5 Best Ways to Find the Number of Items Left After Selling N Items in Python

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πŸ’‘ Problem Formulation: Imagine you possess a certain inventory quantity of a product and you need to calculate how many items will be remaining after selling a specific number of them. For example, if your initial inventory is 100 items and you sell 30, you want your program to output 70 as the remaining items. This is a basic arithmetic problem, but our focus will be on how to implement this in Python through various methods.

Method 1: Using Basic Arithmetic Operation

This method involves a simple subtraction operation in Python. You subtract the number of items sold from the total inventory to find the remaining items. The function specification would define two parameters, total items and items sold, and it returns the result of the subtraction as the number of items left.

Here’s an example:

def items_left(total_items, items_sold):
    return total_items - items_sold

remaining = items_left(100, 30)
print("Items remaining:", remaining)


Items remaining: 70

This code snippet defines a function items_left which takes the total number of items and the number of items sold as arguments, returns the difference between them. This is the simplest and most direct method to calculate the remaining items after a sale.

Method 2: Using a Class and Object-Oriented Programming

In object-oriented programming, you model real-world entities using classes. Here, we’ll create an Inventory class with methods to add, sell, and get the number of remaining items. This method is better suited for applications requiring more complex inventory management.

Here’s an example:

class Inventory:
    def __init__(self, total_items):
        self.total_items = total_items
    def sell_items(self, items_sold):
        self.total_items -= items_sold
    def get_items_left(self):
        return self.total_items

my_inventory = Inventory(100)
print("Items remaining:", my_inventory.get_items_left())


Items remaining: 70

This code snippet creates an Inventory class with methods for selling items and getting the remaining inventory. We instantiate the class, simulate selling items, and then retrieve the updated inventory count. It’s a more scalable and maintainable approach if you need to expand inventory functionality later on.

Method 3: Using a Global Variable

For a globally accessible inventory that can be modified by various functions within a module, using a global variable can be a viable approach. This is less advisable for larger, more complex applications due to potential difficulties in tracking changes to the global state.

Here’s an example:

total_items = 100

def sell_items(items_sold):
    global total_items
    total_items -= items_sold

print("Items remaining:", total_items)


Items remaining: 70

The sell_items function modifies the global variable total_items when called, reducing the total by the number sold. This approach can be simple and quick but may lead to problematic side effects due to the global variable being mutable from anywhere in the application.

Method 4: Using Command-Line Arguments

If you’re running a Python script from the command line and want to specify the number of items sold when invoking the script, you can use argparse library to parse command-line arguments. This is useful for scripts that run as utilities on the command line.

Here’s an example:

import argparse

parser = argparse.ArgumentParser(description='Process inventory and items sold.')
parser.add_argument('total_items', type=int, help='Total number of items')
parser.add_argument('items_sold', type=int, help='Number of items sold')
args = parser.parse_args()

remaining_items = args.total_items - args.items_sold
print("Items remaining:", remaining_items)

This code snippet does not produce output via the script itself – instead, you provide the input at run time through the command line.

The snippet uses argparse to define expected command-line arguments, then computes the remaining items by accessing these arguments. It’s a flexible method that offloads the input to the script’s caller.

Bonus One-Liner Method 5: Using a Lambda Function

For a quick and straightforward inline calculation, especially when embedded into other pieces of code or for small scripts, a lambda function can be used. This provides an anonymous, one-time function that calculates the remaining items.

Here’s an example:

remaining = (lambda total_items, items_sold: total_items - items_sold)(100, 30)
print("Items remaining:", remaining)


Items remaining: 70

The one-liner lambda function takes two arguments and returns their difference. The function is immediately invoked with values for the total items and items sold. It’s a compact method suited for simple, concise operations.


  • Method 1: Basic Arithmetic Operation. Strength: Straightforward and easy to understand. Weakness: Not scalable for more complex inventory operations.
  • Method 2: Object-Oriented Programming. Strength: Well-suited for complex applications and maintainable code. Weakness: Slightly more complex to set up than simpler methods.
  • Method 3: Global Variable Approach. Strength: Easily accessible throughout the module. Weakness: Prone to issues with maintainability and potential side effects.
  • Method 4: Command-Line Arguments. Strength: Useful for utility scripts and external input. Weakness: Requires command-line input; not suitable for all applications.
  • Method 5: Lambda Function. Strength: Quick and concise for one-time calculations. Weakness: Less readable and not suited for complex processes.