Lesson 19 of 21 · 54 min · UG / professional

Classes, instances and methods

A class is a named record plus behaviour. __init__ fills fields. A method is a function that belongs with the data. Fifty motors should not be v1, a1, v2, a2.

Lab motor

Group fields; pass the whole record.

You will be able to

✓ See a class as a named record plus behaviour
✓ Treat __init__ as the constructor — here a factory function that returns a dict
✓ Keep methods as functions that take the record (self on a laptop)

A lab motor is not seven loose names

A motor has volts, amps and a power figure. Fifty motors should not be v1, a1, v2, a2. On a laptop you write class Motor: with __init__ to fill fields and methods to compute watts. This sandbox does not run class. We keep the dict as the runnable record and a factory motor(volts, amps) as the ancestor of __init__. Callers never remember whether the key is "volts" or "V".

An instance is one motor built from the class (or from the factory). A method is a function that belongs with the data: watts(m) here, m.watts() on a laptop. Construction in one place means the rest of the lab bench only calls motor(...).

Laptop: class, __init__, method

class Motor:
    def __init__(self, volts, amps):
        self.volts = volts
        self.amps = amps

    def watts(self):
        return self.volts * self.amps

m = Motor(12, 2)
print(m.watts())

Behaviour lives next to the data — not next to HTTP

watts should return a number from the fields. Do not put file writes inside watts. Power is physics; writing a CSV is an edge. Objects go wrong when every method becomes a small program. Fifty instances in a list is the point of a class; fifty copy-pasted helpers is the failure.

Words that matter

Class: A named type for records plus functions that belong with them.
Instance: One motor, one student — a value built from the class.
__init__: Laptop constructor — fills fields when you call Motor(12, 2).
Method: A function stored on the class, called on an instance.

Common mistakes

Avoid: Fifty motors as v1, a1, v2, a2, ...

Do this: A list of motor records from one constructor.

Run it step by step

Each box is a real program. Press Run, change a number, Run again — the output must follow your code.

1. Step 1 — one constructor-shaped function

Callers never type the keys. Keys stay stable.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

2. Step 2 — behaviour as a function on the record

This is a method waiting for class syntax. 12 * 2 = 24.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

On a full Python install — class Motor

One type, many instances. __init__ fills fields. Methods use self.

stdlib — no pip

Two motors, one formula.

Real library code (not run in this browser sandbox)

class Motor:
    def __init__(self, volts, amps):
        self.volts = volts
        self.amps = amps

    def watts(self):
        return self.volts * self.amps

bench = [Motor(12, 2), Motor(24, 1)]
for m in bench:
    print(m.watts())

Example program — Two motors, one formula

A list of records — fifty would look the same.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

Your turn — Student record

student(roll, name) returns a dict with both fields. Print student(19, "Ira") so both 19 and Ira appear.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

Self-assessment

Check your understanding before you mark the lesson complete.

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