Your First Python Program

Mr. Diaz runs the winter concert. Every December he counts chairs twice — once to work out how many the room needs, and once because he does not trust the first count at ten o’clock at night. This program counts them once, correctly, and prints something he can read while carrying a stack.

The program

# Chair setup for Mr. Diaz, who runs the winter concert.
# Change the four numbers at the top; everything else follows.
 
rows = 8
chairs_per_row = 12
ensemble_chairs = 6
chairs_in_storage = 120
 
audience_chairs = rows * chairs_per_row
total_needed = audience_chairs + ensemble_chairs
chairs_left_over = chairs_in_storage - total_needed
 
print("Concert chair setup")
print("-------------------")
print(f"Audience: {rows} rows of {chairs_per_row} = {audience_chairs} chairs")
print(f"Ensemble: {ensemble_chairs} chairs")
print(f"Needed:   {total_needed} chairs")
print(f"Storage:  {chairs_in_storage} chairs")
print(f"Spare:    {chairs_left_over} chairs")

Save it as chairs.py and run it. You should see exactly this:

Concert chair setup
-------------------
Audience: 8 rows of 12 = 96 chairs
Ensemble: 6 chairs
Needed:   102 chairs
Storage:  120 chairs
Spare:    18 chairs

How it works

Python starts at the top and runs one line at a time to the bottom. It never jumps around and never guesses.

The first block gives names to the four numbers that might change. The second block does the arithmetic, and each line uses the names created above it — total_needed can use audience_chairs because that line has already run. The third block prints. Splitting a program into “facts, working out, output” like this is the input-process-output shape from What a Program Is, with the input hard-coded for now.

Three details worth naming:

• The lines starting with # are comments. Python ignores them completely; they are for the next person to open the file, which is usually you in three weeks.
• f"..." is an f-string: anything inside { } is replaced by its value when the line runs.
• The names are long on purpose. chairs_per_row costs a second to type and saves a minute to read — see Writing Code Others Can Read.

Running it, the boring reliable way

Save the file, then in a terminal opened in the same folder type python3 chairs.py and press return. If you get can't open file ... No such file or directory, you are in a different folder from the file — that is the most common first-day problem, and Setting Up Python walks through it.

The editor you type in and the terminal you run from are your development environment. An IDE bundles them together and adds a debugger, which you will meet in Using the Debugger; nothing in this course needs more than an editor, a terminal, and Python itself.

Change it

1. One line. Mr. Diaz wants wider rows: change chairs_per_row to 14. The audience line becomes 112, the total 118, and the spare drops to 2 — three printed numbers changed, and you edited one.
2. A few lines. The chairs travel on trolleys that hold eight. Work out total_needed // 8 and total_needed % 8, and print Trolley loads: 12 stacks of 8, plus 6 loose chairs. // divides and throws away the remainder; % keeps only the remainder.
3. A real change. Set chairs_in_storage to 100 and run it. The program cheerfully reports a spare of -2 chairs, which is not a thing. You cannot fix that yet — a program that reacts differently to different numbers needs Making Decisions — but write down what it should say, because that sentence is a specification, and you just wrote your first one.

Curriculum connection

A1.3

use assignment statements correctly with both arithmetic and string expressions in computer programs;

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A4.2

use workplace and professional conventions (e.g., naming, indenting, commenting) correctly to write programs and internal documentation;

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C3.1

demonstrate an understanding of an integrated software development environment and its main components (e.g., source code editor, compiler, debugger);

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