Exercises: Objects

A map needs two markers that can change independently.

Specification: Define Point with __init__(self, x, y) and store both parameters as attributes. Make start = Point(2, 6) and finish = Point(9, 4). Change only start.x to 5. Read and print both objects' x attributes to prove that finish.x is untouched.

Worked sample reads:

start.x  -> 5
start.y  -> 6
finish.x -> 9
finish.y -> 4

A sign shop needs each order to carry its code, width, and height together.

Specification: Define Rectangle with __init__(self, code, width, height). Store all three parameters as attributes. Make two rectangles with the sample values and read their attributes.

Worked sample calls and reads:

small = Rectangle("N-12", 6, 4)
large = Rectangle("S-30", 12, 7)
small.code   -> "N-12"
small.width  -> 6
large.height -> 7

The sign shop wants each rectangle to calculate its own measurements.

Specification: Define Rectangle with __init__(self, width, height) and store width and height as attributes. Add area(self) and perimeter(self) methods. Each method must use the object's attributes and return a number.

Worked sample calls:

tile = Rectangle(9, 4)
tile.area()      -> 36
tile.perimeter() -> 26
Rectangle(2.5, 6).area() -> 15.0

A route planner needs one point to measure its straight-line distance to another point.

Specification: Define Point with x and y attributes. Add distance(self, other). Compute dx and dy, then return (dx * dx + dy * dy) ** 0.5. Do not import a square-root tool.

Worked sample calls:

Point(1, 2).distance(Point(4, 6)) -> 5.0
Point(2, 1).distance(Point(2, 6)) -> 5.0
Point(1, 1).distance(Point(1, 1)) -> 0.0

A warehouse display needs a short region code and a readable rectangle description.

Specification: Define Rectangle with code, width, and height attributes. Add short_code(self) that returns the genuine attribute slice self.code[0:2]. Add describe(self) that returns an f-string in the form "code: width by height".

Worked sample calls:

crate = Rectangle("NE-48", 7, 3)
crate.short_code() -> "NE"
crate.describe()   -> "NE-48: 7 by 3"
Rectangle("SW-9", 2, 2).short_code() -> "SW"

A game needs a counter that remembers its label and changing count.

Specification: Define Counter with __init__(self, label). Store the label and start count at 0. Add increment(self, amount) that adds amount to self.count and returns the new count. Make two counters and change only one of them.

Worked sample calls and reads:

blue = Counter("blue")
gold = Counter("gold")
blue.increment(3) -> 3
blue.increment(2) -> 5
blue.count        -> 5
gold.count        -> 0

A map editor needs to rebuild a point after the program ends.

Specification: Define Point with x and y attributes plus save(self, path). The method must open path in "w" mode, write each attribute on its own f-string line, then return path. Define a separate module-level function load_point(path). It must open path in explicit "r" mode, use readline(), .strip(), and int() to rebuild both values, then return a new Point.

Use only ex7_point.txt. In the same cell, make and save the point before you load it, so the exercise is self-contained.

Worked sample calls and reads:

original = Point(14, 9)
original.save("ex7_point.txt") -> "ex7_point.txt"
loaded = load_point("ex7_point.txt")
loaded.x -> 14
loaded.y -> 9

A sign maker wants a rectangle drawn with number signs.

Specification: Define Rectangle with __init__(self, width, height) and draw(self). Add a short purpose comment. Store the dimensions. In draw, append one "#" * self.width row for each unit of height, then return "\n".join(rows). Do not leave trailing spaces.

Worked sample calls:

Rectangle(4, 2).draw() -> "####\n####"

An account owner needs deposits and withdrawals to change one balance.

Specification: Define BankAccount with __init__(self, owner, balance), deposit(self, amount), and withdraw(self, amount). Add a short purpose comment. Store owner and balance. A deposit adds the amount and returns the balance. If a withdrawal is larger than the balance, return "Insufficient funds" without changing it; otherwise subtract the amount and return the balance.

Worked sample calls:

account = BankAccount("Idris", 50)
account.deposit(30) -> 80
account.withdraw(100) -> "Insufficient funds"
account.balance -> 80
account.withdraw(25) -> 55

A crossing signal cycles through three colors.

Specification: Define TrafficLight with __init__(self) that starts self.state at "green" and next(self) that changes the state using an if/elif/else chain: green to yellow, yellow to red, red to green. Add a short purpose comment. Return the new state.

Worked sample calls:

light = TrafficLight()
light.state -> "green"
light.next() -> "yellow"
light.next() -> "red"
light.next() -> "green"

A music player keeps songs in the order they were added.

Specification: Define Playlist with __init__(self), a list attribute self.songs, add(self, title) that appends a title, and song_count(self) that returns the list length.

Worked sample calls:

playlist = Playlist()
playlist.add("Tide")
playlist.add("Ember")
playlist.song_count() -> 2
playlist.songs -> ["Tide", "Ember"]

A vending machine collects credit until it can release an item.

Specification: Define VendingMachine with __init__(self, price) and insert(self, amount). Start credit at 0. Each insert adds to credit. If credit is less than price, return an f-string "Insert N more" with the remaining amount. Otherwise return an f-string "Vend! Change: N" with the change and reset credit to 0.

Worked sample calls:

machine = VendingMachine(3)
machine.insert(1) -> "Insert 2 more"
machine.insert(1) -> "Insert 1 more"
machine.insert(2) -> "Vend! Change: 1"
machine.credit -> 0

A workshop tracks quantities by item name.

Specification: Define Stockroom with dictionary attribute self.stock = {}. add(self, name, amount) uses if name in self.stock to increase an existing quantity or stores amount for a new name. remove(self, name, amount) subtracts the amount.

