元类来参数化继承 [英] Metaclass to parametrize Inheritance

问题描述

我已经阅读了一些关于Python元类的教程。我从来没有使用过，但我需要一个相对简单的东西，所有的教程似乎面向更复杂的用例。我基本上想创建一个模板类，它有一些预先指定的body，但它的基类作为参数。因为我从C ++ / D模板的想法，这里是一个例子我想要编写的代码看起来像在C ++：

I've read some tutorials on Python metaclasses. I've never used one before, but I need one for something relatively simple and all the tutorials seem geared towards much more complex use cases. I basically want to create a template class that has some pre-specified body, but takes its base class as a parameter. Since I got the idea from C++/D templates, here's an example of what the code I want to write would look like in C++:

template<class T>
    class Foo : T {
        void fun() {}
    }

推荐答案

做一个相当简单的翻译，不使用元类，应该这样做。除了相对不复杂，因为这一点，它也可以工作，大部分不变，在Python 2& 3。

Doing a reasonably straightforward translation, which doesn't use metaclasses, should do it. Besides being relatively uncomplicated because of this, it'll also work, unchanged for the most part, in both Python 2 & 3.

from __future__ import print_function  # for Py 2 & 3 compatibility

def template(class_T):
    class Foo(class_T):
        def fun(self):
            print('%s.fun()' % self.__class__.__name__)
    Foo.__name__ += '_' + class_T.__name__  # rename the subclass returned
    return Foo

class Base1:
    def bar(self):
        print('Base1.bar()')

class Base2:
    def bar(self):
        print('Base2.bar()')

Foo_Base1 = template(Base1)
print('Foo_Base1 base classes:', Foo_Base1.__bases__)

Foo_Base2 = template(Base2)
print('Foo_Base2 base classes:', Foo_Base2.__bases__)

d1 = Foo_Base1()
d1.fun()
d1.bar()

d2 = Foo_Base2()
d2.fun()
d2.bar()

输出：

Foo_Base1 base classes: (<class __main__.Base1 at 0x00A79C38>,)
Foo_Base2 base classes: (<class __main__.Base2 at 0x00A79DC0>,)
Foo_Base1.fun()
Base1.bar()
Foo_Base2.fun()
Base2.bar()

在代码中（无意义地命名） template（）函数是通常称为类工厂的示例。有关详细信息，请参阅我对问题的回答工厂？。

Explanation: In the code the (unimaginatively-named) template() function is an example of what is commonly called a "class factory". For more information, see my answer to the question "What exactly is a Class Factory?".

编辑：添加了代码为每个子类创建不同的类名，返回的是@ aaronasterling的洞察如果返回的类总是具有相同的名称，则调试时的潜在混淆。

Added code to create different class names for each subclass returned—inspired by @aaronasterling's insight about potential confusion when debugging if the class returned always had the same name.

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