How to trace Python code by hand

Code tracing is reading a program like a machine would, step by step. It is the foundation of debugging and of many exam questions.

The method

1. Read top to bottom. Statement order matters.
2. Track every variable. Write each assignment and update.
3. Evaluate expressions in order. Multiplication before addition, left to right for ties.
4. Count loop iterations. Ask how many times the body runs.

Example

total = 0
for i in range(4):
    total += 1
print(total)

The loop body runs 4 times, so total becomes 4.

Common traps

• Indices start at 0.
• % is the remainder, // is floor division.
• A loop that never ends is an infinite loop.

Practise with randomised code-tracing questions below.