Python类及元类的创建流程

代码

class MetaCls(type):
    print('0>>>', 'MetaCls', 0)
    def __init__(self, name, bases=None, dict=None):
        print('5>>>',self, '__init__',1)
        print('     MetaCls','__init__',id(self))
        super(MetaCls,self).__init__(name, bases, dict)
    
    def __new__(cls, *args, **kwagrs):
        print('4>>',cls, '__new__',2)
        print('     args',*args)
        print('     kwagrs',**kwagrs)
        print('     MetaCls','__new__',id(cls))
        print('     MetaCls','__mro__', MetaCls.__mro__)
        return super(MetaCls, cls).__new__(cls, *args, **kwagrs)
        
    print('1>>>', 'MetaCls',3)        
        
print("========1========")

class Pa(metaclass=MetaCls):
    print('2>>>', 'Pa',4)
    def __init__(self, num):
        self.num = num
        print('7>>>', self, '__init__',5)
        print('     Pa','__init__',id(self))
    
    def __new__(cls, *args, **kwagrs):
        print('6>>>', cls, '__new__',6)
        print('     args',*args)
        print('     kwagrs',**kwagrs)
        print('     Pa','__new__',id(cls))
        print('     Pa','__mro__',Pa.__mro__)
        return super(Pa, cls).__new__(cls)
        # return object.__new__(cls)
    print('3>>>', 'Pa',7)

print("========2========")

son = Pa(10000)
print('8>>>',8)
print('     ',type(son))
print('9>>>',9)
print('     ',son.num)
print('     son',id(son))
print('10>>>',10)
metaclass = MetaCls('meta',(),{})
print('     metaclass', metaclass)
    

运行结果

0>>> MetaCls 0
1>>> MetaCls 3
========1========
2>>> Pa 4
3>>> Pa 7
4>> <class '__main__.MetaCls'> __new__ 2
     args Pa () {'__module__': '__main__', '__qualname__': 'Pa', '__init__': <function Pa.__init__ at 0xf62558>, '__new__': <function Pa.__new__ at 0xb9d520>, '__classcell__': <cell at 0xe00998: empty>}
     kwagrs
     MetaCls __new__ 12179720
     MetaCls __mro__ (<class '__main__.MetaCls'>, <class 'type'>, <class 'object'>)
5>>> <class '__main__.Pa'> __init__ 1
     MetaCls __init__ 16757312
========2========
6>>> <class '__main__.Pa'> __new__ 6
     args 10000
     kwagrs
     Pa __new__ 16757312
     Pa __mro__ (<class '__main__.Pa'>, <class 'object'>)
7>>> <__main__.Pa object at 0x926870> __init__ 5
     Pa __init__ 9594992
8>>> 8
      <class '__main__.Pa'>
9>>> 9
      10000
     son 9594992
10>>> 10
4>> <class '__main__.MetaCls'> __new__ 2
     args meta () {}
     kwagrs
     MetaCls __new__ 12179720
     MetaCls __mro__ (<class '__main__.MetaCls'>, <class 'type'>, <class 'object'>)
5>>> <class '__main__.meta'> __init__ 1
     MetaCls __init__ 11834720
     metaclass <class '__main__.meta'>

再看type和object的关系和约定

>>> object.__bases__
()

>>> object.__bases__.__bases__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute '__bases__'

>>> object.__bases__.__class__
<class 'tuple'>

>>> object.__class__
<class 'type'>

>>> type.__bases__
(<class 'object'>,)

>>> type.__class__
<class 'type'>

>>> ().__bases__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute '__bases__'

>>> ().__class__
<class 'tuple'>

>>> ().__class__.__bases__
(<class 'object'>,)

type和object具有的方法不一样

>>> dir(type)
['__abstractmethods__', '__annotations__', '__base__', '__bases__', '__basicsize__', '__call__', '__class__', '__delattr__', '__dict__', '__dictoffset__', '__dir__', '__doc__', '__eq__', '__flags__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__instancecheck__', '__itemsize__', '__le__', '__lt__', '__module__', '__mro__', '__name__', '__ne__', '__new__', '__or__', '__prepare__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__ror__', '__setattr__', '__sizeof__', '__str__', '__subclasscheck__', '__subclasses__', '__subclasshook__', '__text_signature__', '__weakrefoffset__', 'mro']

>>> dir(object)
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

看代码和运行结果，可以完全理解python的执行过程。再补充几点，

• __new__不一定需要重写。
• 元类可以继承于object或者type或者其他类。如果没有显式的写成父类，默认是继承于object。
• 如果元类要调用父类的__new__和__init__，传递的参数要和父类方法的一致。注意：type和object中的方法和参数是不一定相同，比如__new__的参数。官方文档对object的说明，官方文档对type的说明
• object没有__call__。
• __new__中一定有return返回类或实例，__init__中没有return。
• 推荐阅读，其中核心之一是

class Foo(object, metaclass=MyMetaclass, kwarg1=value1):
…
它在元类中转换为
def new(cls, clsname, bases, dct, kwargs1=default):
…