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Lab 01 — Build a CLI Calculator

Hands-on Lab 30 min Beginner

Objectives​

By the end of this lab, you will be able to:

  1. Use input() to collect numeric data and convert it
  2. Apply all arithmetic operators correctly
  3. Format output with f-strings including alignment and precision
  4. Handle edge cases (division by zero) with conditionals
  5. Organize code with functions

Prerequisites​

  • Python 3.10+ installed
  • Lessons 01.1, 01.2, 01.3 completed

Step 1 — Create the Project​

mkdir ~/python-course/lab-01
cd ~/python-course/lab-01

Create calculator.py:

#!/usr/bin/env python3
"""
Lab 01 — CLI Calculator
A complete command-line calculator demonstrating Python fundamentals.
"""

def get_numbers():
    """Prompt the user for two numbers and return them as floats."""
    while True:
        try:
            a = float(input("Enter the first number: "))
            b = float(input("Enter the second number: "))
            return a, b
        except ValueError:
            print("❌ Invalid input — please enter numeric values.\n")


def calculate(a, b):
    """Perform all operations and return results as a dict."""
    results = {
        "addition":       a + b,
        "subtraction":    a - b,
        "multiplication": a * b,
        "division":       a / b if b != 0 else None,
        "floor_division": a // b if b != 0 else None,
        "modulo":         a % b if b != 0 else None,
        "power":          a ** b,
    }
    return results


def display_results(a, b, results):
    """Print a formatted results table."""
    width = 50
    print("\n" + "=" * width)
    print(f"{'CALCULATION RESULTS':^{width}}")
    print("=" * width)
    print(f"  {'Inputs':<20} a = {a:>10}   b = {b:>10}")
    print("-" * width)

    ops = [
        ("Addition",        "+", results["addition"]),
        ("Subtraction",     "-", results["subtraction"]),
        ("Multiplication",  "×", results["multiplication"]),
        ("Division",        "÷", results["division"]),
        ("Floor Division",  "//", results["floor_division"]),
        ("Modulo",          "%", results["modulo"]),
        ("Power",           "**", results["power"]),
    ]

    for name, symbol, value in ops:
        if value is None:
            display = "undefined (division by zero)"
        else:
            display = f"{value:,.6g}"
        print(f"  {name:<18} ({symbol}): {display:>20}")

    print("=" * width)

    # Extra statistics
    if results["division"] is not None:
        ratio = results["division"]
        print(f"\n  a / b expressed as:")
        print(f"    Float : {ratio:.10f}")
        print(f"    2 dec : {ratio:.2f}")
        print(f"    Sci   : {ratio:.3e}")
        print(f"    Percent: {ratio:.2%}")
    print()


def run_calculator():
    """Main loop — allows multiple calculations."""
    print("=" * 50)
    print(f"{'Python CLI Calculator — Lab 01':^50}")
    print("=" * 50)

    while True:
        a, b = get_numbers()
        results = calculate(a, b)
        display_results(a, b, results)

        again = input("Calculate again? (y/n): ").strip().lower()
        if again != "y":
            break

    print("\nGoodbye! 👋")


if __name__ == "__main__":
    run_calculator()

Step 2 — Run and Test​

python3 calculator.py

Verification ✅​

Test with a = 17, b = 5:

==================================================
            Python CLI Calculator — Lab 01
==================================================
Enter the first number: 17
Enter the second number: 5

==================================================
              CALCULATION RESULTS
==================================================
  Inputs               a =         17   b =          5
--------------------------------------------------
  Addition         (+):                   22
  Subtraction      (-):                   12
  Multiplication   (×):                   85
  Division         (÷):                  3.4
  Floor Division   (//)：                   3
  Modulo           (%):                    2
  Power            (**):              1419857
==================================================
  a / b expressed as:
    Float : 3.4000000000
    2 dec : 3.40
    Sci   : 3.400e+00
    Percent: 340.00%

Step 3 — Test Edge Cases​

Run again with b = 0:

Enter the first number: 10
Enter the second number: 0
  Division         (÷):      undefined (division by zero)
  Floor Division   (//):     undefined (division by zero)
  Modulo           (%):      undefined (division by zero)

Verification ✅​

The calculator should gracefully display "undefined" instead of crashing.

Step 4 — Test Invalid Input​

Enter the first number: abc
❌ Invalid input — please enter numeric values.

Enter the first number:

Verification ✅​

The try/except ValueError block catches the error and prompts again.

Bonus Challenge​

Add a history feature:

  1. Store each calculation in a list of tuples [(a, b, results), ...]
  2. At the end, print a summary showing all calculations performed
  3. Calculate the average result of all addition operations
history = []

# Inside the loop, after display_results:
history.append((a, b, results))

# After the loop:
print(f"\n{'HISTORY':^50}")
for i, (a, b, res) in enumerate(history, 1):
    print(f"  #{i}: {a} + {b} = {res['addition']}")

Cleanup​

No cleanup required — this is a local Python script.

Summary​

In this lab you:

  • Used float(input(...)) with error handling for robust number input
  • Applied all 7 arithmetic operators including edge case (division by zero)
  • Formatted a results table with f-strings: alignment (:<, :>), precision (.2f), scientific notation (.3e), and percentage (.2%)
  • Organized code with focused functions following the single responsibility principle