📘 Lesson 28 · Advanced

Project: Python Calculator

Build a command-line calculator with functions, error handling, and a menu loop.

Project Overview

Now that you've learned the fundamentals, let's build something real. This calculator runs in the terminal and performs basic arithmetic with proper error handling. It uses concepts from throughout the course — functions, loops, conditionals, and exception handling — all working together.

💡

Concepts used: functions, while loops, if/elif/else, try/except, f-strings, dictionaries, lambda.

Core Functions

Define one function per operation. This keeps the code organised, each piece testable, and the main program loop clean. Notice divide() checks for zero before attempting division.

ops.py

def add(a, b):      return a + b
def subtract(a, b): return a - b
def multiply(a, b): return a * b
def divide(a, b):
    if b == 0: return "Error: divide by zero"
    return a / b

print(add(10, 5))
print(divide(10, 0))

▶ Output

15
Error: divide by zero

Full Interactive Calculator

The main loop shows a menu, reads the choice, validates input, performs the operation, and loops again. A dictionary maps choices to lambdas — this avoids a long if/elif chain and is more Pythonic. The try/except catches non-numeric input without crashing.

calculator.py

def calculator():
    print("=== Python Calculator ===")
    ops = {{
        "1": ("+", lambda a,b: a+b),
        "2": ("-", lambda a,b: a-b),
        "3": ("×", lambda a,b: a*b),
        "4": ("÷", lambda a,b: a/b if b!=0 else "undefined")
    }}
    while True:
        print("\n1:Add 2:Sub 3:Mul 4:Div 5:Quit")
        c = input("Choose: ")
        if c == "5": print("Goodbye!"); break
        if c not in ops: print("Invalid"); continue
        try:
            a = float(input("First: "))
            b = float(input("Second: "))
        except ValueError:
            print("Enter valid numbers"); continue
        sym, fn = ops[c]
        print(f"  {a} {sym} {b} = {fn(a,b)}")

calculator()

▶ Output

=== Python Calculator ===

1:Add 2:Sub 3:Mul 4:Div 5:Quit
Choose: 3
First: 6
Second: 7
  6.0 × 7.0 = 42.0

✅

Extend this with a history list that stores past calculations, or a memory feature (M+, MR, MC) — great portfolio additions!

🧠 Quick Check

Which exception handles bad input when converting to float?

TypeError

ValueError

InputError

ConvertError