Building a Calculator in Python

A calculator you build yourself

Every phone has a calculator app, and somebody had to program it. In this lesson you'll build your own — first a simple version, then a full one that lets you keep doing sums until you're finished. Along the way you'll practise the maths operators, reading input, making decisions, and looping. It's a perfect first real project.

If you've met variables and basic input and output, you have everything you need. Let's go.

The maths operators

Python knows arithmetic out of the box. Here are the four you'll use most:

print(8 + 3)    # addition      -> 11
print(8 - 3)    # subtraction   -> 5
print(8 * 3)    # multiplication-> 24
print(8 / 3)    # division      -> 2.6666666666666665

Two things to notice. Multiplication uses * (a star), not x. And division with / gives a decimal answer, even when it divides evenly — 6 / 2 is 3.0, not 3. You can store any result in a variable:

total = 100 + 50
print(total)    # 150

Reading numbers from the user

A calculator is useless if it can only work with numbers you typed into the code. We want the user to supply them. The input() function asks a question and gives back whatever they type — but always as text:

typed = input("Enter a number: ")
print(typed + typed)

If you typed 5, that surprisingly prints 55, not 10! That's because typed is the string "5", and adding strings glues them together. To do real maths we must convert the text to a number with float():

number = float(input("Enter a number: "))
print(number + number)   # now typing 5 gives 10.0

float() turns "5" into the number 5.0. We use float (rather than int) so the calculator handles decimals like 3.5 too.

Choosing an operation with if / elif / else

A real calculator lets you pick +, -, * or /. We ask which one and use if / elif / else to decide what to do:

a = float(input("First number: "))
op = input("Operation (+ - * /): ")
b = float(input("Second number: "))

if op == "+":
    print(a + b)
elif op == "-":
    print(a - b)
elif op == "*":
    print(a * b)
elif op == "/":
    print(a / b)
else:
    print("Unknown operation.")

The program checks the symbol the user typed and runs the matching line. The final else catches anything that isn't one of the four operators, so a typo doesn't silently do nothing. If you'd like a refresher, see making decisions with if.

The divide-by-zero trap

There's one sum that breaks maths itself: dividing by zero. In Python it doesn't give a wrong answer — it crashes your whole program with a ZeroDivisionError. We must guard against it:

if op == "/":
    if b == 0:
        print("Cannot divide by zero!")
    else:
        print(a / b)

The inner if b == 0 checks the divisor before dividing. If the second number is zero, we print a polite message instead of crashing. Always protect division this way.

Worked example: the full calculator

Now we combine everything — and wrap it in a while loop so the user can do as many calculations as they like, then quit when they're done.

def calculate(a, op, b):
    """Return the result of a op b, or an error message string."""
    if op == "+":
        return a + b
    elif op == "-":
        return a - b
    elif op == "*":
        return a * b
    elif op == "/":
        if b == 0:
            return "Error: cannot divide by zero"
        return a / b
    else:
        return "Error: unknown operation"

def run_calculator():
    print("Simple Calculator — type q at any time to quit.")
    while True:
        first = input("\nFirst number: ")
        if first == "q":
            break
        a = float(first)

        op = input("Operation (+ - * /): ")
        b = float(input("Second number: "))

        result = calculate(a, op, b)
        print(f"{a} {op} {b} = {result}")

    print("Goodbye!")

run_calculator()

How it works:

1. calculate(a, op, b) is a function that does one job: it takes two numbers and an operator and returns the answer. Keeping the maths in its own function makes the program tidy and testable. Notice the divide-by-zero check returns a message instead of crashing.
2. run_calculator holds the loop. while True: repeats forever until we break.
3. Before converting, we check if first == "q": so the user can quit by typing q. Putting the check first means we never try to float("q"), which would crash.
4. Each pass reads the operator and second number, calls calculate, and prints a neat line like 8.0 * 3.0 = 24.0.

Run it and you've got a real, reusable calculator. Type a few sums, then q to leave.

Try it yourself

Make your calculator smarter:

• Add a power operation. In Python, a b means "a to the power of b" (so 2 3 is 8). Add an elif op == "**": branch.
• Add a remainder operation using % (so 7 % 2 is 1).
• Keep a running history: each time you calculate, append the result line to a list, and when the user types q, print the whole history before saying goodbye.
• Challenge: protect the program from non-number input by wrapping float(...) in a try / except ValueError so a typo shows "Please type a number" instead of crashing.

When your calculator works, see how the same return-based thinking powers other programs in functions that return values.