Lesson 4 — Storing Data

Variables

Python variables require no type declaration — just assign a value. This flexibility is powerful but requires discipline in naming and type awareness.

Assignment is Declaration

In Python, you create a variable by assigning a value. There is no var, int, or String keyword. Python infers the type from the value.

PYTHON — Assignment 
# All of these create and initialise a variable:
name       = "Thandi Mokoena"
age        = 22
mark       = 74.5
enrolled   = True
address    = None   # None is Python's equivalent of null

print(name)
print(f"Age: {age}, Mark: {mark}%, Enrolled: {enrolled}")

Dynamic Typing

A Python variable can be reassigned to a completely different type at runtime. This is called dynamic typing. It gives flexibility but requires care — accidentally assigning the wrong type to a variable is a common source of bugs.

PYTHON — Dynamic typing and type hints 
x = 42         # x is int
print(type(x)) # <class 'int'>

x = "hello"    # x is now str — perfectly valid in Python
print(type(x)) # <class 'str'>

x = [1, 2, 3]  # x is now list
print(type(x)) # <class 'list'>

# Type hints — optional but recommended for clarity:
from typing import Optional

name:   str           = "Sipho"
age:    int           = 24
mark:   float         = 81.0
active: bool          = True
notes:  Optional[str] = None   # may be str or None

Multiple Assignment

PYTHON — Multiple assignment 
# Assign the same value to multiple variables:
a = b = c = 0

# Assign different values on one line (tuple unpacking):
x, y, z = 10, 20, 30
print(x, y, z)  # 10 20 30

# Swap without a temp variable (idiomatic Python):
a, b = 5, 10
a, b = b, a
print(a, b)  # 10 5

# Underscore for values you don't need:
first, _, last = (1, 2, 3)
print(first, last)  # 1 3

# Extended unpacking with *:
head, *tail = [1, 2, 3, 4, 5]
print(head)  # 1
print(tail)  # [2, 3, 4, 5]

Naming Conventions (PEP 8)

  • Variables and functions — snake_case: first_name, total_score, is_enrolled.
  • Classes — PascalCase: LearnerProfile, BankAccount.
  • Constants — UPPER_SNAKE_CASE: VAT_RATE, MAX_RETRIES.
  • Private — _single_underscore: _internal_counter.
  • Very private — __double_underscore: __secret (name-mangled).

Practice Task

Your Turn

Create variables for: your full name (str), age (int), city (str), course (str), GPA (float), whether you have a laptop (bool), and an optional scholarship amount (could be None or a float). Use type hints on all of them. Use extended unpacking to split a list of 5 marks into the first mark and the rest. Print everything with f-strings.

Common Mistakes

  • Variables are case-sensitive — Name and name are different.
  • Starting a variable name with a digit — 1st_place is a SyntaxError.
  • Using Python keywords as names — list, type, print are valid names but shadow built-ins.
  • Reassigning a variable to an incompatible type by accident — type hints help IDEs catch this.

Professional Tip

Descriptive snake_case names are Pythonic. number_of_enrolled_learners over n. Your future self and colleagues will thank you.

Mini Quiz

What naming convention do Python variables and functions use?