Inheritance, composition and _privacy

You will be able to

• ✓ Share one report() across device types instead of copying the line
• ✓ Prefer has-a (a lab holds a sensor) when is-a would only add a field
• ✓ Treat _name as a convention: please do not touch from outside

Gate sensors and lab sensors both report celsius

The report line is the same; the device name differs. Share report(dev). Copy-paste of the print into six files is how one file gets “Celcius” and five do not. Inheritance on a laptop is this idea with extra syntax: a base type that knows how to report, and subtypes that fill in id and source. You do not need that syntax to practise the design.

When the difference is only a string ("gate" vs "lab"), a field is enough. When the difference is behaviour (USB vs Wi-Fi read), you will want two functions or two classes. Do not build a class tree because a diagram looked busy — build it when two callers were about to copy a formula.

Has-a is often simpler than is-a; _privacy is a please

A lab bench has a sensor: lab["sensor"]["c"]. That is composition — a record inside a record. Inheritance says a LabSensor is-a Sensor. Has-a is usually the campus default. _raw on a dict (or _raw on a laptop instance) means “implementation leftover; outside code should call report().” It is a convention, not a lock. Double underscore name-mangling on a laptop is still not security; it is a hint against accidental clashes.

Reuse the contract, not the pygame colour. report returns a string. The kiosk prints it. The log file writes it.

class Sensor:
    def __init__(self, id, c):
        self.id = id
        self._raw = c

    def report(self):
        return f"{self.id}: {self._raw} C"

class LabSensor(Sensor):
    pass

lab = {"id": "BENCH-1", "sensor": LabSensor("DHT-04", 31)}
print(lab["sensor"].report())

Words that matter

Inheritance

A laptop feature: a type that extends another type’s behaviour.

Composition

A record that contains another record — has-a.

_name

Convention: please do not use from outside; call the public function.

Common mistakes

Example program — Difference in a field, not a second reporter

kind is data. One report.

Your turn — Shared line

report(name, c) returns a string with both the name and C. Print report("Lab", 30) so Lab and 30 appear.

Self-assessment

Check your understanding before you mark the lesson complete.