2016年10月3日 星期一

[ 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

Answer: 
Maybe a bit of example code will help: Notice the difference in the call signatures of fooclass_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  
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(<__main__ .a="" 0x7f382470a490="" at="" object="">, 1)

With @classmethod, the class of the object instance is implicitly passed as the first argument instead of self. 
>>> a.class_foo(1)
executing class_foo(, 1)
>>> 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. One use people have found for class methods is to create inheritable alternative constructors

With @staticmethod, 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('hi')
executing static_foo(hi)

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

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