Python for Everyone: Chapters 1–3, free to read

Chapter 2: Variables and Data Types

Version: Python 3.13

The magic box

In the last chapter you wrote print("My name is Alex") . Now imagine you have to print your name in 50 places. Will you type it 50 times? And if the name changes, will you fix it in 50 places? The answer is the variable — a named magic box that holds data inside it.

name = "Alex"
print(name)
print(name)
print(name)

Here name is the name of the box, and "Alex" is what's inside. Put something in the box once, then use it as many times as you like. To change the name, change it in just one place:

name = "Alex"
print(name)
# Alex
name = "Maria"
# the thing inside the box changed
print(name)
# Maria

Three boxes, three types — the value inside decides the type. See? The = sign does not mean "equal" like in math — it means "put the thing on the right into the box on the left". Programmers call this assignment.

What can go in the box? — Data types

A box holds more than text. It can hold many kinds of things. Each kind is called a data type:

age = 24
# int

— whole number (integer)

price = 99.99
# float — decimal number
name = "Alex"
# str

— text (string)

is_student = True
# bool

— true/false (boolean)

Want to know what's inside a box? Ask type() :

age = 24
name = "Alex"
print(type(age))
# <class 'int'>
print(type(name))
# <class 'str'>

NOTE

The T in True and the F in False are capital — Python will not recognize true . This is the most common beginner mistake.

Rules for naming

You can't name a box just anything — there are 3 rules:

my_name = "Alex"
# OK — lowercase, underscore
myName = "Alex"
# works, but not the Python convention
2name = "Alex"
# WRONG — cannot start with a number
my-name = "Alex"
# WRONG — a hyphen means minus!

The rules: a name must start with a letter or _ ; inside, letters, numbers and _ are fine; spaces and hyphens are not. And don't use Python's own words ( print , if , for ) as names. A good name is half the understanding: age = 24 is a thousand times better than x = 24 .

A fun truth

In many other languages, when you create a box you must declare "this box will hold only numbers". Python needs none of that — the box figures out what's inside by itself. The same box can hold a number first and text later:

x = 10
print(type(x))
# int
x = "Now I am text"
print(type(x))
# str — the box changed, no problem at all!

The same box, a new kind of value — Python does not lock a variable to one type.

Try it yourself

1. Create three variables: your name ( str ), your age ( int ), whether you are a student ( bool ) — then print all three. Answer

name = "Alex"
age = 24
is_student = True
print(name)
print(age)
print(is_student)

2. What will this code output? (Think before you run it!)

a = 5
b = a
a = 10
print(b)

Answer `5`. When you do `b = a`, the value `a` has at that moment (5) is copied into `b`. Later, when `a` changes, `b` does not — they are two separate boxes. 3. Find the mistake: my-age = 24

print(my-age)

Answer You can't use a hyphen (`-`) in a name — Python thinks you are subtracting `age` from `my`. Correct: `my_age = 24` You know the box now — next we do math with what's inside it: addition, subtraction, division — and there is a fun trap in division that we'll look at too!

Chapter 3: Numbers and Math

Version: Python 3.13

Python as a calculator

Open Python, type numbers, and it becomes a calculator. In the VS Code terminal, type python and press Enter — a >>> sign appears. This is interactive mode: type, press Enter, get the answer at once.

>>> 2 + 3
5
>>> 10 - 4
6
>>> 3 * 7
21

To get out, type exit() .

Operators — the math signs

Notice two things: 1. / always gives a decimal — even 4 / 2 gives 2.0 , not 2 ! Use // when you want whole-number division. 2. % (remainder) is surprisingly useful. Is a number even or odd?

print(10 % 2)
# 0 → even
print(7 % 2)
# 1 → odd

The division trap

A fun question: if 10 chocolates are shared equally among 3 people, how many does each person get?

print(10 / 3)
# 3.3333333333333335
print(10 // 3)
# 3 (3 each)
print(10 % 3)
# 1 (1 left over)

Plain /, whole-number //, and remainder % — three different questions, three different answers. Three operators together tell the whole story!

Which comes first? — Precedence

2 + 3 * 4 = how much? 20 or 14? The math rule applies here too — multiplication before addition: the

answer is 14 .

print(2 + 3 * 4)
# 14
print((2 + 3) * 4)
# 20 — brackets always come first
print(2 ** 3 ** 2)
# 512 — powers run right to left: 2^(3^2)

Python does brackets and powers first, multiplication next, addition last — the BODMAS rule from school, built in. An easy way to remember: brackets → powers → multiply/divide → add/subtract. When in doubt, add brackets — being clear beats being wrong.

Changing type — conversion

"5" (text) and 5 (number) are not the same thing:

print("5" + "3")
# 53 (two pieces of text joined)
print(5 + 3)
# 8 (two numbers added)

To turn text into a number:

num = int("42")
# "42" → 42
price = float("9.99")
text = str(100)
# "9.99" → 9.99
# 100 → "100"

int(), float() and str() change a value's type — but only when the value makes sense as that type.

GOTCHA

int("abc") will crash! int() only works on text that looks like a number. Remember this when converting text a

user typed — in Chapter 12 we learn how to handle it safely.

Shortcuts — change yourself

count = 10
count = count + 5
# old value + 5
print(count)
# 15

This pattern is so common that it has a shortcut:

count = 10
count += 5
# same thing: count = count + 5
count -= 3
# count = count - 3
count *= 2
# count = count * 2

Handy functions

print(round(3.14159, 2))
# 3.14 — 2 places after the decimal
print(abs(-7))
# 7 — absolute value
print(min(4, 9, 2))
# 2
print(max(4, 9, 2))
# 9
print(pow(2, 10))
# 1024 — same as 2**10

Try it yourself

1. A rectangle has length 12 and width 5 — find its area. Answer

length = 12
width = 5
area = length * width
print(area)
# 60

2. If 100 dollars are shared equally among 7 people, how much does each person get, and how much is left? Answer

print(100 // 7)
# 14 (each person)
print(100 % 7)
# 2 (left over)

3. What is the output?

x = 5
x += 3
x *= 2
print(x)

Answer `16`. Step by step: x = 5 → `x += 3` makes x = 8 → `x *= 2` makes x = 16. Numbers are done — now we play with text (strings): pulling the first letter out of a name, reversing text, and the magic of f-strings!