Python 约定 : function constructor for a private class

Question

我在 Python 的线程模块源代码中注意到了这一点:

def Event(*args, **kwargs):
  return _Event(*args, **kwargs)

class _Event(_Verbose):
  ...

我是否正确地假设这是在其他语言中模仿“密封”类 (c#) 或“最终”类 (java) 的尝试？这是 Python 中的常见模式吗？在 Python 中是否有任何其他方法可以解决此问题？

Answer 1

我不喜欢这种标识符的选择。类名通常以大写字母开头，因此您认为可以说 isinstance(x, Event)，但实际上不能。我认为像 make_event 这样明确地将函数标识为函数的名称会更好。

这个问题已经discussed在邮件列表中，BDFL states :

This started out as an experiment in API design, where I tried to make things look as much like the similar Java API as possible (I didn't want to invent yet anotherwobbly wheel). I specifically wanted these not to be classes so that people wouldn't start subclassing them. At the time PEP-8 wasn't well established (if at all) and I wanted the factory functions to look like classes. I think in 2.7 / 3.1 we can change the factory functions to conform to PEP-8 (leaving the old names in for a couple of release).

他提到的更改尚未完成，但我认为可以肯定地说，如今命名方案被视为误导，不应扩散。

当您说引入这些工厂函数是为了模仿密封类时，您说得对。 BDFL says

Allowing them to be subclassed makes it harder to replace them on some platforms with equivalent but faster implementations.

我不认为这种方案太常见。大多数时候，没有动机用等效的实现替换类，如果一个类不是为子类化而设计的，您可以在文档中提及。