2017年7月29日 星期六

[ Python 常見問題 ] What is the difference between @staticmethod and @classmethod in Python?

Source From Here 
Question 
What is the difference between a function decorated with @staticmethod and one decorated with @classmethod? 

How-To 
Maybe a bit of example code will help: Notice the difference in the call signatures of foo, class_foo and static_foo: 
  1. class A(object):  
  2.     def foo(self,x):  
  3.         print "executing foo(%s,%s)"%(self,x)  
  4.   
  5.     @classmethod  
  6.     def class_foo(cls,x):  
  7.         print "executing class_foo(%s,%s)"%(cls,x)  
  8.   
  9.     @staticmethod  
  10.     def static_foo(x):  
  11.         print "executing static_foo(%s)"%x      
  12.   
  13. a=A()  
Below is the usual way an object instance calls a method. The object instance, a, is implicitly passed as the first argument: 
>>> a.foo(1) 
executing foo(,1)

With classmethods, the class of the object instance is implicitly passed as the first argument instead of self: 
>>> a.class_foo(1) 
executing class_foo(,1)

You can also call class_foo using the class. In fact, if you define something to be a classmethod, it is probably because you intend to call it from the class rather than from a class instance. A.foo(1) would have raised a TypeError, but A.class_foo(1) works just fine: 
>>> A.class_foo(1) 
executing class_foo(,1)

One use people have found for class methods is to create inheritable alternative constructors. 

With staticmethods, neither self (the object instance) nor cls (the class) is implicitly passed as the first argument. They behave like plain functions except that you can call them from an instance or the class: 
>>> a.static_foo(1) 
executing static_foo(1) 
>>> A.static_foo(1) 
executing static_foo(1)

Staticmethods are used to group functions which have some logical connection with a class to the class. 

foo is just a function, but when you call a.foo you don't just get the function, you get a "partially applied" version of the function with the object instance a bound as the first argument to the function. foo expects 2 arguments, while a.foo only expects 1 argument. a is bound to foo. That is what is meant by the term "bound" below: 
>>> print(a.foo) 
>

With a.class_foo, a is not bound to class_foo, rather the class A is bound to class_foo. 
>>> print(a.class_foo) 
>

Here, with a staticmethod, even though it is a method, a.static_foo just returns a good 'ole function with no arguments bound. static_foo expects 1 argument, and a.static_foo expects 1 argument too. 
>>> print(a.static_foo) 
 
>>> print(A.static_foo) 


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