Constructors

We have seen that classes can share data via class variables. Also it is possible to assign specific data to instances. To generalize and unify instance variables constructors are used.

Constructors are called to initialize an object. They can define and change the data of an instance. Constructors are special class functions with the following signature def __init__(self, arg1, arg2).

The first argument self is a special argument referring to the current instance of the class.

Examples:

class Person:
    last_name = "Doe"

    def __init__(self, first_name, age=None):
        self.first_name = first_name
        self.age = age


john = Person("John")
jack = Person("Jack", 33)

print(john.last_name)  # prints out Doe
print(john.first_name)  # prints out John

print(jack.last_name)  # prints out Doe
print(jack.first_name)  # prints out Jack
print(jack.age)  # prints out 33

print(john.age)  # john's age is not set. This time it will just print None.

# changing a class variables still changes the value for all instances
Person.last_name = "Taylor"

print(jack.last_name)  # prints out Taylor
print(john.last_name)  # prints out Taylor