为什么 Python 3.x 的 super() 有魔力?

问题描述

在 Python 3.x 中，super() 可以不带参数调用:

In Python 3.x, super() can be called without arguments:

class A(object):
    def x(self):
         print("Hey now")

class B(A):
    def x(self):
        super().x()

>>> B().x()
Hey now

为了完成这项工作，执行了一些编译时魔法，其结果之一是以下代码(将 super 重新绑定到 super_)失败:

In order to make this work, some compile-time magic is performed, one consequence of which is that the following code (which rebinds super to super_) fails:

super_ = super

class A(object):
    def x(self):
        print("No flipping")

class B(A):
    def x(self):
        super_().x()

>>> B().x()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in x
RuntimeError: super(): __class__ cell not found

为什么 super() 在没有编译器帮助的情况下无法在运行时解析超类?是否存在这种行为或其根本原因可能会咬到粗心的程序员的实际情况?

Why is super() unable to resolve the superclass at runtime without assistance from the compiler? Are there practical situations in which this behaviour, or the underlying reason for it, could bite an unwary programmer?

...并且，作为一个附带问题:Python 中是否还有其他函数、方法等示例可以通过将它们重新绑定到不同的名称来破坏?

... and, as a side question: are there any other examples in Python of functions, methods etc. which can be broken by rebinding them to a different name?

推荐答案

添加了新的魔法 super() 行为以避免违反 D.R.Y.(不要重复自己)原则，请参阅 PEP 3135.必须通过将类作为全局引用来显式命名类也容易出现与 super() 本身相同的重新绑定问题:

The new magic super() behaviour was added to avoid violating the D.R.Y. (Don't Repeat Yourself) principle, see PEP 3135. Having to explicitly name the class by referencing it as a global is also prone to the same rebinding issues you discovered with super() itself:

class Foo(Bar):
    def baz(self):
        return super(Foo, self).baz() + 42

Spam = Foo
Foo = something_else()

Spam().baz()  # liable to blow up

这同样适用于使用类装饰器，其中装饰器返回一个新对象，该对象重新绑定类名:

The same applies to using class decorators where the decorator returns a new object, which rebinds the class name:

@class_decorator_returning_new_class
class Foo(Bar):
    def baz(self):
        # Now `Foo` is a *different class*
        return super(Foo, self).baz() + 42

神奇的 super() __class__ 单元通过让您访问原始类对象很好地回避了这些问题.

The magic super() __class__ cell sidesteps these issues nicely by giving you access to the original class object.

PEP 由 Guido 启动，他最初设想super 成为关键字，以及使用单元格查找当前类的想法也是他的.当然，将其作为关键字的想法是 初稿的一部分PEP.

The PEP was kicked off by Guido, who initially envisioned super becoming a keyword, and the idea of using a cell to look up the current class was also his. Certainly, the idea to make it a keyword was part of the first draft of the PEP.

然而，实际上是 Guido 本人随后 走开了从关键字想法太神奇"，改为提出当前的实现.他预计 super() 可能是个问题:

However, it was in fact Guido himself who then stepped away from the keyword idea as 'too magical', proposing the current implementation instead. He anticipated that using a different name for super() could be a problem:

我的补丁使用了一个中间解决方案:它假设你需要 __class__每当您使用名为 'super' 的变量时.因此，如果您(全球)将 super 重命名为 supper 并使用 supper 而不是 super，它将不起作用没有参数(但如果你传递它，它仍然可以工作__class__ 或实际的类对象)；如果你有一个不相关的名为 super 的变量，一切正常，但方法将使用用于单元格变量的调用路径稍慢.

My patch uses an intermediate solution: it assumes you need __class__ whenever you use a variable named 'super'. Thus, if you (globally) rename super to supper and use supper but not super, it won't work without arguments (but it will still work if you pass it either __class__ or the actual class object); if you have an unrelated variable named super, things will work but the method will use the slightly slower call path used for cell variables.

所以，最后是 Guido 自己宣称使用 super 关键字感觉不对，而提供一个神奇的 __class__ 单元格是可以接受的折衷方案.

So, in the end, it was Guido himself that proclaimed that using a super keyword did not feel right, and that providing a magic __class__ cell was an acceptable compromise.

我同意实现的神奇隐式行为有点令人惊讶，但 super() 是该语言中应用最多的函数之一.看看所有误用的 super(type(self), self) 或 super(self.__class__, self) 在 Internet 上找到的调用；如果曾经从派生类中调用过任何代码 你会得到一个无限递归异常.至少简化的 super() 调用没有参数，避免了 那个 问题.

I agree that the magic, implicit behaviour of the implementation is somewhat surprising, but super() is one of the most mis-applied functions in the language. Just take a look at all the misapplied super(type(self), self) or super(self.__class__, self) invocations found on the Internet; if any of that code was ever called from a derived class you'd end up with an infinite recursion exception. At the very least the simplified super() call, without arguments, avoids that problem.

至于重命名的super_；只需在您的方法中引用 __class__ ，它就会再次起作用.如果您在方法中引用 super 或 __class__ 名称，则会创建单元格:

As for the renamed super_; just reference __class__ in your method as well and it'll work again. The cell is created if you reference either the super or __class__ names in your method:

>>> super_ = super
>>> class A(object):
...     def x(self):
...         print("No flipping")
... 
>>> class B(A):
...     def x(self):
...         __class__  # just referencing it is enough
...         super_().x()
... 
>>> B().x()
No flipping

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