Build a Python CMD Application 🐍

A Beginner's Guide to a Simple Car Rental System Using CRUD Principles.

Back to Basics: Command-Line Power! 💻

Web applications are powerful, but many essential programs run directly in the command line (CMD) or terminal. These applications are fast, lightweight, and great for learning core programming logic.

In this tutorial, we will build a complete car rental management system that runs in your terminal. We will implement the four fundamental **CRUD** operations: **C**reate (add a car), **R**ead (view cars), **U**pdate (change a car's status), and **D**elete (remove a car).

Step 0: The Goal & Our Data Structure 🎯

Project Goal

To create a menu-driven CMD program where a user can manage a list of rental cars. All you need is Python installed—no extra libraries are required.

The Data Structure: A List of Dictionaries

To keep things simple, we won't use a database. Instead, we'll store our car data in memory using a Python **list**. Each item in the list will be a **dictionary**, where each dictionary represents one car. This structure is intuitive and easy to manage.

# This is how our data will be structured.
cars = [
    {'id': 1, 'make': 'Toyota', 'model': 'Camry', 'year': 2022, 'status': 'Available'},
    {'id': 2, 'make': 'Honda', 'model': 'Accord', 'year': 2023, 'status': 'Rented'},
    # ... more cars can be added here
]

Step 1: Setting Up the Main Program Structure 뼈대

Every good CMD application needs a main loop that keeps the program running and a menu to show the user their options. Let's create this skeleton first.

# We'll start with some sample data.
cars = [
    {'id': 1, 'make': 'Toyota', 'model': 'Camry', 'year': 2022, 'status': 'Available'},
    {'id': 2, 'make': 'Honda', 'model': 'Civic', 'year': 2023, 'status': 'Rented'}
]
next_id = 3 # To keep track of the next available ID for new cars

def display_menu():
    print("\n--- Car Rental System Menu ---")
    print("1. View All Cars")
    print("2. Add a New Car")
    print("3. Update Car Status")
    print("4. Delete a Car")
    print("5. Exit")

# Main application loop
while True:
    display_menu()
    choice = input("Enter your choice (1-5): ")
    
    if choice == '1':
        # Call function to view cars (we'll build this next)
        print("Viewing cars...")
    elif choice == '2':
        # Call function to add a car
        print("Adding a car...")
    elif choice == '3':
        # Call function to update a car
        print("Updating a car...")
    elif choice == '4':
        # Call function to delete a car
        print("Deleting a car...")
    elif choice == '5':
        print("Exiting the program. Goodbye!")
        break
    else:
        print("Invalid choice. Please enter a number between 1 and 5.")

Step 2: The "Read" Operation - Viewing Cars 📖

This is the simplest operation. We'll create a function that iterates through our `cars` list and prints the details of each car in a nicely formatted table.

def view_cars(car_list):
    print("\n--- List of All Cars ---")
    if not car_list:
        print("There are no cars in the system.")
        return

    # Print table headers
    print(f"{'ID':<5}{'Make':<15}{'Model':<15}{'Year':<10}{'Status':<15}")
    print("-"*60)
    
    for car in car_list:
        print(f"{car['id']:<5}{car['make']:<15}{car['model']:<15}{car['year']:<10}{car['status']:<15}")

Now, we can replace the placeholder `print("Viewing cars...")` in our main loop with a call to this function: `view_cars(cars)`.

Step 3: The "Create" Operation - Adding a Car ➕

This function will ask the user for the new car's details, create a new dictionary, and append it to our list.

def add_car(car_list):
    global next_id # Use the global variable for the ID
    print("\n--- Add a New Car ---")
    make = input("Enter car make: ")
    model = input("Enter car model: ")
    while True:
        try:
            year = int(input("Enter car year: "))
            break
        except ValueError:
            print("Invalid year. Please enter a number.")

    new_car = {
        'id': next_id,
        'make': make,
        'model': model,
        'year': year,
        'status': 'Available' # New cars are available by default
    }
    car_list.append(new_car)
    next_id += 1 # Increment the ID for the next car
    print(f"Success! Car '{make} {model}' added with ID {new_car['id']}.")

We'll replace the placeholder in the main loop with `add_car(cars)`.

Step 4: The "Update" Operation - Changing Status 🔄

Updating allows us to change existing data. In our system, the most common update will be changing a car's status from "Available" to "Rented," or vice-versa.

def update_car_status(car_list):
    print("\n--- Update Car Status ---")
    view_cars(car_list) # Show cars so user can pick an ID
    if not car_list:
        return
        
    try:
        car_id = int(input("Enter the ID of the car to update: "))
    except ValueError:
        print("Invalid ID. Please enter a number.")
        return

    car_to_update = None
    for car in car_list:
        if car['id'] == car_id:
            car_to_update = car
            break

    if car_to_update:
        print(f"Current status of car ID {car_id}: {car_to_update['status']}")
        new_status = input("Enter new status (e.g., Rented, Available): ")
        car_to_update['status'] = new_status
        print("Status updated successfully!")
    else:
        print("Error: Car with that ID not found.")

Replace the update placeholder in the main loop with `update_car_status(cars)`.

Step 5: The "Delete" Operation - Removing a Car 🗑️

This function will remove a car's dictionary from our list entirely.

def delete_car(car_list):
    print("\n--- Delete a Car ---")
    view_cars(car_list) # Show cars so user can pick an ID
    if not car_list:
        return
        
    try:
        car_id = int(input("Enter the ID of the car to delete: "))
    except ValueError:
        print("Invalid ID. Please enter a number.")
        return

    car_to_delete = None
    for car in car_list:
        if car['id'] == car_id:
            car_to_delete = car
            break

    if car_to_delete:
        car_list.remove(car_to_delete)
        print(f"Success! Car with ID {car_id} has been deleted.")
    else:
        print("Error: Car with that ID not found.")

Finally, replace the delete placeholder in the main loop with `delete_car(cars)`.

Step 6: The Complete Program 🏁

Here is the full Python script with all the functions integrated. You can copy this code, save it as a `.py` file (e.g., `rental.py`), and run it from your terminal using `python rental.py`.

# --- Data Storage ---
cars = [
    {'id': 1, 'make': 'Toyota', 'model': 'Camry', 'year': 2022, 'status': 'Available'},
    {'id': 2, 'make': 'Honda', 'model': 'Civic', 'year': 2023, 'status': 'Rented'}
]
next_id = 3

# --- CRUD Functions ---

def view_cars(car_list):
    print("\n--- List of All Cars ---")
    if not car_list:
        print("There are no cars in the system.")
        return
    print(f"{'ID':<5}{'Make':<15}{'Model':<15}{'Year':<10}{'Status':<15}")
    print("-"*60)
    for car in car_list:
        print(f"{car['id']:<5}{car['make']:<15}{car['model']:<15}{car['year']:<10}{car['status']:<15}")

def add_car(car_list):
    global next_id
    print("\n--- Add a New Car ---")
    make = input("Enter car make: ")
    model = input("Enter car model: ")
    while True:
        try:
            year = int(input("Enter car year: "))
            break
        except ValueError:
            print("Invalid year. Please enter a number.")
    new_car = {'id': next_id, 'make': make, 'model': model, 'year': year, 'status': 'Available'}
    car_list.append(new_car)
    next_id += 1
    print(f"Success! Car '{make} {model}' added with ID {new_car['id']}.")

def update_car_status(car_list):
    print("\n--- Update Car Status ---")
    view_cars(car_list)
    if not car_list: return
    try:
        car_id = int(input("Enter the ID of the car to update: "))
    except ValueError: