Storing Information: Variables

Programs constantly work with pieces of information, such as numbers, text, or the results of calculations. Instead of using raw data directly everywhere, it's much more practical to give that data a name and refer to it by that name. Variables fulfill this role. A variable is like a labeled container or a named placeholder in your computer's memory where you can store a value.

What is a Variable?

In Python, a variable is a symbolic name that refers to a value. You use variables to store data so that you can reference and manipulate it later in your program. The term "variable" implies that the value stored can change or vary during the program's execution.

Imagine you're organizing items in boxes. You wouldn't just put things in unmarked boxes; you'd label them ("Winter Clothes", "Books", "Kitchen Utensils"). Variables serve a similar purpose in programming: they provide descriptive names for memory locations that hold data.

Creating Variables: The Assignment Statement

Creating a variable in Python is straightforward using the assignment operator, which is the equals sign (=). The structure is:

variable_name = value

Here's how it works:

1. You choose a name for your variable (following certain rules, discussed below).
2. You use the = sign.
3. You provide the value you want to store in the variable.

Let's look at a few examples:

message = "Hello, Python Learner!"
count = 10
price = 49.99
is_active = True

In these examples:

• message is a variable storing a string of text.
• count is a variable storing an integer (a whole number).
• price is a variable storing a floating-point number (a number with a decimal).
• is_active is a variable storing a boolean value (True).

Python is dynamically typed, meaning you don't need to explicitly declare the type of data a variable will hold. Python automatically determines the type based on the value you assign to it. When you assign "Hello, Python Learner!" to message, Python understands that message now holds a string.

Using Variables

Once you've created (or declared and initialized) a variable, you can use its name in your code to access the value it holds.

message = ""
print(message) # Output:

items = 5
cost_per_item = 2.50
total_cost = items * cost_per_item # Use variables in calculations
print(total_cost) # Output: 12.5

Using variables makes your code more readable and easier to maintain. If the cost_per_item changes, you only need to update it in one place (where the variable is assigned), rather than searching for every instance of the number 2.50 in your code.

Naming Variables: Rules and Conventions

Choosing good variable names is important for writing clear and understandable code. Python has specific rules and widely accepted conventions for naming variables:

Rules (Must be followed):

• Variable names must start with a letter (a-z, A-Z) or an underscore (_). They cannot start with a number.
• The rest of the name can contain letters, numbers (0-9), and underscores.
• Variable names are case-sensitive. myVariable, myvariable, and MYVARIABLE are distinct variables.
• Variable names cannot be the same as Python keywords. Keywords are reserved words that have special meaning in Python (like if, else, for, while, def, class, True, False, None).

Conventions (Should be followed for readability):

• Use descriptive names that indicate the variable's purpose (e.g., user_name instead of un, total_price instead of tp).
• Use snake_case for variable names consisting of multiple words. This means all letters are lowercase, and words are separated by underscores (e.g., first_name, number_of_items).
• Avoid using single-letter names like l, O, or I as they can be easily confused with numbers 1 and 0. Short names like x, y, or i are often acceptable in specific contexts like loop counters or mathematical coordinates, but descriptive names are generally preferred.

Here are examples of valid and invalid variable names:

• Valid: name, age, user_input, _internal_counter, price_2024
• Invalid: 2nd_place (starts with a number), user-name (contains a hyphen), class (is a keyword)

Changing Variable Values (Reassignment)

You can change the value stored in a variable by assigning a new value to it using the assignment operator (=) again.

# Assign an initial value
score = 100
print("Initial score:", score) # Output: Initial score: 100

# Update the value
score = 150
print("Updated score:", score) # Output: Updated score: 150

# Assign a value based on the current value
score = score + 20 # Read the current value (150), add 20, store the result (170) back
print("Final score:", score)   # Output: Final score: 170

Each time you assign a new value, the old value associated with that variable name is replaced.

Variables are fundamental building blocks in Python. They allow you to label and store data, making your programs flexible and easier to understand. As you progress, you'll see variables used constantly to manage information flow within your code.