Worked sample calls:

room = Stockroom()
room.add("rivet", 10)
room.add("washer", 4)
room.remove("rivet", 3)
room.stock -> {"rivet": 7, "washer": 4}

A student record collects scores before calculating an average.

Specification: Define Student with __init__(self, name), list attribute self.scores, add_score(self, score) that appends, and average(self) that returns sum(self.scores) / len(self.scores). Assume at least one score before averaging.

Worked sample calls:

student = Student("Yusra")
student.add_score(80)
student.add_score(90)
student.average() -> 85.0

A board game needs a blank grid and a text picture of its current pieces.

Specification: Define GameBoard with __init__(self, size) that builds self.cells as a size-by-size list of lists containing "."; build each row with append, then append each row. place(self, row, col, mark) writes one cell. render(self) loops over rows, joins each row with "".join(row), appends each text row, and returns "\n".join(rows).

Worked sample calls:

board = GameBoard(3)
board.place(0, 0, "X")
board.place(2, 2, "O")
board.render() -> "X..\n...\n..O"

A builder wants the combined area of several rectangles.

Specification: Define Rectangle with __init__(self, width, height) and area(self). Define total_area(rectangles) that loops over a list of Rectangle objects, adds each area() to a running total, and returns that total.

Worked sample calls:

total_area([Rectangle(2, 3), Rectangle(4, 5)]) -> 26

A navigator needs the nearest point from a list of candidates.

Specification: Define Point with x, y, and distance(self, other) using (dx * dx + dy * dy) ** 0.5. Define closest(origin, points) for a nonempty list: start with the first point as best, replace it only when another distance is strictly smaller, and return the Point object. Print the result as f"{best.x} {best.y}".

Worked sample calls:

best = closest(Point(0, 0), [Point(3, 4), Point(1, 1), Point(6, 8)])
print(f"{best.x} {best.y}") -> 1 1

A method call fails before it can calculate an area.

Specification: Study the broken code and the exact error. Define a repaired Rect with __init__(self, width, height) and area(self) that returns width times height.

class Rect:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area():
        return self.width * self.height

print(Rect(3, 4).area())

TypeError: Rect.area() takes 0 positional arguments but 1 was given

Worked sample calls:

Rect(3, 4).area() -> 12

A crate has the right dimensions, but one attribute name is misspelled.

Specification: Study the broken code and the exact error. Define a repaired Crate whose volume(self) returns width times height times depth.

class Crate:
    def __init__(self, width, height, depth):
        self.width = width
        self.hieght = height
        self.depth = depth

    def volume(self):
        return self.width * self.height * self.depth

print(Crate(2, 3, 4).volume())

AttributeError: 'Crate' object has no attribute 'height'

Worked sample calls:

Crate(2, 3, 4).volume() -> 24

Two objects start with the same label but keep separate counts.

Specification: Predict the exact output of the code. Then define Counter with increment(self, amount) as shown and trace the two objects.

class Counter:
    def __init__(self, label):
        self.label = label
        self.count = 0

    def increment(self, amount):
        self.count = self.count + amount
        return self.count

a = Counter("x")
b = Counter("x")
b.increment(5)
print(a.count, b.count)

Worked sample calls:

print(a.count, b.count) -> 0 5

# Define Counter, trace both objects, then predict the printed line.

A shipping system labels rectangles by area.

Specification: Define Rectangle with __init__(self, width, height) and store width and height as attributes. Add area(self). Add size_band(self) that calls self.area() and returns "large" when the area is at least 60, "medium" when the area is at least 25, and "small" otherwise. Use one if/elif/else ladder.

Worked sample calls:

Rectangle(10, 7).size_band() -> "large"
Rectangle(7, 5).size_band()  -> "medium"
Rectangle(4, 3).size_band()  -> "small"

A museum wants to save a visitor counter and rebuild it tomorrow.

Specification: Define Counter with __init__(self, label, count) and store label and count as attributes. Add increment(self, amount) and save(self, path). The save method writes the label and count as two f-string lines. Define a module-level load_counter(path) that opens the file in explicit "r" mode and returns a new Counter with both saved attributes.

Use only ex22_counter.txt. Save before loading in the same cell.

Worked sample calls and reads:

visitors = Counter("north gate", 12)
visitors.increment(4) -> 16
visitors.save("ex22_counter.txt") -> "ex22_counter.txt"
restored = load_counter("ex22_counter.txt")
restored.label -> "north gate"
restored.count -> 16

A rover moves across a grid while facing one of four compass directions.

Specification: Define Rover starting at (0, 0) facing "N". Define run(self, commands) to read each F, R, or L. For right turns use "NESW"[("NESW".find(self.facing) + 1) % 4]; for left use the same index with - 1. F moves one step in the current direction. Use an if/elif/else chain. Define position(self) that returns f"({self.x}, {self.y}) facing {self.facing}".

Worked sample calls:

rover = Rover()
rover.run("FFRFF")
rover.position() -> "(2, 2) facing E"

A workshop wants to restore tomorrow’s inventory from a saved file.

Specification: Define Stockroom with dictionary attribute stock and add(self, name, amount) using membership for an existing name. Define save(self, path) to write each name,qty record on its own line in explicit "w" mode and return path. Define module-level load_stockroom(path) to read the file in explicit "r" mode, split each stripped line at ",", convert the quantity with int, and rebuild a Stockroom. Save before loading. Use ex24_stockroom.txt.

Worked sample calls:

room = Stockroom()
room.add("rivet", 10)
room.add("washer", 4)
room.stock["rivet"] = 7
room.save("ex24_stockroom.txt") -> "ex24_stockroom.txt"
loaded = load_stockroom("ex24_stockroom.txt")
loaded.stock -> {"rivet": 7, "washer": 4}