需求描述：类一旦初始化后，禁止更新变量值，且只能初始化一次（单例）。

定义装饰器 readOnlyProperties

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def readOnlyProperties(*attrs):
    """
    变量只读
    """
    def class_rebuilder(cls):
        """The class decorator example"""
        class NewClass(cls):
            """This is the overwritten class"""
            def __setattr__(self, name, value):

                if name not in attrs:
                    pass
                elif name not in self.__dict__:
                    pass
                else:
                    raise AttributeError("Can't touch {}".format(name))

                super().__setattr__(name, value)

        return NewClass

    return class_rebuilder

定义Class : Singleton

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class Singleton(type):
    """
    单例模式
    """
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

3.1定ass: Setting

class Setting 包含三个实例变量DEBUG、MEDIA_PATH,LOG_PATH。

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@readOnlyProperties(['DEBUG','MEDIA_PATH','LOG_PATH'])
@dataclass
class Setting(object, metaclass=Singleton):
    DEBUG: bool
    MEDIA_PATH: str
    LOG_PATH: str

input0

实例化2个不同实例s0,s1。
更新s1.LOG_PATH和s1.DEBUG。

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s0 = Setting(DEBUG=False, MEDIA_PATH='1', LOG_PATH='2')
s1 = UIAutoMeta(DEBUG=False, MEDIA_PATH='5', LOG_PATH='6')
print(s0)
s1.LOG_PATH='3'
s1.DEBUG=True
print(s0)
print(s1)

output0

禁止更新实例变量。

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AttributeError: Can't update the key:LOG_PATH to value:3

input1

实例化2个不同实例s0,s1。

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s0 = Setting(DEBUG=False, MEDIA_PATH='1', LOG_PATH='2')
s1 = UIAutoMeta(DEBUG=False, MEDIA_PATH='5', LOG_PATH='6')
print(s0)
s1.LOG_PATH='3'
s1.DEBUG=True
print(s0)
print(s1)

output1

s1和s2为同一个对象。

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readOnlyProperties.<locals>.class_rebuilder.<locals>.NewClass(DEBUG=False, MEDIA_PATH='1', LOG_PATH='2')
readOnlyProperties.<locals>.class_rebuilder.<locals>.NewClass(DEBUG=False, MEDIA_PATH='1', LOG_PATH='2')

Python3.7版本

python3.7以后的版本可以这样写
定义一个dataclass，拥有name和age两个属性
实例化一个People，然后尝试对其age进行重新赋值。

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from dataclasses import dataclass

@dataclass(frozen=True)
class People:
    name: str
    age: int


davida = People(name='davida', age=28)

davida.age = 29

输出

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Traceback (most recent call last):
  File "E:/workspaces/python/pytorchLearn/ttt.py", line 18, in <module>
    davida.age = 29
  File "<string>", line 3, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field 'age'