functools.partial 类方法 [英] functools.partial on class method

问题描述

我正在尝试使用另一个更通用的类方法来定义一些类方法，如下所示:

I'm trying to define some class methods using another more generic class method as follows:

class RGB(object):
    def __init__(self, red, blue, green):
        super(RGB, self).__init__()
        self._red = red
        self._blue = blue
        self._green = green

    def _color(self, type):
        return getattr(self, type)

    red = functools.partial(_color, type='_red')
    blue = functools.partial(_color, type='_blue')
    green = functools.partial(_color, type='_green')

但是当我尝试调用任何这些方法时，我得到:

But when i attempt to invoke any of those methods i get:

rgb = RGB(100, 192, 240)
print rgb.red()
TypeError: _color() takes exactly 2 arguments (1 given)

我猜 self 没有传递给 _color 因为 rgb.red(rgb) 有效.

I guess self is not passed to _color since rgb.red(rgb) works.

推荐答案

您是在 函数 上创建部分，而不是方法.functools.partial() 对象不是描述符，它们本身不会添加 self 参数，也不能作为方法本身.您可以只包装绑定的方法或函数，它们根本不适用于未绑定的方法.这是记录:

You are creating partials on the function, not the method. functools.partial() objects are not descriptors, they will not themselves add the self argument and cannot act as methods themselves. You can only wrap bound methods or functions, they don't work at all with unbound methods. This is documented:

partial 对象类似于 function 对象，因为它们是可调用的、弱可引用的，并且可以具有属性.有一些重要的区别.例如，__name__ 和 __doc__ 属性不会自动创建.此外，在类中定义的 partial 对象的行为类似于静态方法，并且不会在实例属性查找期间转换为绑定方法.

partial objects are like function objects in that they are callable, weak referencable, and can have attributes. There are some important differences. For instance, the __name__ and __doc__ attributes are not created automatically. Also, partial objects defined in classes behave like static methods and do not transform into bound methods during instance attribute look-up.

使用 property 代替；这些是描述符:

Use propertys instead; these are descriptors:

class RGB(object):
    def __init__(self, red, blue, green):
        super(RGB, self).__init__()
        self._red = red
        self._blue = blue
        self._green = green

    def _color(self, type):
        return getattr(self, type)

    @property
    def red(self): return self._color('_red')
    @property
    def blue(self): return self._color('_blue')
    @property
    def green(self): return self._color('_green')

从 Python 3.4 开始，您可以使用新的 functools.partialmethod() 对象 在这里；当绑定到一个实例时它会做正确的事情:

As of Python 3.4, you can use the new functools.partialmethod() object here; it'll do the right thing when bound to an instance:

class RGB(object):
    def __init__(self, red, blue, green):
        super(RGB, self).__init__()
        self._red = red
        self._blue = blue
        self._green = green

    def _color(self, type):
        return getattr(self, type)

    red = functools.partialmethod(_color, type='_red')
    blue = functools.partialmethod(_color, type='_blue')
    green = functools.partialmethod(_color, type='_green')

但是这些必须被调用，而 property 对象可以用作简单的属性.

but these'd have to be called, whilst the property objects can be used as simple attributes.

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