Level 6 · Stock left after a day of sales

Unpacking pairs in a loop

Each sale is a tuple, a fixed pair of values: the product name and the quantity. When every item in a list has the same shape, you can unpack it straight in the for line:

for name, qty in sales:
    print(name, qty)
# Hazel Linen Throw Pillow 6
# Northfield Brass Bud Vase 2
# ...

That is clearer than sale[0] and sale[1], and Python raises an error if a row ever has the wrong number of values, which is a useful early warning on a messy feed.

Updating a dictionary in place

A dictionary of stock levels is a running balance. Each sale reads the current number for that key and writes back the new one:

left = dict(stock)                 # a copy, so the morning count stays intact
left["Hazel Linen Throw Pillow"] = left["Hazel Linen Throw Pillow"] - 6    # 34

The pillow appears three times in today’s sales: 6, 3 and 2 units. Each pass through the loop subtracts from what the previous pass left, so it goes 40, 34, 31, 29. The same line can be written with the shorthand left[name] -= qty.

dict(stock) makes a new dictionary with the same keys and values. Working on a copy means you still have the morning numbers if someone asks “what did we start with?”, and you can rerun the calculation without reloading anything. If you wrote left = stock, both names would point at the same dictionary and the morning count would be overwritten.

Check the totals

A quick reconciliation catches most mistakes. The morning stock adds up to 85 units, and the sales add up to 23. Whatever you end with must add up to 85 - 23 = 62:

sum(left.values())       # 62

If your numbers don’t reconcile, a sale was skipped or counted twice.

Common mistakes

• Subtracting from the morning number on every pass (stock[name] - qty). Only the last sale of each product survives.
• Writing left = stock instead of a copy, and then wondering why the morning count changed.
• Forgetting that a product name in sales must match the key exactly. “Hazel linen throw pillow” with a lowercase l would raise a KeyError.