why should i use @classmethod when i can call the constructor in the without the annotation?

Question

I was looking into the advantages of @classmethods and figured that we can directly call the constructor from any method, in that case, why do we need a class method. Are there some advantages which i have missed.

Why this code, what are the advantages?

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

    @classmethod
    def fromBirthYear(cls, name, year): 
        return cls(name, date.today().year - year) 

and not this code :-

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

    def fromBirthYear(name, year): 
        return Person(name, date.today().year - year) 

Answer 1

Because if you derive from Person, fromBirthYear will always return a Person object and not the derived class.

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

    def fromBirthYear(name, year):
        return Person(name, year)


class Fred(Person):
    pass
print(Fred.fromBirthYear('bob', 2019))

Output:

<__main__.Person object at 0x6ffffcd7c88>

You would want Fred.fromBirthYear to return a Fred object.

In the end the language will let you do a lot of things that you shouldn't do.