Python：staticmethod vs classmethod

Being educated under Java background, static method and class method are the same thing.

But not so in Python, there is subtle difference:

Say function a() is defined in Parent Class, while Sub Class extends Parent Class

If function a() has @staticmethod decorator, Sub.a() still refers to definition inside Parent Class. Whereas,

If function a() has @classmethod decorator, Sub.a() will points definition inside Sub Class.

Let’s talk about some definitions here:

@staticmethod function is nothing more than a function defined inside a class. It is callable without instantiating the class first. It’s definition is immutable via inheritance.

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

Let's assume an example of a class, dealing with date information (this is what will be our boilerplate to cook on):

classDate(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

This class obviously could be used to store information about certain dates (without timezone information; let's assume all dates are presented in UTC).

Here we have

__init__

, a typical initializer of Python class instances, which receives arguments as a typical

instancemethod

, having the first non-optional argument (

self

) that holds reference to a newly created instance.

Classmethod

We have some tasks that can be nicely done using

classmethod

s.

Let's assume that we want to create a lot of

Date

class instances having date information coming from outer source encoded as a string of next format ('dd-mm-yyyy'). We have to do that in different places of our source code in project.

So what we must do here is:

Parse a string to receive day, month and year as thee integer variables or a 3-item tuple consisting of that variable.

Instantiate

Date

by passing those values to initialization call.

This will look like:

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

For this purpose, C++ has such feature as overloading, but Python lacks that feature- so here's when

classmethod

applies. Lets create another "constructor".

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

date2 =Date.from_string('11-09-2012')

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

We've implemented date string parsing in one place and it's reusable now.

Encapsulation works fine here (if you think that you could implement string parsing as a single function elsewher, this solution fits OOP paradigm far better).

cls

is an object that holds class itself, not an instance of the class. It's pretty cool because if we inherit our

Date

class, all children will have

from_string

defined also.

Staticmethod

What about

staticmethod

? It's pretty similar to

classmethod

but doesn't take any obligatory parameters (like

classmethod

or

instancemethod

does).

Let's look at the next use case.

We have a date string that we want to validate somehow. This task is also logically bound to

Date

class we've used so far, but still doesn't require instantiation of it.

Here is where

staticmethod

can be useful. Let's look at the next piece of code:

@staticmethod
def is_date_valid(date_as_string):
day, month, year = map(int, date_as_string.split('-'))try:assert0<= day <=31assert0<= month <=12assert0<= year <=3999exceptAssertionError:returnFalsereturnTrue

So, as we can see from usage of

staticmethod

, we don't have any access to what the class is- it's basically just a function, called syntactically like a method, but without access to the object and it's internals (fields and another methods), while classmethod does.

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