为什么包含类的名称不被识别为一个返回值的功能注释？ [英] Why is the name of the containing class not recognized as a return value function annotation?

问题描述

我要使用 Python函数注释来指定静态工厂方法的返回值的类型。我明白这是所需要的用例之一的注解。

I was going to use Python function annotations to specify the type of the return value of a static factory method. I understand this is one of the desired use cases for annotations.

class Trie:
    @staticmethod
    def from_mapping(mapping) -> Trie:
        # docstrings and initialization ommitted
        trie = Trie()
        return trie

PEP 3107 指出：

功能注释只不过是在编译时关联任意Python前$ P $与函数的各个部分pssions的一种方式了。

Function annotations are nothing more than a way of associating arbitrary Python expressions with various parts of a function at compile-time.

特里是在Python有效的前pression，不是吗？ Python不同意或者说，找不到名称：

Trie is a valid expression in Python, isn't it? Python doesn't agree or rather, can't find the name:

高清from_mapping（映射） - ＆GT;特里：结果
    NameError：名称'特里'没有定义

这是值得注意的是，这个错误不会发生，如果基本类型（如对象或 INT ）或一个标准库类型（如 collections.deque ）指定的。

It's worth noting that this error does not happen if a fundamental type (such as object or int) or a standard library type (such as collections.deque) is specified.

是什么原因造成这个错误，我该如何解决这个问题？

推荐答案

特里是一个有效的前pression，并评估与此名称相关的当前值名称特里。但是，这名未被定义，然而 - 一类对象的 的类主体已运行completition后只绑定至名归。你会注意到在这个非常简单的例子相同的行为：

Trie is a valid expression, and evaluates to the current value associated with the name name Trie. But that name is not defined yet -- a class object is only bound to its name after the class body has run to completition. You'll note the same behavior in this much simpler example:

class C:
    myself = C
    # or even just
    C

通常情况下，解决方法是设置类已经被定义之后，类主体以外的类属性。这不是一个很好的选择在这里，虽然它的工作原理。或者，您可以在初始定义中使用的任何占位符值，然后替换它在 __注释__ （这是合法的，因为这是一个普通的字典）：

Normally, the workaround would be setting the class attribute after the class has been defined, outside the class body. This is not a really good option here, though it works. Alternatively, you could use any placeholder value in the initial definition, then replace it in the __annotations__ (which is legal because it's a regular dictionary):

class C:
    def f() -> ...: pass
print(C.f.__annotations__)
C.f.__annotations__['return'] = C
print(C.f.__annotations__)

它的确实的感觉，而哈克虽然。根据您的使用情况下，有可能改为使用定点对象（如 CONTAINING_CLASS =对象（）），并离开国际preting，要什么实际处理注释。

It does feel rather hacky though. Depending on your use case, it might be possible to instead use a sentinel object (e.g. CONTAINING_CLASS = object()) and leave interpreting that to whatever actually processes the annotations.

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