python classmethod staticmethod

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

1

Though classmethod and staticmethod are quite similar, there’s a slight difference in usage for both entities: classmethod must have a reference to a class object as the first parameter, whereas staticmethod can have no parameters at all.

Let’s look at all that was said in real examples.

Boilerplate

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

class Date(object):    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.

Class Method

We have some tasks that can be nicely done using classmethods.

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 three 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 elsewhere, 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.

Static method

What about staticmethod? It’s pretty similar to classmethod but doesn’t take any obligatory parameters (like a class method or instance method 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('-'))        return day <= 31 and month <= 12 and year <= 3999    # usage:    is_date = Date.is_date_valid('11-09-2012')

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.

2

Rostyslav Dzinko’s answer is very appropriate. I thought I could highlight one other reason you should choose @classmethod over @staticmethod.

In the example above, Rostyslav used the @classmethod from_string as a Factory to create Date objects from otherwise unacceptable parameters. The same can be done with @staticmethod as is shown in the code below:

class Date:  def __init__(self, month, day, year):    self.month = month    self.day   = day    self.year  = year  def display(self):    return "{0}-{1}-{2}".format(self.month, self.day, self.year)  @staticmethod  def millenium(month, day):    return Date(month, day, 2000)new_year = Date(1, 1, 2013)               # Creates a new Date objectmillenium_new_year = Date.millenium(1, 1) # also creates a Date object. # Proof:new_year.display()           # "1-1-2013"millenium_new_year.display() # "1-1-2000"isinstance(new_year, Date) # Trueisinstance(millenium_new_year, Date) # True

Thus both new_year and millenium_new_year are instances of Date class.

But, if you observe closely, the Factory process is hard-coded to create Date objects no matter what. What this means is that even if the Date class is subclassed, the subclasses will still create plain Date object (without any property of the subclass). See that in the example below:

class DateTime(Date):  def display(self):      return "{0}-{1}-{2} - 00:00:00PM".format(self.month, self.day, self.year)datetime1 = DateTime(10, 10, 1990)datetime2 = DateTime.millenium(10, 10)isinstance(datetime1, DateTime) # Trueisinstance(datetime2, DateTime) # Falsedatetime1.display() # returns "10-10-1990 - 00:00:00PM"datetime2.display() # returns "10-10-2000"

because it’s not a DateTime object but a Date object. Check the implementation of the millenium method on the Date class
datetime2 is not an instance of DateTime? WTF? Well that’s because of the @staticmethod decorator used.

In most cases, this is undesired. If what you want is a Factory method that is aware of the class that called it, then @classmethod is what you need.

Rewriting the Date.millenium as (that’s the only part of the above code that changes)

@classmethoddef millenium(cls, month, day):    return cls(month, day, 2000)

ensures that the class is not hard-coded but rather learnt. cls can be any subclass. The resulting object will rightly be an instance of cls. Let’s test that out.

datetime1 = DateTime(10, 10, 1990)datetime2 = DateTime.millenium(10, 10)isinstance(datetime1, DateTime) # Trueisinstance(datetime2, DateTime) # Truedatetime1.display() # "10-10-1990 - 00:00:00PM"datetime2.display() # "10-10-2000 - 00:00:00PM"

The reason is, as you know by now, @classmethod was used instead of @staticmethod

3

@classmethod means: when this method is called, we pass the class as the first argument instead of the instance of that class (as we normally do with methods). This means you can use the class and its properties inside that method rather than a particular instance.

@staticmethod means: when this method is called, we don’t pass an instance of the class to it (as we normally do with methods). This means you can put a function inside a class but you can’t access the instance of that class (this is useful when your method does not use the instance).

4

@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.

Python does not have to instantiate a bound-method for object.
It eases the readability of the code: seeing @staticmethod, we know that the method does not depend on the state of object itself;
@classmethod function also callable without instantiating the class, but its definition follows Sub class, not Parent class, via inheritance, can be overridden by subclass. That’s because the first argument for @classmethod function must always be cls (class).

Factory methods, that are used to create an instance for a class using for example some sort of pre-processing.
Static methods calling static methods: if you split a static methods in several static methods, you shouldn’t hard-code the class name but use class methods