect


def namedtuple(cls):
    argspec = inspect.getargspec(cls.__init__)
    assert argspec.varargs is None
    assert argspec.keywords is None 
    non_self_args = argspec.args[1:]

    # Now we can create the new class definition, based on a namedtuple.
    bases = (collections.namedtuple(cls.__name__, non_self_args), cls)
    namespace = {'__doc__': cls.__doc__}
    newcls = type(cls.__name__, bases, namespace)

    # Here we set up the new class's __new__, which hands off to namedtuple's
    # after setting defaults.
    @classmethod
    def new(*args, **kwargs):
        kls, _kls_again = args[:2]              # The class is passed twice...?
        # Resolve default assignments with this utility from inspect.
        values = inspect.getcallargs(cls.__init__, None, *args[2:], **kwargs)
        values = [values[_] for _ in non_self_args]
        obj = super(newcls, kls).__new__(kls, *values)
        cls.__init__(obj, *values)              # Allow initialization to occur
        return obj
    # The @classmethod annotation is necessary because otherwise we get an
    # error like "unbound method new takes a class instance".

    newcls.__new__ = new

    return newcls

Answer 1

__new__ 被视为 static 方法，而不是 class 方法，调用时直接在 class 上查找通过 Python。 Python 明确地将 class 对象作为第一个参数传入。见 documentation:

__new__() is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument

通过将其设为 class 方法，该方法绑定到 class 并且 class 除了显式 [=] 外还自动传入 50=]，这就是你得到两次的原因。 classmethod 对象是 descriptor（staticmethod、function 和 property 对象也是如此）并且它是对 class 或实例的查找触发绑定行为。

你不应该使用 classmethod。 Python 在创建 class 时使其成为静态方法，因此如果您要使用

def new(*args, **kwargs):
    # ...
namespace = {'__doc__': cls.__doc__, '__new__': new}
newcls = type(cls.__name__, bases, namespace)

而不是事后将其设置在 class 上，然后省略 @classmethod 装饰器就足够了：

def new(*args, **kwargs):
    kls = args[0]
    # Resolve default assignments with this utility from inspect.
    values = inspect.getcallargs(cls.__init__, None, *args[1:], **kwargs)
    values = [values[_] for _ in non_self_args]
    obj = super(newcls, kls).__new__(kls, *values)
    cls.__init__(obj, *values)              # Allow initialization to occur
    return obj

或者，手动将其设为 staticmethod：

@staticmethod
def new(*args, **kwargs):
    # ...

newcls.__new__ = new

考虑到 namedtuple 生成的 class 是 不可变的 ，因此如果隐藏 class __init__ 方法尝试使用与 __init__ 参数相同的名称设置参数，你会得到一个 AttributeError 异常。

@class 方法在 Python 中的 class 之外做了什么？

What's @classmethod do outside of a class in Python?

python

annotations

metaprogramming

class-method

namedtuple