ZERO / HERO
The build crew's checklist

Constructors

The analogy

Before anyone moves in, the crew runs a checklist: pour the foundation, set the address, pick the paint. The constructor is that checklist — it runs exactly once, the moment the house is built, and guarantees nothing is missing.

Visualizer

The build checklist

step 1 / 7
CONSTRUCTOR CHECKLIST — runs once
1. allocate memory for the object
2. validate the arguments
3. assign colour
4. assign address
5. mark the object ready

new House("white", "12 Elm St") is called. Nothing exists yet.

class House:
    def __init__(self, colour, address):
        if not colour:
            raise ValueError("a house needs a colour")
        self.colour = colour
        self.address = address
        self.ready = True      # invariants now hold

# __init__ runs once, at creation. Never again.
Check yourself

How often does a constructor run per object?