Reading a traceback

The most useful thing on the screen

A traceback looks alarming and is actually a set of directions. Read it from the bottom:

Traceback (most recent call last):
  File "shop.py", line 22, in <module>
    print(total_price(basket))
  File "shop.py", line 15, in total_price
    return sum(item["price"] for item in items)
KeyError: 'price'

The last line is what went wrong: a dictionary had no 'price' key. The frames above it are the route Python took to get there, most recent last — so the second-to-last frame is usually where your bug lives.

Three habits worth forming:

1. Read the last line first.
2. Find the last frame in your own code — frames inside libraries are usually downstream of a mistake in yours.
3. The line number is where it surfaced, not always where it started; a None that crashes here may have been created somewhere else.

Your turn: total_price below crashes on items with no price. Fix it so missing prices count as zero.