classes are objects
the following instruction
>>> class ObjectCreator(object):
... pass
...creates an object with the name ObjectCreator!
This object (the class) is itself capable of creating objects (called instances).
type to create class objects
type is used to create class objects. It works this way: type(name, bases, attrs)
Where:
name: name of the class
bases: tuple of the parent class (for inheritance, can be empty)
attrs: dictionary containing attributes names and values
eg. create class FooChild:
>>> class Foo(object):
... bar = True
>>>
>>>
>>> def echo_bar(self):
... print(self.bar)
...
>>> FooChild = type('FooChild', (Foo,), {'echo_bar': echo_bar})
type accepts a dictionary to define the attributes of the class. So:
>>> class Foo(object):
... bar = True
Can be translated to:
>>> Foo = type('Foo', (), {'bar':True})
Metaclass
Metaclasses are the 'stuff' that creates classes
You've seen that type lets you do something like this: MyClass = type('MyClass', (), {})
It's because the function type is in fact a metaclass.
type is the metaclass Python uses to create all classes behind the scenes. type is just the class that creates class objects
type is a metaclass, a class and an instance at the same time
Everything is an object in Python, and they are all either instance of classes or instances of metaclasses.
Except for type. Or, more for type
typeis an instance of classtypeitself
>>> isinstance(1, int)
True
>>> type(1)
<class 'int'>
>>>
>>> isinstance(type, type)
True
>>> type(type)
<class 'type'>
- class
type's metaclass is classtypeitself
>>> age = 1
>>> age.__class__
<class 'int'>
>>> age.__class__.__class__
<class 'type'>
>>>
>>> type.__class__
<class 'type'>
>>> type.__class__.__class__
<class 'type'>
refer to https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python
网友评论