getatribute怎么写才能返回dict中的变量？

Question

我有一个class，里面有一个dict。我想通过直接 "instance.key" 而不是 "instance.d[key]" 来访问字典中的变量。怎么做？

class A:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.fields = {'c': 3, 'd': 4}


def main():
    a = A()
    print(a.a) # 1
    print(a.b) # 2
    print(a.c) # currently raise Exception, I want it to print 3
    print(a.d) # currently raise Exception, I want it to print 4


if __name__ == '__main__':
    main()

Answer 1

您可以覆盖 object.__getattr__(self, name) 方法（可以定义此类方法以自定义 class 实例的属性访问的含义）：

class A:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.fields = {'c': 3, 'd': 4}

    def __getattr__(self, item):
        return self.__dict__.get(item) or self.__dict__['fields'].get(item)

a = A()
print(a.a) # 1
print(a.b) # 2
print(a.c) # 3
print(a.d) # 4

参考link：__getattr__

Answer 2

你还可以修改__dict__:

class A:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.fields = {'c': 3, 'd': 4}
        self.__dict__ = {**self.__dict__, **self.fields}

现在：

def main():
    a = A()
    print(a.a)
    print(a.b)
    print(a.c)
    print(a.d)


if __name__ == '__main__':
    main()

给出：

Answer 3

按照@RomanPerekhrest 的建议实施__getattr__。您可能遇到的唯一问题是您的实例属性覆盖了您的字典条目。如果这不是预期的行为，您应该实施 __getattribute__ 而不是 __getattr__

像这样：

class A:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.e = 5
        self.fields = {'c': 3, 'd': 4, 'e': 6}

    def __getattribute__(self, key):
        fields = super().__getattribute__('fields')
        if key == 'fields':
            return fields

        try:
            return fields[key]
        except KeyError:
            return super().__getattribute__(key)

a = A()
print(a.a) # 1
print(a.b) # 2
print(a.c) # 3
print(a.d) # 4
print(a.e) # 6 (Note that it returns the entry of the dict rather that the instance attribute)
print(a.f) # Raises exception (not in the instance attributes nor in the dict)

getatribute怎么写才能返回dict中的变量？

How to write getatrribute so that variables in dict can be returned?

python

getattr