Python Calculator Project

1Calculator Project: overview

A calculator program reads an expression such as 12 * 3, splits it into two numbers and an operator, runs the matching operation and prints the result — reporting bad input instead of crashing.

Before this lesson: Python while Loops, Python Dictionaries, Python Functions, Python Exceptions: try, except, else and finally

2Calculator Project: diagram

Quick reference: Calculator Project

• Python Simple Calculator Program — A simple Python calculator reads two numbers and an operator with `input()`, then uses `if/elif` (or a dictio…

4Calculator Project: syntax

import operator

OPS = {"+": operator.add, "-": operator.sub}   # symbol -> function

def parse(line):                 # text -> (a, op, b) or ValueError
    a, op, b = line.split()
    return float(a), op, float(b)

while True:
    line = input("calc> ")
    if line == "q":
        break
    try:
        a, op, b = parse(line)
        print(OPS[op](a, b))
    except ZeroDivisionError:
        print("cannot divide by zero")
    except ValueError as error:
        print("Error:", error)

5Calculator Project: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

import operator

OPS = {"+": operator.add, "-": operator.sub,
       "*": operator.mul, "/": operator.truediv}

def parse(line):
    parts = line.split()
    if len(parts) != 3 or parts[1] not in OPS:
        raise ValueError("write it like 12 * 3 (operators: + - * /)")
    return float(parts[0]), parts[1], float(parts[2])

def calculate(a, op, b):
    return OPS[op](a, b)

while True:
    line = input("calc> ")
    if line == "q":
        break
    try:
        a, op, b = parse(line)
        print(f"= {calculate(a, op, b):g}")
    except ZeroDivisionError:
        print("Error: cannot divide by zero")
    except ValueError as error:
        print("Error:", error)

Expected outputcalc> 12 * 3
= 36
calc> 7 / 2
= 3.5
calc> 5 / 0
Error: cannot divide by zero
calc> ten + 2
Error: could not convert string to float: 'ten'
calc> 4 ^ 2
Error: write it like 12 * 3 (operators: + - * /)
calc> q

7Calculator Project: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1The operator module provides the arithmetic operators as ordinary functions: add, sub, mul and truediv.
2. Line 3-4OPS maps each symbol to the function that does that maths. Functions are values, so a dictionary can store them.
3. Line 6-7parse() splits the line on whitespace: "12 * 3" becomes ["12", "*", "3"].
4. Line 8-9If there are not exactly three pieces, or the middle one is not a known operator, parse() raises ValueError with a hint about the right format.
5. Line 10float() converts both numbers and the function returns a tuple. float("ten") would raise its own ValueError here.
6. Line 12-13calculate() looks up the function for op in the dictionary and calls it with a and b.
7. Line 15-16The loop runs until told to stop. input() shows the prompt calc> and reads one line — on this page, from the Input box.
8. Line 17-18Typing q ends the loop with break, which ends the program.
9. Line 19-21The normal path: parse, calculate, print. The :g format drops a pointless .0, so 36.0 prints as 36.
10. Line 22-235 / 0 raises ZeroDivisionError inside calculate(). It is caught here, a message is printed and the loop continues.
11. Line 24-25Any ValueError — from the format check or from float() — is caught too, and its message tells the user what went wrong.

8Calculator Project: common mistakes

a = input("First number: ")
b = input("Second number: ")
print("Sum:", a + b)

a = float(input("First number: "))
b = float(input("Second number: "))
print("Sum:", a + b)

def divide(a, b):
    result = a / b
    if b == 0:
        return "cannot divide by zero"
    return result

print(divide(5.0, 0.0))

def divide(a, b):
    if b == 0:
        return "cannot divide by zero"
    return a / b

print(divide(5.0, 0.0))

line = "5 / 0"
try:
    a, op, b = line.split()
    print(float(a) / float(b))
except:
    print("Something went wrong")

line = "5 / 0"
try:
    a, op, b = line.split()
    print(float(a) / float(b))
except ZeroDivisionError:
    print("Error: cannot divide by zero")
except ValueError as error:
    print("Error:", error)

9Calculator Project: exercises

Write your answer, then press Check. The checks run in your browser.

import operator

OPS = {"+": operator.add, "-": operator.sub,
       "*": operator.mul, "/": operator.truediv}

def parse(line):
    parts = line.split()
    if len(parts) != 3 or parts[1] not in OPS:
        raise ValueError("write it like 12 * 3")
    return float(parts[0]), parts[1], float(parts[2])

def calculate(a, op, b):
    return OPS[op](a, b)

print(calculate(10, "%", 3))
print(calculate(2, "**", 5))

import operator

OPS = {"+": operator.add, "-": operator.sub,
       "*": operator.mul, "/": operator.truediv,
       "%": operator.mod, "**": operator.pow}

def parse(line):
    parts = line.split()
    if len(parts) != 3 or parts[1] not in OPS:
        raise ValueError("write it like 12 * 3")
    return float(parts[0]), parts[1], float(parts[2])

def calculate(a, op, b):
    return OPS[op](a, b)

print(calculate(10, "%", 3))
print(calculate(2, "**", 5))

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "undefined"

for text in ["8 / 2", "3 / 0", "x / 2"]:
    try:
        left, op, right = text.split()
        print(safe_divide(float(left), float(right)))
    except ValueError:
        print("bad number")

4.0
undefined
bad number

2 more exercises → 4 more examples

Further reading

• Python tutorial

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Python Input and OutputPython FunctionsPython Exceptions: try, except, else and finallyPython Expense Tracker Project

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