Python Static method vs class method

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本文详细解释了Python中的静态方法和类方法的区别与用途，通过实例展示了它们在解析日期字符串、创建可继承的构造函数等方面的使用场景。

Static method vs class method in Python

标签： python

2014-04-14 19:50 970人阅读评论(0) 收藏举报

 
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Python中@static method和@class method对于刚接触的人而言，有很多迷茫的地方。随之，搜索了一些文章，然后整理如下：

@staticmethod function is nothing more than a function definedinside a class. It is callable without instantiating the class first. The definition is immutable via inheritance.

@classmethod function also callable without instantiating the class,but its definition follows Sub class, not Parent class, via inheritance, can be overridden bysubclass. That’s because thefirst argument for @classmethodfunction must always be cls (class).

[python]view plain copy
 
 class A(object):  
     def foo(self,x):  
         print "executing foo(%s,%s)"%(self,x)  
  
     @classmethod  
     def class_foo(cls,x):  
         print "executing class_foo(%s,%s)"%(cls,x)  
  
     @staticmethod  
     def static_foo(x):  
         print "executing static_foo(%s)"%x      
   
 a=A()  

With classmethods, the class of the object instance is implicitly passed as thefirst argument instead of self.

a.class_foo(1)

A.class_foo(1)

One use people havefound 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. Theybehave like plain functions except that you can call them from an instance orthe class:

a.static_foo(1)

A.static_foo('hi')

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

======================================================================

[python]view plain copy
 
 class Date(object):  
     day = 0  
     month = 0  
     year = 0  
   
     def __init__(self, day=0, month=0, year=0):  
         self.day = day  
         self.month = month  
         self.year = year  

So what we must do hereis:

1. Parse a string toreceive day, month and year as thee integer variables or a 3-item tupleconsisting of that variable.

2. Instantiate Date by passing those values to initialization call.

This will look like:

[python]view plain copy
 
 day, month, year = map(int, string_date.split('-'))  
 date1 = Date(day, month, year)  

For this purpose, C++ has such feature as overloading, but Pythonlacks that feature- so here's when classmethod applies.Let’s create another "constructor".

[python]view plain copy
 
 @classmethod  
 def from_string(cls, date_as_string):  
     day, month, year = map(int, date_as_string.split('-'))  
     date1 = cls(day, month, year)  
     return date1  
   
 2 = Date.from_string('11-09-2012')  

Let's look morecarefully at the above implementation, and review what advantages we have here:

1. We've implemented datestring parsing in one place and it's reusable now.

2. Encapsulation works finehere (if you think that you could implement string parsing as a single functionelsewher, this solution fits OOP paradigm far better).

3. cls is an object that holds class itself, not an instance of the class. It's prettycool because if we inherit our Date class, allchildren will have from_string defined also.

Summary:

@classmethod

1. Factory method. That are used to create an instance for a classusing for example some sort of preprocessing.

2. Static method calling static methods. If you split a static methodin several static methods, you shouldn’t hard-code the class name but use classmethods.

@staticmethod

1. Avoid thecost of instantiate a bound-method for each object.

2. The methoddoes not depend on the state of object itself.

3. Method override.

Ref：

http://stackoverflow.com/questions/38238/what-are-class-methods-in-python-for

http://stackoverflow.com/questions/136097/what-is-the-difference-between-staticmethod-and-classmethod-in-python

http://stackoverflow.com/questions/12179271/python-classmethod-and-staticmethod-for-beginner

http://julien.danjou.info/blog/2013/guide-python-static-class-abstract-methods