Meaning of @classmethod and @staticmethod for beginner? [duplicate]

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.

Example

class Date(object):    def __init__(self, day=0, month=0, year=0):        self.day = day        self.month = month        self.year = year    @classmethod    def from_string(cls, date_as_string):        day, month, year = map(int, date_as_string.split('-'))        date1 = cls(day, month, year)        return date1    @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 <= 3999date2 = Date.from_string('11-09-2012')is_date = Date.is_date_valid('11-09-2012')

Explanation

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

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 a 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 an outer source encoded as a string with format 'dd-mm-yyyy'. Suppose we have to do this in different places in the source code of our project.

So what we must do here is:

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

This will look like:

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

For this purpose, C++ can implement such a feature with overloading, but Python lacks this overloading. Instead, we can use classmethod. Let's 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 date1date2 = Date.from_string('11-09-2012')

Let's look more carefully at the above implementation, and review what 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 that you could implement string parsing as a single function elsewhere, this solution fits the OOP paradigm far better).
3. cls is an object that holds the 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 the Date class we've used so far, but 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 its internals (fields and another methods), while classmethod does.

Rostyslav Dzinko's answer is very appropriate. I thought I could highlight one other reason you should choose @classmethod over @staticmethod when you are creating an additional constructor.

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 the 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 objects (without any properties 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 for more details.

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 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, that @classmethod was used instead of @staticmethod

@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).