3.10 class methods

import datetime

In previous section we saw that we can make a method linked to class by removing the keyword self from its input arguments and in this way we can call this method from class such as ClassName.MethodName().

class Insan:
    __counter = 0

    def __init__(self):
        type(self).__counter += 1

    def PopulationCount():
        return Insan.__counter


Insan.PopulationCount()

0

Let’s say, we also want to count male population, then we can add another static method as follows:

class Insan:
    __counter = 0
    __MaleCounter = 0

    def __init__(self, gender):
        if gender == 'male':
            type(self).__MaleCounter += 1

        type(self).__counter += 1

    @staticmethod
    def PopulationCount():
        return Insan.__counter

    @staticmethod
    def MaleCount():
        return Insan.__MaleCounter


ali = Insan('male')
fatima = Insan('female')

Insan.PopulationCount()

2

Insan.MaleCount()

1

If we want to now add female counter in above class, one way of doing it is to indirectly calculating it from total counter and male counter as follows

class Insan:
    __counter = 0
    __MaleCounter = 0

    def __init__(self, gender):
        if gender == 'male':
            type(self).__MaleCounter += 1

        type(self).__counter += 1

    @staticmethod
    def PopulationCount():
        return Insan.__counter

    @staticmethod
    def MaleCount():
        return Insan.__MaleCounter

    def FemaleCount():
        return Insan.__counter - Insan.__MaleCounter


ali = Insan('male')
fatima = Insan('female')

print(Insan.PopulationCount())

2

print(Insan.MaleCount())

1

Insan.FemaleCount()

1

Another way of achieving this is to use the decorator @classmethod. The use of this decorator makes sure that when we call this method, the first default implicit argument, that python provides to this method is the class itself. Thus we can make use of some static methods of the class without initializing the class

class Insan:
    __counter = 0
    __MaleCounter = 0

    def __init__(self, gender):
        if gender == 'male':
            type(self).__MaleCounter += 1

        type(self).__counter += 1

    @staticmethod
    def PopulationCount():
        return Insan.__counter

    @staticmethod
    def MaleCount():
        return Insan.__MaleCounter

    @classmethod
    def FemaleCount(cls):
        return cls.__counter - cls.__MaleCounter


ali = Insan('male')
fatima = Insan('female')
zeinab = Insan('female')

print(Insan.PopulationCount())

3

print(Insan.MaleCount())

1

Insan.FemaleCount()

2

So until here class methods are behaving same as static methods in addition to that we can access other static methods of the class. It may not make a lot of sense but we will come to such use of class method once we cover inheritance. Now Consider the following example:

class Student(object):

    def __init__(self, name, age):
        self.name = name
        self.age = age


ali = Student('ali', 12)
print(type(ali))

<class '__main__.Student'>

But let’s say, for a particular student we don’t know his/her age but we know the birth year. Thus we would like to build the class from birth year directly as well.

class Student(object):

    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def fromBirthYear(cls, name, BirthYear):
        current_year = datetime.date.today().year
        age = current_year - BirthYear
        return cls(name, age)


ali = Student('ali', 12)
print(type(ali))

<class '__main__.Student'>

hasan = Student.fromBirthYear('hasan', 1997)
print(type(hasan))

<class '__main__.Student'>

print(hasan.age)

27

Thus we used the class method to build/initiate the class from birth year. We could have performed the conversion of BirthYear to age outside the class as well but this way provides a more user friendly interface of the class and the code is more organized.