Trace It

Python does not read a program the way you read a paragraph. It reads one line, does exactly that, and moves on — carrying the values of every variable with it. Tracing is doing that job by hand, on paper, with a table. It is slow, it is unglamorous, and it is the single most reliable way to find out what a loop is actually doing.

The program

total = 0
for number in range(1, 6):
    if number % 2 == 0:
        total = total + number
print(total)

The trace table

One row per pass through the loop. Fill it in before you run anything.

Pass	number	number % 2 == 0	total after the pass
1	1	False	0
2	2	True	2
3	3	False	2
4	4	True	6
5	5	False	6

The program prints 6. Notice what the table exposes that reading never does: range(1, 6) produced 1 through 5 and stopped before 6, and total only changed on two of the five passes.

How to run it

1. Draw the columns: one per variable, plus any condition being tested.
2. Walk the program line by line, writing every change down. If you find yourself skipping ahead, you have stopped tracing.
3. Compare tables with a neighbour before running the code.
4. Run it. A disagreement between your table and Python’s answer is worth more than a page of correct tracing.

Tracing by hand is what a debugger automates, as Using the Debugger shows — but doing it on paper first is how you learn to read a loop without one.