Lesson 02 — Sequence and Tracing

Computers follow an order

Python usually executes instructions top to bottom, one line at a time.

print("First")
print("Second")
print("Third")

Output

First
Second
Third

Reversing two lines changes the result.

A trace is a record of what a program does, step by step. Before running a program, try predicting its output.

Line executed	Output added
print("Checking battery")	Checking battery
print("Starting motor")	Starting motor

print() normally ends with a new line, so each call displays its message on a separate line.

Practice: Startup Sequence

A robot should display these three messages in the following order:

Booting...
Checking sensors...
Ready.

The starter code contains the right messages but in the wrong order. Move the instructions into the correct sequence. View starter.

Stop and think

• Can all three messages be correct while the overall output is wrong?
• Why should a programmer test the order of output as well as the words themselves?