Detailed description of python's metaclass

Metaclasses are generally used to create classes. When executing a class definition, the interpreter must know the correct metaclass of the class. The interpreter will first look for the classattributemetaclass. If this attribute exists, it will assign this attribute to this class as its metaclass. If this attribute is not defined, it will look up the metaclass in the parent class. If the metaclass attribute is not found yet, the interpreter will check the global variable named metaclass, and if it exists, use it as the metaclass . Otherwise, the class is a traditional class, with types.ClassType as its metaclass.

When executing the class definition, the correct (usually default) metaclass of this class will be checked. The metaclass (usually) passes three parameters (to the constructor): class name, from the base classInherit Tuples of data, and (class) attribute dictionaries.

When is the metaclass created?

#!/usr/bin/env python  
   
print '1. Metaclass declaration'  
class Meta(type):  
    def init(cls, name, bases, attrd):  
        super(Meta,cls).init(name,bases,attrd)  
        print '3. Create class %r' % (name)  
   
print '2. Class Foo declaration'  
class Foo(object):  
    metaclass=Meta  
    def init(self):  
        print '*. Init class %r' %(self.class.name)  
   
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print '4. Class Foo f1 instantiation'  
f1=Foo()  
   
print '5. Class Foo f2 instantiation'  
f2=Foo()  
   
print 'END'  
输出

Result:

1. Metaclass declaration

2. Class Foo declaration

3. Create class 'Foo'

4 . Class Foo f1 instantiation

*. Init class 'Foo'

5. Class Foo f2 instantiation

*. Init class 'Foo'

END

It can be seen that when the class is declared, the methods in metaclass are executed. In the future, when the class object is defined, only the init() method of the class is called. The init() method in MetaClass init() is only executed once when the class is declared.

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