元类

一切的类都是通过type这个类来创建的，例如：

class Foo:
    a = 1
    pass


print(type(Foo))
print(Foo.__dict__)


def __init__(self, name, age):
    self.name = name
    self.age = age


f1 = type('crazy', (object,), {'a': 1, '__init__': __init__})
print(type(f1))
print(f1.__dict__)
ff = f1('alex',18)
print(ff.name)
print(ff.age)
输出：
<class 'type'>
{'__module__': '__main__', 'a': 1, '__dict__': <attribute '__dict__' of 'Foo' objects>, '__weakref__': <attribute '__weakref__' of 'Foo' objects>, '__doc__': None}
<class 'type'>
{'a': 1, '__init__': <function __init__ at 0x00978A00>, '__module__': '__main__', '__dict__': <attribute '__dict__' of 'crazy' objects>, '__weakref__': <attribute '__weakref__' of 'crazy' objects>, '__doc__': None}
alex
18

class Mytype(type):
    def __init__(self, a, b, c):
        print(a)
        print(b)
        print(c)

    def __call__(self, *args, **kwargs):
        obj = object.__new__(self)
        self.__init__(obj, *args, **kwargs)
        return obj


class Foo(metaclass=Mytype):  # Foo = Mytype(Foo,'Foo',(),{})
    def __init__(self, name, age):
        self.name = name
        self.age = age


f1 = Foo('alex', 28)
print(f1.__dict__)
输出：
Foo
()
{'__module__': '__main__', '__qualname__': 'Foo', '__init__': <function Foo.__init__ at 0x00698A00>}
{'name': 'alex', 'age': 28}