Data types and variables, the four types you'll use 95% of the time.

Section · 01

The four types you'll actually use

Python has more types than this, but four cover almost every line of code you’ll write in the first year:

name = "Ada Lovelace"        # str   — text
age = 36                     # int   — whole number
balance = 1287.45            # float — number with a decimal
is_active = True             # bool  — True or False

Plus a fifth that means “no value yet”:

last_login = None            # NoneType — used as a placeholder

Check what type something is with the type() function. This is useful when a value isn’t doing what you expected:

type(name)        # <class 'str'>
type(age)         # <class 'int'>
type(balance)     # <class 'float'>
type("36")        # <class 'str'>  — quotes win, even if it looks like a number
type(36)          # <class 'int'>

That second-to-last one is the trap. "36" and 36look identical to you but are completely different to Python — one is text, one is a number. You can’t do math on text.

Section · 02

Variables are just names for values

A variable is a label pointing at a value. You bind one with =:

subtotal = 49.99
tax_rate = 0.0725
total = subtotal + (subtotal * tax_rate)
print(total)    # 53.61...

Two things to know about =:

1. It’s not equality. The single =is assignment — “point the name on the left at the value on the right.” The double ==is equality — “are these two things equal?” Mixing them up is a daily occurrence at first.

2. The right side runs first. Python evaluates everything to the right of =, then assigns the result. That’s why x = x + 1works — the right side becomes “old x plus one”, then the result is bound to x.

Section · 03

The rules for naming

Python is strict about what counts as a valid name. Three hard rules:

user_id = 1          # OK — letters, digits, underscores
_internal = "..."    # OK — leading underscore is fine
score2 = 100         # OK — digits anywhere except the start

2score = 100         # SYNTAX ERROR — can't start with a digit
user-id = 1          # SYNTAX ERROR — hyphens are not allowed (that's minus)
class = "Math 101"   # SYNTAX ERROR — 'class' is a reserved word

And one rule that isn’t enforced by Python but is enforced by every other Python developer who’ll read your code:

# Use snake_case for variables and functions
user_age = 32              # ✓
first_login_at = "..."     # ✓

# NOT camelCase or PascalCase for ordinary variables
userAge = 32               # technically works, but everyone will hate it
FirstLoginAt = "..."       # looks like a class, will confuse readers

Capitalization matters: userId, UserId, and USERIDare three different variables. This causes bugs you’ll spend 20 minutes looking for before you spot the typo. Be consistent.

Section · 04

Converting between types

When data comes in from outside your program — a form, a file, a web request — it almost always arrives as a string. Your job is to convert it to the right type before doing anything with it:

age_str = input("Your age? ")       # always a string
age = int(age_str)                  # str → int

price_str = "19.99"
price = float(price_str)            # str → float

count = 7
label = str(count)                  # int → str, for printing/concatenation

Each conversion can fail. int("hello") raises a ValueError. float("19.99") works; int("19.99")does not (you can’t go directly from a decimal string to an int — convert to float first, then int).

The classic beginner bug

# This looks fine but produces "55" instead of 10:
five_str = input("First number: ")    # user types "5", gets "5"
also_five = input("Second number: ")  # user types "5", gets "5"
print(five_str + also_five)           # "55"  — string concatenation!

# Convert first:
n1 = int(input("First number: "))     # int
n2 = int(input("Second number: "))    # int
print(n1 + n2)                        # 10   — numeric addition

+ means different things depending on the types on either side. Two strings? Concatenation. Two numbers? Addition. A string and a number? TypeError. Python won’t guess what you meant.