Debugging Your Code

Follow the Execution Order

When a program contains several statements, Python does not execute them all at once. It runs them in order from top to bottom. One statement completes before the next statement begins. This execution order gives you a reliable way to understand what a script will do and to investigate behavior that is not what you expected.

Trace Each Statement

To trace a script, examine each statement in sequence and ask two questions: What does this statement do, and does it produce output? Also note whether it changes a variable. This separates the internal work of the program from the visible result on the screen.

score = 6
print(score)
score = 10
print(score)

6
10

number = 2
print(number)
number = 7
print(number)

Store Values or Display Results

An assignment statement changes what value is stored for a variable, but the assignment does not by itself place text on the screen. A print() statement displays a result. Values assigned in earlier statements remain available for later statements, so a later print() can display a value that was stored earlier or a newer value that replaced it.

Mistakes Beginners Make

• Expecting an assignment to display its value

  The assignment stores 25 but does not produce visible output.

  Fix: Use print(value) when the value needs to appear on the screen.

• Reading the final value into every earlier print() statement

  Each print() runs at its position in the sequence. The first print() runs before amount changes to 8.

  Fix: Trace the current value at each line. The output is 3 followed by 8.

• Skipping assignment statements while tracing

  The second assignment makes a value available to the later print() statement.

  Fix: Record what each assignment stores, even when it produces no output.

• Looking only at statements that produce output

  The final output depends on the later assignment, even though neither assignment displays anything.

  Fix: Mark both output and value changes while moving from top to bottom.

Practice the Trace

label = 4
print(label)
label = 11
print(label)

A Reliable Debugging Routine

1. Begin with the first statement and move downward one statement at a time.
2. Describe what the current statement does.
3. Record whether the statement changes a variable.
4. Record whether the statement produces visible output.
5. When a print() statement appears, use the value stored at that exact point in the sequence.
6. Compare your predicted output with the program's actual output to locate where your understanding differs.

This routine turns debugging into a trace rather than a guess. The important information is not only what a variable contains at the end, but also what it contained when each statement executed. Since statements run sequentially, an earlier print() can display an earlier value even when a later assignment changes that variable.

Key Takeaways

1. A script executes statements from top to bottom, with each statement completing before the next begins.
2. Assignment statements store values but produce no visible output.
3. print() statements display results using the value available when they execute.
4. Values stored earlier remain available to later statements until a later assignment changes the variable.
5. Line-by-line tracing helps you predict output and locate unexpected behavior.