Metaclasses — usually not

You will be able to

• ✓ Know a class is an object whose type is type
• ✓ Treat a metaclass as custom class creation — almost never needed
• ✓ Reach for a factory or a decorator first

Internship code almost never needs a metaclass

On a laptop, class Room: makes an object whose type is type. A metaclass customises that birth: inject a campus stamp, register plugins, forbid a missing method. Frameworks sometimes do this. Your attendance CSV does not.

If you wanted every sensor class to carry campus = "RSIL Demo", a class attribute, a decorator, or a factory will do. A metaclass is how you get a 2 a.m. stack trace nobody on the floor can read.

Look, then put it back

The sample below is so you can recognise metaclass= in a library, not so you start the next intern project with one. Copy it onto a laptop if you must. This Run box has no type() gymnastics — only a registry dict, which is the factory you actually wanted.

class Kind(type):
    def __new__(cls, name, bases, body):
        body.setdefault("campus", "RSIL Demo")
        return super().__new__(cls, name, bases, body)

class Sensor(metaclass=Kind):
    pass

print(Sensor.campus)

# Prefer this instead:
def make_sensor(sid):
    return {"id": sid, "campus": "RSIL Demo"}

Words that matter

type

The usual type of classes — classes are objects too.

Metaclass

A class of a class; customises class creation.

Registry / factory

The first tool when you wanted ‘magic at birth’.

Common mistakes

Example program — Register, then build

Plugin energy without type().

Your turn — Campus stamp

Print a line that includes both RSIL Demo and factory.

Self-assessment

Check your understanding before you mark the lesson complete.