PCC·02 · Chapter record

Variables & Simple Data Types

Chapter 2 — the data you'll work with in every program

Done

Five sessions covering the data types you'll use constantly: variables, strings, numbers, and the comments that explain them. Each one explains the concept, lists the key terms, then gives you an exercise — try it yourself before revealing the code.

2.1

Variables & the Traceback

Concept

A variable is a name connected to a value.
It's often described as a box you store a value in, but it's more accurate to think of it as a label attached to a value — you can move the label to a new value at any time.
Variable names can only contain letters, numbers, and underscores, and can't start with a number.
Spaces aren't allowed — use underscores instead — and you should avoid Python's reserved keywords and built-in function names like print.
Misspell a variable name Python hasn't seen before, and you get a NameError, shown as a traceback pointing at the exact line.

Key Terms

variable: a name connected to a value
NameError: raised when Python can't find a variable or function by the name you used
traceback: the report Python prints showing where and why an error happened

Exercise

Activity 1

Assign a greeting to a variable, then print that message.

Activity 2

Print that same variable, then reassign it to a new message and print it again.

2.2

Strings: Methods & f-strings

Concept

A string is a series of characters inside single or double quotes.
One of the simplest things you can do with a string is change its case using a method — an action Python performs on a value, written with a dot after it.
name.title() capitalizes each word, name.upper() makes it all caps, and name.lower() makes it all lowercase.
To insert a variable's value into a string, place an f immediately before the opening quote and wrap the variable name in braces: f"{first_name} {last_name}".
These are called f-strings — Python replaces each {name} with that variable's value, and you can even call a method right inside the braces, like f"Hello, {full_name.title()}!".

Key Terms

string: a series of characters inside quotes
method: an action performed on a value, written as value.method()
f-string: a string prefixed with f that inserts {variable} values directly

Exercise

Activity 2

Use a variable for someone's name, and print a short, friendly greeting to them.

Activity 3

Print that same name in lowercase, uppercase, and title case.

Activity 4

Print a quote you like along with who said it, keeping the quotation marks in the output.

Activity 5

Redo the quote exercise, but build the full quote-plus-author line into its own variable first, then print that variable.

2.3

Whitespace & Stripping

Concept

Whitespace is any nonprinting character — spaces, tabs (\t), and newlines (\n).
To a program, 'python' and 'python ' are different strings, even though they look the same to you — extra whitespace can quietly break comparisons like checking a username at login.
Python removes it with three methods: rstrip() strips trailing whitespace, lstrip() strips leading whitespace, and strip() strips both.
Each one only changes the value temporarily — the original variable is untouched unless you reassign the result back to it, like favorite_language = favorite_language.rstrip().

Key Terms

whitespace: nonprinting characters — spaces, tabs (\t), newlines (\n)
rstrip() / lstrip() / strip(): remove whitespace from the right, left, or both sides

Exercise

Activity 6

Store a name with extra whitespace at both ends (try mixing in \t and \n). Print it once as-is, then print it again after each of lstrip(), rstrip(), and strip().

2.4

Numbers: Arithmetic & Multiple Assignment

Concept

Python handles the usual arithmetic — + - * / — plus ** for exponents, and it respects order of operations, so 2 + 3 * 4 is 14, not 20.
Numbers with a decimal point are floats; whole numbers are ints.
Mix a float into any operation, or use /, and the result is always a float — even when the answer is a whole number, like 4 / 2 → 2.0.
Assign several variables in one line by separating both the names and the values with commas: x, y, z = 0, 0, 0.
By convention, a variable meant to never change is written in ALL_CAPS, like MAX_CONNECTIONS = 5000 — Python won't enforce that, but every Python programmer reads it as a signal not to reassign it.

Key Terms

int / float: a whole number vs. a number with a decimal point
**: the exponent operator — 3 ** 2 is 9
multiple assignment: assigning several variables in one line: x, y, z = 0, 0, 0
constant: a variable meant to stay fixed, written in ALL_CAPS by convention

Exercise

Activity 7

Write four print() calls, one each using +, -, *, and /, that all result in the number 8.

Activity 8

Store your favorite number in a variable, then print a message built around it.

2.5

Comments

Concept

A comment is a note in plain English that Python ignores completely — it starts with a hash mark (#), and everything after it on that line doesn't run.
As programs get longer, comments help explain your overall approach so that you (or anyone else) can understand it later without re-reading every line.
The best comments explain why you did something, not what the code does — the code itself already shows what.
A comment like # apply a 10% loyalty discount is more useful than # multiply by 0.9, which just restates the line below it.

Key Terms

comment: a note starting with # that Python ignores when running the file

Exercise

Final challenge

Add a comment above one line of an earlier exercise explaining why that line exists, not what it does.