Python--OOP advanced--meta class

type()function can either return the type of an object or create a new type

DEF fn (self, name= "World"):    print ("Hello,%s!"% name)    Hello = Type ("Hello", (object,), Dict (hello=  fn))  

h = Hello () >>> H.hello () Hello, world!>>> type (h) out[165]: __main__. hello>>> Type (Hello) out[166]: type

To create a class object, the type() function passes through 3 parameters in turn:

1. The name of the class;
2. Inherited parent class Collection, note that Python supports multiple inheritance, if there is only one parent class, do not forget the single element of tuple notation;
3. Class's method name and function binding, where we bind the function fn to the method name hello .

Metaclass

in addition to using type() In addition to dynamically creating classes, you can also use meta-classes (Metaclass) to control the creation behavior of classes. 

Once you define Metaclass, you can create a class and finally create an instance.

Metaclass allows you to create classes or modify classes. In other words, you can think of a class as an "instance" created by Metaclass.

For a simple example, this metaclass can add a method to our custom MyList add :

Definition ListMetaclass , according to the default habit, the class name of Metaclass always ends with metaclass so that it is clear that this is a metaclass:

# Metaclass is a template for a class, so you must derive from the ' type ' type: class Listmetaclass (type):     def __new__ (CLS, name, bases, attrs):        attrs['add'lambda self , Value:self.append (value)        return type. __new__ (CLS, name, bases, Attrs) class MyList (list, metaclass=listmetaclass):    Pass

>>> L = MyList()>>> L.add(1)>> L[1]

__new__()The parameters that are received by the method are:

1. The object of the class that is currently ready to be created;

2. The name of the class;

3. The collection of parent classes that the class inherits;

4. The collection of methods for the class.

Application examples

Orm

ORM (Object Relational Mapping), which is objects-relational mapping, is the mapping of a row of a relational database to an object, which is a class that corresponds to a table, so that writing code is simpler and does not have to manipulate SQL statements directly.

Python--OOP advanced--meta class