01-variables

01 · Variables

Part 1 — Fundamentals · Estimated time: 20–30 min · No prior lessons required

A variable is a name you give to a value so you can use it again later. Think of it as a labelled sticky note attached to a piece of data: the label is the name, and the note points to the value.

What you'll learn

• What a variable is and how to create one with =
• Why Python is dynamically typed (you never declare the type)
• How to reassign, update, and combine variables
• How to assign several variables at once and swap two values
• The rules (must follow) and conventions (should follow) for naming

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1. Creating your first variable

You create a variable with a single equals sign, =. The name goes on the left, the value goes on the right.

name = "Ada"
age = 36

print(name)                       # Ada
print(f"{name} is {age} years old.")   # Ada is 36 years old.

That f"..." is an f-string. The f before the quotes lets you drop variables straight into text using {curly braces}. You'll use f-strings constantly — get comfortable with them now.

▶️ Run it: python examples/01_first_variable.py

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2. Python figures out the type for you

In languages like Java or C++ you must announce a variable's type (int age = 36;). Python doesn't — it looks at the value and works it out. This is called dynamic typing.

x = 100        # x is an integer right now
x = "hello"    # ...and now the SAME name points to a string. Totally fine.

print(type(x))     # <class 'str'>

type(value) tells you what kind of value a name currently points to. It's a handy tool when something behaves unexpectedly.

▶️ Run it: python examples/02_dynamic_typing.py

💡 Try it yourself: make a variable temperature = 21.5 and print type(temperature). What does Python call a number with a decimal point?

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3. Reassigning and updating

A variable can change as your program runs. You can even use a variable's old value to compute its new value.

score = 0
score = 10            # reassign to a brand-new value
score = score + 5     # use the old value to build the new one  -> 15
score += 5            # shorthand for "score = score + 5"        -> 20

The += shortcut also has friends: -=, *=, /=.

▶️ Run it: python examples/03_reassigning_and_updating.py

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4. Assigning many at once (and swapping)

x, y, z = 10, 11, 12     # three names, three values, one line
a = b = c = 0            # give several names the same starting value

left, right = "L", "R"
left, right = right, left # swap two values in ONE line — a Python superpower
print(left, right)        # R L

▶️ Run it: python examples/04_multiple_assignment_and_swapping.py

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5. Naming: rules vs. conventions

Rules — break these and Python raises an error:

• A name cannot start with a digit: 2cats is invalid, cats2 is fine.
• A name can only contain letters, digits, and underscores — no spaces, no hyphens, no symbols like @.
• Names are case-sensitive: total and Total are two different variables.

Conventions — Python still runs, but the community expects:

• Use snake_case for normal variables: user_name, total_price.
• Use UPPER_CASE for values you treat as constants: MAX_SCORE = 100.
• CamelCase is reserved for class names (you'll meet those in the OOP part).
• Names wrapped in double underscores like __name__ are special to Python. Don't invent your own.

user_name = "Sam"     # good
MAX_SCORE  = 100       # "constant" — by convention, don't change it
# 2cats = 10           # ❌ SyntaxError: can't start with a digit
# user-name = 10       # ❌ hyphen is not allowed

▶️ Run it: python examples/05_naming.py

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Common mistakes

Mistake	What happens	Fix
name = "Sam" then later type Name	NameError: name 'Name' is not defined	Match the case exactly.
2morrow = "day"	SyntaxError	Start names with a letter: tomorrow.
Confusing = and ==	= assigns; == compares	Use = to store, == to test equality (next lessons).
print(f"Hi {name}") but forgetting the f	prints the literal text Hi {name}	Add the f before the opening quote.

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Recap / cheat-sheet

name = "Ada"            # create
age = 36
age = 37               # reassign
age += 1               # update using the old value
x, y = 1, 2            # multiple assignment
x, y = y, x            # swap
type(name)             # inspect the current type
# snake_case for variables, UPPER_CASE for constants

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Exercises

Work through these in order. Each file explains the task at the top and has a TODO where your code goes. Run a file with python exercises/<file>.py, then compare with the matching file in solutions/ when you're done (or stuck).

1. exercises/01_about_you.py — print a sentence about yourself with an f-string.
2. exercises/02_swap.py — swap two values in one line.
3. exercises/03_rectangle.py — compute a rectangle's area from variables.
4. exercises/04_fix_the_names.py — debug some illegal variable names.

Try each exercise yourself before opening the solution. Getting stuck and experimenting is where the learning happens.

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Next → 02 · Data Types & Numbers (coming soon)