Python Calculator App Project: Step-by-Step Code & Tutorial

### Step 1: Project Setup Create a new project folder named `calculator_app`. Inside this folder, create a file named `main.py`. We will use standard Python built-in features, so no external `pip` installations are required.

### Step 2: Core Logic Implementation Define individual helper functions for addition, subtraction, multiplication, and division. Keeping them separate makes our code modular and easy to test. ```python def add(a: float, b: float) -> float: return a + b def subtract(a: float, b: float) -> float: return a - b def multiply(a: float, b: float) -> float: return a * b def divide(a: float, b: float) -> float: if b == 0: raise ValueError("Cannot divide by zero.") return a / b ```

### Step 3: UI & Interaction Implementation Create a continuous `while True` loop that prompts the user for inputs, parses strings into floats, and dispatches the corresponding function. ```python def main(): print("=== Python CLI Calculator ===") while True: try: num1 = float(input("Enter first number: ")) op = input("Enter operator (+, -, *, /) or 'q' to quit: ") if op.lower() == 'q': print("Exiting calculator. Goodbye!") break num2 = float(input("Enter second number: ")) # Execution logic follows... except ValueError: print("Invalid input. Please enter numbers correctly.") ```

### Step 4: Error Handling & Edge Cases Wrap the arithmetic execution in a `try-except` block to catch `ValueError` when division by zero occurs or when strings cannot be converted to floats. Always inform the user clearly rather than letting the application crash.