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Module 003: Variables and Data Types

Phase: 1. Fundamentals | Estimated time: 2 hours | Milestone Project: No

Prerequisites

Learning Objectives

By the end of this module, you will be able to:

  • Assign values to variables using =
  • Follow Python naming rules for variables
  • Explain what dynamic typing means
  • Identify the four basic data types: int, float, str, bool
  • Use type() to check a value's data type

Why This Matters

Variables are the building blocks of every program. They let you store, label, and reuse data. Understanding data types is critical because different types behave differently — you cannot add a number to a text string without converting first.

Concept Explanation

Variable Assignment

A variable is a named container that holds a value. Use the = sign to assign a value to a name:

name = "Alice"
age = 25
height = 1.68
is_student = True

Think of it as: name ← value. The variable goes on the left, the value on the right.

Once assigned, you can use the variable's name anywhere you would use the value:

name = "Alice"
age = 25
print(name)     # Output: Alice
print(age + 1)  # Output: 26

Naming Rules

Python has strict rules for variable names:

Rule Example ✅ Example ❌
Must start with a letter or underscore name, _count 1name
Can contain letters, digits, underscores user_name, data2 user-name
Case-sensitive name and Name are different —
Cannot be a reserved keyword — if, for, class, while

Python keywords (reserved words) include: if, else, elif, for, while, def, class, import, from, return, True, False, None, and, or, not, in, is, with, as, try, except, finally, raise, yield, lambda, pass, break, continue.

Use descriptive names — student_count is better than sc.

Dynamic Typing

Python is dynamically typed. A variable can hold any type of value, and the type can change over time:

value = 42          # value is an int
print(value)
value = "now text"  # value is now a str
print(value)

The same variable can hold an integer, then a string, then a float. This is flexible but requires care — it can lead to runtime errors if you assume a type that isn't there.

Basic Data Types

Python has four fundamental data types you will use constantly:

int — Integer

Whole numbers, positive or negative, with no decimal point:

count = 10
negative = -5
zero = 0
large = 1_000_000  # underscores for readability

float — Floating-point number

Numbers with a decimal point:

pi = 3.14159
temperature = -2.5
scientific = 1.5e10  # 1.5 × 10^10

str — String

Text enclosed in quotes:

greeting = "Hello"
name = 'Alice'
multi = "She said, \"Hi!\""

bool — Boolean

Only two values: True or False:

is_sunny = True
is_raining = False

Note: True and False must be capitalized. true and false cause errors.

The type() Function

Use type() to discover the data type of any value:

print(type(42))       # <class 'int'>
print(type(3.14))     # <class 'float'>
print(type("Hello"))  # <class 'str'>
print(type(True))     # <class 'bool'>

This is especially useful when debugging or when you are unsure what type a variable holds.

Common Pitfalls

  1. Using unassigned variables: print(x) where x has never been assigned causes NameError.
  2. Spelling True / False wrong: true and false (lowercase) are not valid Python.
  3. Using a keyword as a variable name: if = 5 causes SyntaxError.
  4. Forgetting that type() returns a type object, not a string: type(x) == "int" is False; use type(x) == int or isinstance(x, int).
  5. Assuming division between two ints returns an int: 10 / 3 returns 3.333... (a float).

Hands-On Walkthrough

Create a file profile.py that stores information about yourself using variables:

# My profile
first_name = "Alex"
last_name = "Smith"
age = 30
height = 1.75
likes_python = True

print("First name:", first_name)
print("Last name:", last_name)
print("Age:", age)
print("Height:", height)
print("Likes Python:", likes_python)

Now add some type() calls to verify the types:

print(type(first_name))
print(type(age))
print(type(height))
print(type(likes_python))

Expected output:

First name: Alex
Last name: Smith
Age: 30
Height: 1.75
Likes Python: True
<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

Experiment: change age to 30.0 and see type(age) change from int to float.

Key Takeaways

  • Use = to assign values to variables.
  • Variable names: start with letter/underscore, contain letters/digits/underscores, case-sensitive.
  • Python is dynamically typed — a variable's type can change.
  • Four basic types: int (whole numbers), float (decimals), str (text), bool (True/False).
  • type() reveals the type of any value or variable.
  • True and False are capitalized keywords.
  • Underscores in numbers (1_000_000) are ignored and improve readability.
  • Division always returns a float.

Further Reading

Next Module

Learn how to convert between types in Module 004: Type Conversion and Casting.