Python:staticmethod vs Classmethod

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

There is subtle difference:

Say function A () is defined in the parent class, while Sub class extends parent class

• If function A () has @staticmethod decorator, SUB.A () still refers to definition inside pare NT Class. Whereas,
• If function A () has @classmethod decorator, SUB.A () would points definition inside Sub Class.

Let's talk on 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 it definition follows Sub class, not Pa Rent 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, the dealing with date information (this is what would be we boilerplate to cook on):

ClassDate(Object):Day=0Month=0 year =0 
 
  Def __ Init__ (self, Day=< Span class= "lit" >0, Month=0 , Year=0): self. Day = day self.= month self.= year           

This class obviously could is used to the store information about certain dates (without timezone information; let's assume Al L dates is presented in UTC).

Here we have __init__ , a typical initializer of the Python class instances, which receives arguments as a typical instancemethod , having th E First non-optional argument ( self ) that holds reference to a newly created instance.

Classmethod

We have some tasks, which can be nicely-done using classmethod s.

Let's assume that we want to create a lot of Date class instances have date information coming from outer source encode D as a string of next format (' dd-mm-yyyy '). We have the different places of our source code in project.

So, we must do here:

1. Parse a string to receive day, month and year as thee integer variables or a 3-item tuple consisting of that variable.
2. Instantiate by Date passing those values to initialization.

This would look like:

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

For this purpose, C + + has such feature as overloading and Python lacks that feature-so here's when classmethod applies. Lets Create another "constructor".

@classmethod
DefFrom_string(Cls,Date_as_string):Day, Month, year = Map (int, date_as_string< Span class= "pun". split ( date1 Span class= "pun" >= Cls (day Month, Year) return< Span class= "PLN" > Date1date2 =date. From_string ( ' 11-09-2012 '      

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

1. We ' ve implemented date string parsing in one place and it's reusable now.
2. Encapsulation works fine here (if you think it could implement string parsing as a single function elsewher Lution fits OOP paradigm far better).
3. clsis a object that holds class itself, not a instance of the class. It's pretty cool because if we inherit our Date class and all children would have from_string defined also.

Staticmethod

What's about staticmethod ? It's pretty similar to and classmethod doesn ' t take any obligatory parameters (like classmethod or instancemethod does).

Let's look at the next use case.

We have a date string, which we want to validate somehow. This was also logically bound Date to class we ' ve used so far, but still doesn ' t require instantiation of it.

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

@staticmethod
DefIs_date_valid(Date_as_string):Day,Month,Year=Map(Int, Date_as_string. ( '-' try:assert0<= day <=31assert0<= month <=12assert0<=  year <=3999except Assertionerror:returnfalsereturntrue               

So, as we can see from usage of staticmethod , we don ' t has any access to what the class is-it ' s basically just a function, call Ed syntactically like a method, but without access to the object and it's internals (fields and another methods), while CL Assmethod 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

Python:staticmethod vs Classmethod