🧮 Python Calculator

A simple calculator program that performs arithmetic operations on two numbers, supports result rounding, and maintains a history of calculations using file storage.

---

📘 Features

Calculator

• Performs basic arithmetic operations:
  • Addition
  • Subtraction
  • Multiplication
  • Division
• Supports custom rounding precision (e.g., 10 / 3 → 3.33 when rounding to 2 digits)
• Allows updating rounding digits at runtime

History

• Saves expressions and results to a text file
• Restores previously saved expressions
• Clears all saved history

---

🧩 Class Overview

Calculator

Method	Description
__init__(round_digits)	Initializes calculator with a rounding precision
addition(a, b)	Returns the sum of a and b
subtraction(a, b)	Returns the difference of a and b
multiplication(a, b)	Returns the product of a and b
division(a, b)	Returns the quotient of a and b
set_round_digits(digits)	Updates the rounding precision
round_digits (property)	Returns the current rounding digits

History

Method	Description
save(expression: str)	Saves a string expression to a file
restore() -> list[str]	Returns a list of saved expressions
clear()	Clears the history file

---

🧪 Tests

Unit tests validate:

• Correct arithmetic results and rounding
• Division by zero handling
• File-based history operations (save, restore, clear)

To run the tests:

python -m unittest test_calculator.py

You must include spaces between the numbers and operator.
Invalid expressions are rejected with an explanatory message.

---

💻 The app.py Program

app.py is the main application script that brings everything together.
It acts as the user interface for your calculator system.

Here's what it does step-by-step:

1. Displays a colorful welcome screen with usage instructions.
2. Creates:
   • A Calculator object (from utils.py) to perform operations.
   • A History object to manage a text file (History.txt).
3. Listens for user input in an infinite loop:
   • h → Displays the last 5 saved expressions (most recent first)
   • c → Clears the history file
   • r → Lets the user set how many decimal digits results are rounded to
   • q → Quits the program
   • Any valid arithmetic expression → Computes and prints the result, then saves it to history
4. Validates input expressions using the is_valid_expression() helper.
5. Prints results neatly and appends them to the history file.

---

Example Session

==========================================================================
Welcome to the calculator:

-> h to view history (last 5 operations)
-> c to clear history
-> r to set round off digits (e.g., 2 means 7.33)
-> enter expression like (1 + 1) to get an answer
-> q to quit program
==========================================================================

> 10 / 3
= 3.33

> 5 + 2
= 7.0

> h
--------History---------
-    5 + 2 = 7.0
-    10 / 3 = 3.33