Debugging Example – Finding the Maximum in a List

Below is a Python script that is supposed to find the maximum value in a list of numbers.
However, it contains a bug (or maybe a couple of bugs).
Your task is to run this code, observe what goes wrong, and fix the issues step by step.

def find_maximum(values):
    max_val = 0
    for val in values:
        if val > max_val:
            max_val = val
    return val  # Bug likely here

nums = [4, 1, 9, 3]
print("Max of", nums, ":", find_maximum(nums))

empty_list = []
print("Max of empty list:", find_maximum(empty_list))

Step 1 – Observe the Error

Running the script produces:

Max of [4, 1, 9, 3] : 3
Traceback (most recent call last):
  File "buggy_script.py", line 8, in <module>
    print("Max of empty list:", find_maximum(empty_list))
  File "buggy_script.py", line 5, in find_maximum
    return val
UnboundLocalError: local variable 'val' referenced before assignment

Observation:

1. The first list returns 3 instead of 9 (logic problem).

2. An UnboundLocalError occurs when calling with an empty list.

Step 2 – Investigate the Logic for Non-Empty List

• The loop correctly updates max_val when a larger value is found.

• But at the end, the function returns val, which is just the last loop value, not the maximum.

• For [4, 1, 9, 3], the loop ends with val = 3, so it returns 3.

Step 3 – Understand the Empty List Issue

• For [], the loop never runs.

• val is never assigned, so return val fails with UnboundLocalError.

• Even if we returned max_val instead, starting max_val = 0 could be wrong if all values are negative.

Step 4 – Fix the Bugs

1. Return max_val instead of val.

2. Handle empty list by raising an exception.

3. Initialize max_val with the first element of the list to handle negative numbers.

Revised Function:

def find_maximum(values):
    if len(values) == 0:
        raise ValueError("Cannot find max of an empty list")
    max_val = values[0]
    for val in values:
        if val > max_val:
            max_val = val
    return max_val

Step 5 – Testing the Fix

nums = [4, 1, 9, 3]
print("Max of", nums, ":", find_maximum(nums))

empty_list = []
try:
    print("Max of empty list:", find_maximum(empty_list))
except ValueError as e:
    print("Error:", e)

Expected Output:

Max of [4, 1, 9, 3] : 9
Error: Cannot find max of an empty list

Step 6 – Reflection

• Bug #1: Returned the wrong variable (val instead of max_val).

• Bug #2: No handling for empty input.

• Fixing logic and adding input validation makes the function more robust.

• This debugging process involved:

  • Observing the traceback.

  • Checking variable states.

  • Updating logic to avoid hidden edge case failures.