Python类及元类的创建流程
代码
python
复制代码
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
python
复制代码
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
bash
复制代码
>>> 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
bash
复制代码
>>> 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__。