Calculator Program in Python

What we are building

A calculator takes two numbers and an operator (+, -, *, /) and shows the result. It is a great first project that brings together input, conditions, functions and error handling. We will build it step by step and make it safe against divide by zero.

A basic calculator

a = float(input("First number: "))
op = input("Operator (+ - * /): ")
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("Invalid operator")

We read two numbers (converted to float) and an operator, then use if-elif to run the matching operation. else catches an invalid operator.

Using functions

Putting each operation in a function keeps the code clean and reusable.

def calculate(a, b, op):
    if op == "+": return a + b
    if op == "-": return a - b
    if op == "*": return a * b
    if op == "/": return a / b
    return "Invalid operator"

print(calculate(20, 4, "/"))

Now the logic lives in one calculate function you can call anywhere, which is cleaner than repeating if-elif everywhere.

Handling divide by zero

def calculate(a, b, op):
    if op == "/":
        if b == 0:
            return "Cannot divide by zero"
        return a / b
    # ... other operators

print(calculate(10, 0, "/"))

Dividing by zero would crash the program, so we check if b == 0 first. You could also use a try-except ZeroDivisionError block.

A menu loop

Let the user do many calculations until they choose to quit.

while True:
    a = float(input("First number: "))
    op = input("Operator: ")
    b = float(input("Second number: "))
    print("Result:", calculate(a, b, op))

    again = input("Another? (y/n): ")
    if again != "y":
        break
print("Goodbye")

The while True loop repeats the calculator. After each result we ask if the user wants another; typing anything other than "y" breaks out and ends.

Common mistakes

• Not converting input. input() gives a string — use float() or int() for maths.
• Ignoring divide by zero. Always guard the division with a check or try-except.
• No else for bad operators. Handle invalid input so the program does not fail silently.
• Comparing with =. Use == to compare the operator, not the assignment =.

Summary

• A calculator reads two numbers and an operator, then computes.
• Convert input with float() before doing maths.
• Use if-elif or a calculate function to pick the operation.
• Guard against divide by zero with a check or try-except.
• Add a while loop to let the user repeat.

Next, learn decisions in if-else in Python, or reuse code with Python functions.