Variables and types #

A variable is a named container for a value. You create one with =:

name = "Alice"
age = 30
is_active = True

If you've come across variables before in another language, Python's version will feel familiar — with one notable difference. You don't declare a type upfront. There's no string name or int age. Python looks at the value you assign and figures out the type itself. This is called dynamic typing, and it keeps your code noticeably shorter.

The core types #

Python has five types you'll work with constantly:

str #

Text wrapped in quotes. Single or double — Python treats them identically, so use whichever you prefer and stay consistent.

first_name = "Alice"
last_name = 'Smith'

For multi-line text, triple quotes keep things readable:

bio = """
Alice is a software engineer
based in Melbourne.
"""

Strings are immutable — you can't change individual characters in place. Any operation that looks like it modifies a string is actually creating a new one.

int and float #

The difference is simple: integers are whole numbers, floats have a decimal point.

score = 100
year = 2024

price = 9.99
tax_rate = 0.08

One handy trick — underscores in numbers are ignored by Python but make large values much easier to read:

population = 8_200_000_000  # same as 8200000000

bool #

True or False — always capitalised. Booleans are the backbone of any decision-making in code.

is_logged_in = True
has_paid = False

Worth knowing: under the hood, True equals 1 and False equals 0. This comes up occasionally in arithmetic and comparisons, so it's good to have in the back of your mind.

None #

None represents the absence of a value — not zero, not an empty string, genuinely nothing. It's commonly used to initialise a variable before you have real data to put in it, or to signal that a function has nothing useful to return.

result = None
current_user = None

Checking the type #

type() tells you what type a value is:

print(type("hello"))   # <class 'str'>
print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type(True))      # <class 'bool'>
print(type(None))      # <class 'NoneType'>

In real code you'll more often use isinstance() when you actually need to act on the type:

if isinstance(age, int):
    print("Got a whole number")

Converting between types #

Python won't automatically convert types for you — if you try to add a number to a string, you'll get an error. You need to be explicit:

int("42")       # 42
float("3.14")   # 3.14
str(100)        # "100"
bool(0)         # False
bool("hello")   # True

This comes up more than you might expect. Any time you read input from a user or a file, it arrives as a string — even if they typed a number. Converting it before doing maths on it is a very common pattern:

user_input = "25"         # this came from somewhere as text
age = int(user_input)     # now it's a number you can do maths with
next_year = age + 1       # works fine

Conversion fails loudly if the value doesn't make sense for the target type:

int("hello")    # ValueError: invalid literal for int()
int("3.5")      # ValueError — convert to float first, then int

Dynamic typing and reassignment #

Because types are attached to values rather than variable names, you can reassign a variable to a completely different type:

x = 10
x = "ten"    # perfectly valid in Python
x = True     # also fine

This flexibility is useful but can bite you in larger codebases where you lose track of what a variable is supposed to hold. If that's a concern, Python 3.5+ lets you add type hints as documentation:

name: str = "Alice"
age: int = 30

These don't enforce anything at runtime — they're signals for you, your team, and tools like mypy.

Naming conventions #

Python's community has strong conventions around naming. Following them makes your code immediately readable to other Python developers:

• snake_case for variables and functions: user_name, total_price, is_active
• ALL_CAPS for constants: MAX_RETRIES, BASE_URL, PI
• PascalCase for class names: UserProfile, ShoppingCart

Names are case-sensitive — score, Score, and SCORE are three different variables.

Multiple assignment #

Python lets you assign multiple variables in one line, which is often cleaner than separate statements:

a, b, c = 1, 2, 3

# Swap without a temp variable
a, b = b, a

# Same value across multiple names
x = y = z = 0

Practice #