__getattr__用于python中的静态/类变量 [英] __getattr__ for static/class variables in python

问题描述

我有一个像这样的课程:

I have a class like:

class MyClass:
     Foo = 1
     Bar = 2

无论何时调用MyClass.Foo或MyClass.Bar，我都需要在返回值之前调用一个自定义方法. Python可能吗?我知道创建类的实例并定义自己的__getattr__方法是可能的.但是我的scnenario涉及这样使用此类而不创建任何实例.

Whenever MyClass.Foo or MyClass.Bar is invoked, I need a custom method to be invoked before the value is returned. Is it possible in Python? I know it is possible if I create an instance of the class and I can define my own __getattr__ method. But my scnenario involves using this class as such without creating any instance of it.

此外，我还需要在调用str(MyClass.Foo)时调用自定义的__str__方法. Python提供了这样的选择吗?

Also I need a custom __str__ method to be invoked when str(MyClass.Foo) is invoked. Does Python provide such an option?

推荐答案

__getattr__()和__str__()，因此，如果要为类自定义这些内容，则需要使用以下类:一类的.元类.

__getattr__() and __str__() for an object are found on its class, so if you want to customize those things for a class, you need the class-of-a-class. A metaclass.

class FooType(type):
    def _foo_func(cls):
        return 'foo!'

    def _bar_func(cls):
        return 'bar!'

    def __getattr__(cls, key):
        if key == 'Foo':
            return cls._foo_func()
        elif key == 'Bar':
            return cls._bar_func()
        raise AttributeError(key)

    def __str__(cls):
        return 'custom str for %s' % (cls.__name__,)

class MyClass:
    __metaclass__ = FooType

# in python 3:
# class MyClass(metaclass=FooType):
#    pass


print MyClass.Foo
print MyClass.Bar
print str(MyClass)

打印:

foo!
bar!
custom str for MyClass

不，对象不能截取对其属性之一进行字符串化的请求.为该属性返回的对象必须定义自己的__str__()行为.

And no, an object can't intercept a request for a stringifying one of its attributes. The object returned for the attribute must define its own __str__() behavior.

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