Python Practice 8/9

Fix the bug: read the traceback

So far every lesson asked you to write code from scratch. Real work is more often the opposite: someone else's code is already there, and it's broken. The skill that matters is reading the traceback Python hands you - it names the exact line and the exact exception, if you read it instead of guessing.

The code below throws as soon as you press Run - it isn't waiting for a test to catch it, the bug crashes it immediately. IndexError: list index out of range means the loop asked for an index that doesn't exist in the list - and the line number in the traceback points straight at the loop that asked for it.

Your task: run the code, read the traceback, and fix the bug in average so it returns the mean of the numbers in nums.

You'll practice:

Reading a traceback instead of guessing where the bug is
Tracing an IndexError back to an off-by-one loop bound

# This code throws when you run it - read the traceback and fix the bug.
def average(nums):
    total = 0
    for i in range(len(nums) + 1):
        total += nums[i]
    return total / len(nums)

print(average([10, 20, 30]))