python - 这个 "enumeration"成语是如何工作的？

Question

我已经使用它一段时间了 - 我不记得是在哪里找到它的:

#####
#  An enumeration is handy for defining types of event 
#  (which otherwise would messily be just random strings)

def enum(**enums):
    return type('Enum', (), enums)

#####
#   The control events that the Controller can process
#   Usually this would be in a module imported by each of M, V and C
#   These are the possible values for the "event" parameter of an APP_EVENT message

AppEvents = enum(APP_EXIT = 0,
                 CUSTOMER_DEPOSIT = 1,
                 CUSTOMER_WITHDRAWAL = 2)

它是如何工作的？

Answer 1

当以这种方式调用时，type 动态声明一个类型。来自docs :

With three arguments, return a new type object. This is essentially a dynamic form of the class statement. The name string is the class name and becomes the name attribute; the bases tuple itemizes the base classes and becomes the bases attribute; and the dict dictionary is the namespace containing definitions for class body and becomes the dict attribute.

所以类型名称是'Enum'并且没有基类。

enum 函数有一个 **enums 参数，它接受所有命名参数并将它们放入字典对象中。

在你的例子中，enums 变量是

{
    'APP_EXIT': 0,
    'CUSTOMER_DEPOSIT':  1,
    'CUSTOMER_WITHDRAWAL': 2,
}

那些成为返回类型的属性。

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仅供引用，Enum已添加到 Python 3.4 和 backported到 2.4+。 (另请参见 How can I represent an 'Enum' in Python?)。