Constructors — __init__, Default and Parameterized

A constructor initializes the object so it does not start empty. In Python that method is always named __init__, and you do not call it yourself.

Use the playground at the bottom of this page, then take the quiz.

What you will learn

• What initialization means
• The rules of __init__
• Default constructors and parameterized constructors
• Why one class keeps a single __init__

The picture

Initializing means placing the starting data into the object at creation time.

def __init__(self, m, p):
    self.label = m
    self.seats = p

Rules:

1. The name is always __init__.
2. The PVM calls it automatically when the object is created.
3. It does not return a useful value. Returning anything other than None raises TypeError.
4. A child class can inherit it, and a child class can override it.
5. Per object, it runs once: at creation. A normal method can run many times.

There are two forms.

Form	Signature	Use it when
Default (parameter-less)	__init__(self)	Every object should start with the same values
Parameterized	__init__(self, m, p)	Each object should start with different values

If you write two __init__ methods, the later one replaces the earlier one. Python keeps one constructor per class. To allow both Badge() and Badge("Avery Lane", 4), give the parameters default values:

def __init__(self, m="Guest", p=1):

A method can have any name and runs only when you call it. A constructor runs because the object was created, and its job is to set instance fields.

Example

class Badge:
    def __init__(self, m="Guest", p=1):
        self.label = m
        self.seats = p

guest = Badge()
named = Badge("Avery Lane", 4)
print(guest.label, guest.seats)
print(named.label, named.seats)

guest uses both defaults. named passes a label and a seat count. The playground prints both lines. There is still only one __init__.

Key takeaways

• __init__ initializes the object during creation

• You do not call it by name; the object call does

• A default constructor takes only self

• A parameterized constructor takes values after self

• Default arguments let one __init__ cover both calling styles

• Next: Destructors — what happens when the object is finished

• Previous: Objects