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如何使用python3元类动态生成中间类

metaclass dynamic python-3.x python

Ross MacArthur · 技术社区 · 6 年前

见全部要点 here

考虑这样的情况,我们有一个简单的元类生成 __init__ 类的方法

class TestType(type):

    def __new__(cls, cname, bases, attrs):
        # Dynamically create the __init__ function
        def init(self, message):
            self.message = message

        # Assign the created function as the __init__ method.
        attrs['__init__'] = init

        # Create the class.
        return super().__new__(cls, cname, bases, attrs)


class Test(metaclass=TestType):

    def get_message(self):
        return self.message

现在这些都很好用

test = Test('hello')
assert test.get_message() == 'hello'

但是我们在子类化的时候有问题,因为如果你想子类化 __初始化__ 方法发生的事情当然是子类化的方法被覆盖了。

class SubTest(Test):

    def __init__(self, first, second):
        self.first = first
        self.second = second
        super().__init__(first + ' ' second)

subtest = SubTest('hello', 'there')

TypeError: init() takes 2 positional arguments but 3 were given

我认为解决这个问题的唯一方法就是在 __new__ 方法,并使其成为我们正在创建的类的基础。但我做不到,我试过这样的方法

class TestType(type):

    def __new__(cls, cname, bases, attrs):
        # Dynamically create the __init__ function
        def init(self, message):
            self.message = message

        # If the __init__ method is being subclassed
        if '__init__' in attrs:
            # Store the subclass __init__
            sub_init = attrs.pop('__init__')

            # Assign the created function as the __init__ method.
            attrs['__init__'] = init

            # Create an intermediate class to become the base.
            interm_base = type(cname + 'Intermediate', bases, attrs)

            # Add the intermediate class as our base.
            bases = (interm_base,)

            # Assign the subclass __init__ as the __init__ method. 
            attrs['__init__'] = sub_init

        else:
            # Assign the created function as the __init__ method.
            attrs['__init__'] = init

        # Create the class.
        return super().__new__(cls, cname, bases, attrs)

RecursionError: maximum recursion depth exceeded while calling a Python object

1 回复 | 直到 6 年前

Aran-Fey Kevin 6 年前

无限递归是由于 type 在此行中:

interm_base = type(cname + 'Intermediate', bases, attrs)

如果 bases 是的实例 TestType ,则子类也将是 测试类型 Test 可以毫无问题地创建,但是 SubTest 导致无限递归。

解决方法很简单:创建中间类没有一 __init__ if '__init__' in attrs: 将 False ,避免了无休止的递归。

class TestType(type):
    def __new__(cls, cname, bases, attrs):
        # Dynamically create the __init__ function
        def init(self, message):
            self.message = message

        # If the __init__ method is being subclassed
        if '__init__' in attrs:
            # Create an intermediate class to become the base.
            interm_base = type(cname + 'Intermediate', bases, {})

            # Add the intermediate class as our base.
            bases = (interm_base,)
        else:
            # Assign the created function as the __init__ method.
            attrs['__init__'] = init

        # Create the class.
        return super().__new__(cls, cname, bases, attrs)