- Understand what variables are and why we use them
- Learn how to create and assign values to variables
- Follow Python naming conventions and best practices
- Master variable reassignment and multiple assignment techniques
Variables and Assignment
Variables are the foundation of every program. Think of a variable like a labeled box where you can store something - a number, text, or any other piece of information. You give the box a name so you can easily find and use what's inside later.
In Python, working with variables is straightforward. No complex declarations or type specifications needed - just name your variable and give it a value.
What is a Variable?
A variable is a named location in your computer's memory that stores a value. It's like a label attached to a piece of data.
Variable Concept
VARIABLE NAME VALUE
↓ ↓
age 25
name "Alice"
price 19.99
Why Use Variables?
| Reason | Example |
|---|---|
| Store data for later | Save a user's name to greet them later |
| Make code readable | total_price is clearer than 19.99 * 3 |
| Reuse values | Use the same discount percentage everywhere |
| Change values easily | Update one variable instead of many numbers |
Creating Variables (Assignment)
In Python, you create a variable using the assignment operator (=):
# Basic assignment syntax
variable_name = value
# Examples
age = 25
name = "Alice"
price = 19.99
is_student = True
Key Point: The
=sign in Python means "assign the value on the right to the variable on the left." It's not the same as mathematical equality!
Reading the Assignment Statement
x = 10
Read this as: "x gets the value 10" or "x is assigned 10"
NOT as: "x equals 10" (that's == in Python)
Variable Reassignment
Variables can change their values at any time – that's why they're called "variables"!
# Initial assignment
score = 0
print(score) # Output: 0
# Reassignment
score = 10
print(score) # Output: 10
# Reassign again
score = score + 5 # Add 5 to current score
print(score) # Output: 15
Type Can Change Too!
Unlike some languages, Python allows you to change a variable's type:
x = 10 # x is an integer
print(x) # 10
x = "hello" # Now x is a string
print(x) # hello
x = 3.14 # Now x is a float
print(x) # 3.14
Note: While this flexibility is convenient, it's often better practice to keep variables consistent in their type for code clarity.
Variable Naming Rules
Python has specific rules about what makes a valid variable name:
The Rules
| Rule | Valid Examples | Invalid Examples |
|---|---|---|
| Must start with letter or underscore | name, _count, Age |
1name, @data |
| Can contain letters, numbers, underscores | user_1, total2024 |
my-var, user@name |
| Cannot be a Python keyword | my_class, for_loop |
class, for, if |
| Case-sensitive | Name ≠ name ≠ NAME |
— |
Valid vs Invalid Examples
# VALID variable names
name = "Alice"
age2 = 25
_private = "secret"
firstName = "Bob"
CONSTANT = 3.14
user_email = "test@example.com"
# INVALID variable names
# 2name = "error" # Can't start with number
# my-var = 10 # No hyphens allowed
# for = 5 # 'for' is a keyword
# user@email = "x" # No @ allowed
Naming Conventions (Best Practices)
While Python allows many valid names, following conventions makes code more readable:
Python Naming Conventions
Variables & Functions: snake_case
user_name, total_price, calculate_tax
Constants: UPPER_SNAKE_CASE
MAX_SIZE, PI, TAX_RATE
Classes: PascalCase
UserProfile, ShoppingCart
Private: Leading underscore
_internal_value, _helper_function
Good vs Bad Names
# BAD: Unclear
x = 100
n = "John"
t = 19.99
# GOOD: Descriptive
user_age = 100
customer_name = "John"
product_price = 19.99
# BAD: Too abbreviated
usr_nm = "Alice"
ttl_amt = 50
# GOOD: Readable
user_name = "Alice"
total_amount = 50
Multiple Assignment
Python offers elegant ways to assign multiple variables at once:
Assign Same Value to Multiple Variables
# All three variables get the value 0
a = b = c = 0
print(a) # 0
print(b) # 0
print(c) # 0
Assign Different Values in One Line
# Assign multiple values at once
x, y, z = 1, 2, 3
print(x) # 1
print(y) # 2
print(z) # 3
# Great for related data
name, age, city = "Alice", 25, "Paris"
Swap Variables
This is a Python specialty – no temporary variable needed!
# Traditional way (in other languages)
# temp = a
# a = b
# b = temp
# Python way - elegant!
a = 10
b = 20
a, b = b, a # Swap!
print(a) # 20
print(b) # 10
Checking Variable Types
Use the type() function to see what type a variable holds:
age = 25
name = "Alice"
price = 19.99
is_active = True
print(type(age)) # <class 'int'>
print(type(name)) # <class 'str'>
print(type(price)) # <class 'float'>
print(type(is_active)) # <class 'bool'>
Practical Example: Building a Profile
Let's put it all together with a real example:
# User profile data
first_name = "Alice"
last_name = "Smith"
age = 28
email = "alice.smith@email.com"
is_premium_member = True
account_balance = 150.75
# Calculate full name
full_name = first_name + " " + last_name
# Display profile
print("=" * 40)
print("USER PROFILE")
print("=" * 40)
print("Name:", full_name)
print("Age:", age)
print("Email:", email)
print("Premium Member:", is_premium_member)
print("Balance: $", account_balance)
print("=" * 40)
Output:
========================================
USER PROFILE
========================================
Name: Alice Smith
Age: 28
Email: alice.smith@email.com
Premium Member: True
Balance: $ 150.75
========================================
Key Takeaways
Remember These Points
Variables store data with a name
age = 25
Use = for assignment (not equality)
x = 10 means "x gets 10"
Variables can be reassigned
x = 5 then x = 10 is perfectly fine
Follow naming rules and conventions
• snake_case for variables
• Start with letter or underscore
• No keywords or special characters
Use descriptive names
user_age is better than x
What's Next
Now that you understand variables, let's explore the different types of data you can store in them. In the next lesson, we'll dive into Python's data types – integers, floats, strings, booleans, and more!