Subtracting values of only matching indexes in Pandas Dataframes

I've been working on a program to manage a store inventory using Pandas. For what I am working on now is subtracting the quantity ordered from the original and printing the inventory after the transaction. I created two dataframes to hold the original and the user's cart.

Inventory

Cart

I've tried using the command inventory['Quantity'] = inventory['Quantity'] - cart['Quantity'] to get a final result but it comes up as

Does anyone know how to stop the other rows from coming back as NaN?

Use the following approach, and you can solve your problem:

import pandas as pd

# Create the inventory data frame
inventory_data = {'Items': ['Apple', 'Banana', 'Orange', 'Book', 'Notepad'],
                  'Price': [1.30, 0.57, 1.33, 15.0, 1.0],
                  'Quantity': [90, 100, 50, 50, 100]}
inventory = pd.DataFrame(inventory_data)

# Create the cart data frame
cart_data = {'Items': ['Apples', 'Orange'],
             'Cost': [13.0, 6.65],
             'Quantity': [10, 5]}
cart = pd.DataFrame(cart_data)

# Convert item names to lowercase for matching
inventory['Items'] = inventory['Items'].str.lower()
cart['Items'] = cart['Items'].str.lower()

# Merge the inventory and cart data frames on the "Items" column
merged = pd.merge(inventory, cart, on='Items', how='left')

# Subtract cart quantities from inventory quantities
merged['Quantity'] = merged['Quantity_x'] - merged['Quantity_y']

# Fill NaN values with inventory quantities
merged['Quantity'] = merged['Quantity'].fillna(merged['Quantity_x'])

# Select the necessary columns and rename them
updated_inventory = merged[['Items', 'Price', 'Quantity']].rename(columns={'Quantity': 'Quantity'})

# Convert the item names back to their original case
updated_inventory['Items'] = updated_inventory['Items'].str.capitalize()

# Convert the quantity column to integers
updated_inventory['Quantity'] = updated_inventory['Quantity'].astype(int)

# Print the updated inventory
print(updated_inventory)

Use Series.subtract with fill_value = 0 so that non-matched indices are subtracted by zero.

inventory["Quantity"].subtract(cart["Quantity"], fill_value=0)