Engineering Python · Functions & Files

Light OOP: Shapes

In Engineering Python because a few labs group data with the formulas that belong to it — a class is that grouping, not a second language.

A class names a kind of object. An instance holds that object's data. A method is a function that receives the instance as `self`. Inheritance lets Circle, Triangle and Square reuse a Shape idea and fill in their own area and perimeter — one pattern, not a design-patterns course.

Engineering Python
Medium level
4 concepts

1Class, instance, self

A class is a blueprint. Calling `Square(3)` builds one instance whose `side` is 3. Methods are functions defined inside the class; the instance is passed as `self` so they can read that instance's data.

This is still ordinary Python: `__init__` stores fields, and other methods compute from those fields. You already know functions; `self` is the extra first argument.

Figure. A class is a blueprint. Square(3) builds one instance whose side is 3. area reads self.side and returns self.side * self.side, which is 9. s.area() passes s as self.

One class, one instance

class Square:
    def __init__(self, side):
        self.side = side
    def area(self):
        return self.side * self.side

s = Square(3)
print(s.area())

In `def area(self):`, what is `self`?

The instance the method was called on
A global list of all squares
The class name as a string

`s.area()` passes s as self.

2Inheritance: one idea, specialised formulas

A subclass written `class Circle(Shape):` is a Shape and can add or replace methods. The shared idea here is "this object can answer area and perimeter"; each shape supplies the formula.

You do not need abstract base classes, private fields, or multiple inheritance for this lab. One parent, a few children, two methods.

Figure. A subclass written class Circle(Shape): is a Shape and can add or replace methods. The shared idea is that the object can answer area and perimeter; Square fills side*side and 4*side. You do not need abstract base classes, private fields, or multiple inheritance.

Takeaway

IdeaA subclass written `class Circle(Shape):` is a Shape and can add or replace methods. The shared idea here is "this object can answer area and perimeter"; each shape supplies the formula.

Shape then Square

class Shape:
    def area(self):
        return 0.0
    def perimeter(self):
        return 0.0

class Square(Shape):
    def __init__(self, side):
        self.side = side
    def area(self):
        return self.side * self.side
    def perimeter(self):
        return 4 * self.side

3Circle, triangle, square formulas

Square: area is side*side; perimeter is 4*side. Circle: area is pi*r*r; perimeter (circumference) is 2*pi*r — use `math.pi`. Triangle: area is 0.5*base*height; perimeter is the sum of the three sides.

The class only stores the numbers the formula needs. A circle stores r; a triangle stores base, height, and the three sides if perimeter is required.

No diagram — the idea is carried by the prose, code block or coding lab.

Area and perimeter
Shape	Area	Perimeter
Square	side*side	4*side
Circle	pirr	2pir
Triangle	0.5baseheight	side sum

Square of side 3

A Square instance with side 3. What are area and perimeter?

area = 3 * 39
perimeter = 4 * 312

Pro tip. Same numbers as s.area() and s.perimeter() after Square(3).

4Lab: square area and perimeter

Define Square with area and perimeter, construct side 3, and print both results.

Figure. Square(3) stores side 3. area is side*side = 9. perimeter is 4*side = 12. Methods read self.side — the check can see both 9 and 12.

Takeaway

IdeaDefine Square with area and perimeter, construct side 3, and print both results.

Coding lab. Square methods runs in the app, with checks on your output.

Notes

In Engineering Python because a few labs group data with the formulas that belong to it — a class is that grouping, not a second language.
A class is a blueprint. Calling `Square(3)` builds one instance whose `side` is 3. Methods are functions defined inside the class; the instance is passed as `self` so they can read that instance's data.
A subclass written `class Circle(Shape):` is a Shape and can add or replace methods. The shared idea here is "this object can answer area and perimeter"; each shape supplies the formula.

Exam traps & shortcuts

Run one cell at a time and read stdout before changing more lines.
Names are labels for values; rebinding a name does not rewrite old prints.

Recap

A class groups data with methods; subclasses specialise area and perimeter.

Class, instance, self: A class is a blueprint. Calling `Square(3)` builds one instance whose `side` is 3. Methods are functions defined inside the class; the instance is passed as `self` so they can read
Inheritance: one idea, specialised formulas: A subclass written `class Circle(Shape):` is a Shape and can add or replace methods. The shared idea here is "this object can answer area and perimeter"; each shape supplies the fo
Circle, triangle, square formulas: Square: area is side*side; perimeter is 4*side. Circle: area is pi*r*r; perimeter (circumference) is 2*pi*r — use `math.pi`. Triangle: area is 0.5*base*height; perimeter is the sum
Lab: square area and perimeter: Define Square with area and perimeter, construct side 3, and print both results.

Practise Light OOP: Shapes

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A 2-question practice set that ends the chapter
1 quick check with worked explanations
Timed mocks scored with the real marking scheme
Readiness tracked per topic, kept on your device

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