beginner⏱30 minutesLesson 3 of 10

Variables and Data Types

Learn how to store data using variables and understand Python's core data types: int, float, string, and bool

Variables and Data Types

Variables are containers for storing data. Python is dynamically typed, meaning you don't need to declare a variable's type — Python infers it automatically.

Variables

python
name = "Alice" # String age = 25 # Integer height = 1.68 # Float is_student = True # Boolean

Naming Rules

  • Must start with a letter or underscore
  • Can contain letters, numbers, and underscores
  • Case-sensitive (age ≠ Age)
  • Cannot use Python keywords (like if, for, while)
python
# Valid names user_name = "Bob" _user_id = 42 camelCase = "OK" # Allowed but not preferred # Invalid names 2nd_place = "No" # Starts with number my-var = "No" # Hyphen not allowed class = "No" # Reserved keyword

Naming Conventions (PEP 8)

python
# Use snake_case for variables and functions user_age = 30 total_price = 99.99 # Use UPPER_CASE for constants PI = 3.14159 MAX_SIZE = 100
ℹ️Note

Python follows PEP 8 style guide. Use snake_case for variables, not camelCase.

Numeric Types

Integers (int)

python
count = 10 negative = -5 big_number = 1_000_000 # Underscores improve readability

Floats (float)

python
price = 19.99 pi = 3.14159 scientific = 1.5e-4 # 0.00015

Type Conversion

python
# int to float x = float(10) # 10.0 # float to int (truncates) y = int(3.99) # 3 # string to int z = int("42") # 42

Strings (str)

python
# Single or double quotes first = 'Hello' second = "World" # Multi-line strings poem = """Roses are red, Violets are blue, Python is fun, And so are you.""" # String concatenation greeting = "Hello" + " " + "World" # String interpolation (f-strings) name = "Alice" age = 25 message = f"{name} is {age} years old." # String methods text = " Python is FUN " print(text.lower()) # " python is fun " print(text.upper()) # " PYTHON IS FUN " print(text.strip()) # "Python is FUN" print(text.replace("FUN", "awesome")) # " Python is awesome "

Booleans (bool)

python
is_active = True is_finished = False # Comparison operators return booleans print(10 > 5) # True print(3 == 4) # False

None Type

None represents the absence of a value:

python
result = None print(result) # None

Checking Types

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

Dynamic Typing

A variable's type can change:

python
x = 10 # x is int x = "hello" # x is now str x = 3.14 # x is now float
⚠️Warning

Dynamic typing is flexible but can cause bugs. Use meaningful variable names and be consistent with types.

✅Success

Remember: int for whole numbers, float for decimals, str for text, bool for true/false, and None for nothing.

Practice Questions

  1. What will type(3.14) return?
  2. Convert "100" to an integer.
  3. What's wrong with 2nd_place = "Bob"?
  4. Write an f-string that says "Alice is 30 years old".
  5. What's the difference between 10 and 10.0?
  6. What does "hello".upper() return?
  7. True or False: Python requires you to declare variable types.
  8. What does None represent in Python?
  9. Convert the float 99.9 to an integer — what's the result?
  10. Write a Python variable name using snake_case for a user's email address.
Progress30%
Previous
Installation and Setup
Next
Operators and Expressions

Course Content

1. What is Python?2. Installation and Setup3. Variables and Data Types4. Operators and Expressions5. Conditionals (if/elif/else)6. Loops (for and while)7. Lists and Tuples8. Dictionaries and Sets9. Functions and Scope10. Modules, Packages, and Standard Library