For beginners, how to understand @classmethod and @staticmethod

staticmethod is equivalent to a function defined in a class, so if a method is neither related to an instance nor to a specific class, it is recommended to define it as a static method. This not only makes the code clear at a glance, but also facilitates maintenance. code.

The parameter passed in implicitly to the subclass calling method is the class corresponding to the object, and the class variable of the corresponding class is dynamically generated during the calling process.

Understand that classicmethod and staticmethod classes are a data structure that can create objects. When a class is called, an instance object of the class is created. Once the object is created, Python checks whether the init() method is implemented. If init() has been implemented, then it will be called, passing the instance object as the first parameter (self).

For example, define a class Date that stores date information, and the initialization function is redefined here.

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
    def tellDate(self):
        print 'Today is %s-%s-%s'%(self.day,self.month,self.year)

If we need to call the tell Date method, we must first create an instance, or use Date().tellDate()

date1 = Date()
date1.tellDate()

How to call the function directly using the class name?

You can use @classmethod or @staticmethod to call the function.

Now assume that we need to create many Date instances, and the date information is passed in from the outside in the form of a string. , the format is 'dd-mm-yyyy'. What needs to be done

Parse the date information string into a tuple containing three variables: year, month and day;

When instantiating a Date object, pass in the year, month and day as parameters .

Like this:

day, month, year = map(int, string_date.split('-'))
date1 = Date(day, month, year)

Python cannot be overloaded like C, so we introduce class method, which does not require a self parameter, but requires that the first parameter is cls representing its own class. parameter.

@classmethod
    def from_string(cls, date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        date1 = cls(day, month, year)
        return date1

You can directly use the class name. Method name ()

The benefits of programming like this are:

The string decomposition operation can be used repeatedly, and we only need to pass in Parameter once;

OOP;

cos is the class itself, not an instance of the class. When we use Date as the parent class, its subclasses will also have the from_string method

Staticmethod is very similar to class method, but it does not require self representing its own object and cls parameters of its own class, just like using a function.

@staticmethod
    def is_date_valid(date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        return day <= 31 and month <= 12 and year <= 3999

All programs and running results

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
        
    def tellDate(self):
        print 'Today is %s-%s-%s'%(self.day,self.month,self.year)
    @classmethod
    def from_string(cls, date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        date = cls(day, month, year)
        return date
 
    @staticmethod
    def is_date_valid(date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        return day <= 31 and month <= 12 and year <= 3999
 
if __name__ == '__main__':
    date1 = Date()
    date1.tellDate()
    date2 = Date.from_string("14-04-2016")
    date2.tellDate()
    print Date.is_date_valid("14-04-2016")

The running results are:

Today is 0-0-0
Today is 14-4-2016
True

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