メタクラスの__new__と__prepare__

metaclass.py

# FYI: https://docs.python.jp/3/reference/datamodel.html#metaclass-example

import collections

class OrderedClass(type):

    @classmethod
    def __prepare__(metacls, clsname, bases, **kwds):
        # metacls: メタクラスに指定したtype
        # classname: メタクラスを適用したクラスの名前
        # bases: 継承したクラスのtypeを格納したtuple
        # kwds: クラス定義時にinheritance部(A(...))で与えた名前付き引数を含む辞書
        #  https://docs.python.jp/3/reference/compound_stmts.html#class-definitions
        print(f'prepare:{metacls}') 
        print(f'prepare:{clsname}')
        print(f'prepare:{bases}')
        print(f'prepare:{kwds}')
        d = collections.OrderedDict()
        d['hoge'] = 'piyo'
        return d

    def __new__(metacls, clsname, bases, namespace, **kwds):
        # namespace: メタクラスを適用したクラスの名前空間
        #   https://docs.python.jp/3/reference/datamodel.html#preparing-the-class-namespace
        print(f'new:{metacls}')
        print(f'new:{clsname}')
        print(f'new:{bases}')
        print(f'new:{namespace}')
        print(f'new:{kwds}')
        result = type.__new__(metacls, clsname, bases, dict(namespace))
        result.members = tuple(namespace)
        return result

class B:
    pass

class A(B, metaclass=OrderedClass, a='test'):
    def one(self): pass
    def two(self): pass
    def three(self): pass
    def four(self): pass

a = A()
print(a.members)
print(f'classdict:{A.__dict__}')