🧪 Practice: Errors & Files

Failure handling and persistence, under your fingers. Five exercises, solutions at the bottom.

Exercise 1 — The unbreakable input

Write a function get_positive_number(prompt) that keeps asking until the user enters a valid positive number (decimals allowed). No crash should ever be possible.

code

weight = get_positive_number("Enter weight (kg): ")
print(f"Got: {weight}")

Exercise 2 — Safe division calculator

Ask for two numbers and print the division. Handle BOTH failure modes: non-numeric input and zero denominator — each with its own friendly message.

Exercise 3 — The guest list (JSON persistence)

Create guests.json-backed app:

Load the guest list (empty list if no file)

Ask for a name, add it, save

Print the full list

Run the script 3 times with different names — all names should accumulate.

Exercise 4 — Config loader with defaults

A config file settings.json may or may not exist, and may be missing keys. Write a loader that returns a dict with these defaults filled in:

code

DEFAULTS = {"theme": "dark", "volume": 50, "notifications": True}

If the file exists, its values override defaults; missing keys fall back to defaults.

Exercise 5 — CSV to report

Given marks.csv:

code

name,subject,marks
Aarav,Math,85
Aarav,Science,90
Diya,Math,95
Diya,Science,88

Read it with csv.DictReader and print each student's total marks:

code

Aarav: 175
Diya: 183

*(Hint: accumulate into a dict keyed by name — the counting pattern from Module 5, upgraded.)*

Solutions

Exercise 1:

code

def get_positive_number(prompt):
    while True:
        try:
            value = float(input(prompt))
            if value <= 0:
                print("Must be positive!")
                continue
            return value
        except ValueError:
            print("That's not a number — try again")

weight = get_positive_number("Enter weight (kg): ")
print(f"Got: {weight}")

Exercise 2:

code

try:
    a = float(input("Numerator: "))
    b = float(input("Denominator: "))
    print(f"Result: {a / b}")
except ValueError:
    print("Numbers only, please")
except ZeroDivisionError:
    print("Cannot divide by zero")

Exercise 3:

code

import json
import os

FILE = "guests.json"

guests = []
if os.path.exists(FILE):
    with open(FILE) as f:
        guests = json.load(f)

name = input("Guest name: ")
guests.append(name)

with open(FILE, "w") as f:
    json.dump(guests, f, indent=2)

print("Guests so far:", guests)

Exercise 4:

code

import json
import os

DEFAULTS = {"theme": "dark", "volume": 50, "notifications": True}
settings = DEFAULTS.copy()

if os.path.exists("settings.json"):
    try:
        with open("settings.json") as f:
            saved = json.load(f)
        settings.update(saved)          # saved values override defaults
    except json.JSONDecodeError:
        print("Settings file corrupted — using defaults")

print(settings)

(dict.update(other) merges — other's keys win. The try/except guards against a corrupted file, because a settings file should never crash the app.)

Exercise 5:

code

import csv

totals = {}
with open("marks.csv") as f:
    for row in csv.DictReader(f):
        name = row["name"]
        marks = int(row["marks"])
        if name in totals:
            totals[name] += marks
        else:
            totals[name] = marks

for name, total in totals.items():
    print(f"{name}: {total}")

✅ Module 8 Checkpoint

Errors handled, files persisting, formats parsed. Your programs now survive contact with real users and remember between runs — two traits that separate toys from tools.

Next module: Object-Oriented Python — organizing code the way big projects do.