Iowa State University · Computer Science

Variables and print()

15-minute lesson · module-01-getting-started

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Variables and print()

15-minute lesson · module-01-getting-started

Understand what a Python program is and how it executes top to bottom.
Create string and integer variables and print them.
Predict the output of simple programs before running them.
Recognize when a NameError happens and what causes it.

A program is a list of instructions

When you run a Python program, the computer reads it from top to bottom and carries out each instruction in order. The very first program almost everyone writes prints a friendly message to the screen.

print("Hello, world!")

The word print is a function — a small named tool that does a job. When you write print(...), you are calling the function and giving it something to display: the value inside the parentheses.

Variables hold values

A variable is a name that points to a value so you can use it later. You make a variable using the = sign:

name = "Cyclone"
year = 2026

name is a string — text inside quotes.
year is an integer — a whole number, no quotes.

Once a variable is created you can use it anywhere a value is allowed:

print(name)
print("It is the year", year)

Names matter

Python is case sensitive. Name and name are two different variables. If you type a name Python has never seen before, you get a NameError — Python is essentially saying "I don't know what you mean."

Predict before you run. Reading the program and guessing what will happen is the single most useful habit a new programmer can build. The AI tutor on the right will encourage you to predict first — please do!

Examples

Greeting a person

name = "Adisak"
print("Welcome to Python,", name)

`name` stores the string "Adisak". The print function shows two values separated by a comma — Python adds a space between them automatically.

Checkpoint questions

What does this program print?

x = 5
y = 3
print(x + y)

a)5 + 3
b)8
c)53
d)An error

Which line will cause a NameError?

city = "Ames"
print(City)

a)Line 1 (city = "Ames")
b)Line 2 (print(City))
c)Neither — both work fine

Hands-on coding

Print a welcome line for ISU CS

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ActiveCode

# Create a variable called school that holds the string "Iowa State"
# Create a variable called dept that holds the string "Computer Science"
# Print:  Welcome to Iowa State Computer Science!
# (You should use the variables in the print line.)

Output

(no output yet)

CodeLens — step through execution

How variables and print work step by step

CodeLens — step through frames, objects, and references

CodeLens

Code · Python 3

  1name = "Cyclone"
  2year = 2026
  3print("Hello,", name)
  4print("It is the year", year)
  5

→ line that just executed▶ next line to execute

Print output

(click Trace)

Frames

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Objects

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Edit the code

name = "Cyclone"
year = 2026
print("Hello,", name)
print("It is the year", year)

Debugging interlude

Predict the error first (+50 XP), then fix the code (+100 XP). Hints cost 30 XP each.

🐞

Spot the NameError

One of these lines will cause a NameError. Predict which one and why, then fix the program so it prints the greeting correctly.

Predict · 50 XPFix · 100 XP

city = "Ames"
print("Hello from", City)

Step 1 — Predict the error

Read the program. Which kind of error (or non-error) will Python raise?

End-of-lesson quiz

Which of these creates a variable that holds the number 42?
- a)answer = "42"
- b)answer = 42
- c)answer == 42

Reflection

In your own words, what is the difference between a variable and a value?

Source attribution

Status: adapted

How to Think Like a Computer Scientist: Interactive Edition — source · General Introduction / Algorithms / The First Program / Comments

Adaptation notes. Reading rewritten in our own words. Examples and hidden tests authored specifically for PyTutor AI. Pedagogical sequencing follows Think CS Python — variables introduced after print() so the first program is immediately runnable.

License notes. Verify chapter-level license metadata on Runestone before copying any text verbatim. This lesson contains no verbatim quotations.