4. Variables & Constants

Variables and constants are fundamental concepts in programming. They allow a program to store, reuse, and manipulate data efficiently during execution.

Understanding their distinction is key: variables are changeable containers of data, while constants represent values that are intended to remain fixed throughout the program's lifecycle.

1. Variables, Constants, and Naming

1.1. Variables

A variable is a name that stores a value in the computer's memory.

Variables:

• hold data (numbers, text, etc.)
• can change during program execution
• are created when you assign a value using =

Example:

age = 17       # Integer
price = 4.99   # Float
name = "Ana"   # String

1.2. Constants

A constant is a name used for a value meant to remain unchanged throughout the program.

Example:

TAX_RATE = 0.16
MAX_STUDENTS = 30

1.3. Naming Conventions

All variable names must start with a letter (a-z) or an underscore (_), and they cannot contain spaces or reserved keywords (like print or if).

When using multiple words, two common styles exist:

Style	Variable Example	Constant Example (always ALL_CAPS)
Snake Case (PEP 8)	student_name	MAX_CAPACITY
Camel Case	studentName	MAX_CAPACITY

Consistency is the Rule: While you may choose either Snake Case or Camel Case for variables based on the specific course or team preference, the most important rule is consistency. Do not mix styles in the same project.

For Constants, the convention is universal: always use ALL_CAPS.

1.4. Case Sensitivity in Python

Python is case-sensitive, meaning that uppercase and lowercase letters are treated as different characters. This means:

• Total, total, and TOTAL are three different identifiers.
• A variable must always be used exactly as it was defined.
• Changing a single letter from lowercase to uppercase will cause a NameError.

Example:

score = 95
print(Score)   # Error - "Score" is not the same as "score"

2. Commented Examples

2.1. Variable Assignment and Data Types

# 1. Setup: Assign text data (string)
name = "Ana"
# 2. Setup: Assign a whole number (integer)
result = 25
# 3. Setup: Assign a decimal number (float)
num = 4.5 

Explanation:

• Python automatically infers the data type from the assigned value.
• str is used to store text data.
• int is used for whole numbers.
• float is used for decimal numbers, preserving precision.

2.2. Handling Input and Using a Constant

# 1. Input: Get user data (always returns a string)
tickets_str = input("Enter the number of tickets: ")

# 2. Process: Convert string input to integer for calculations
ticket_count = int(tickets_str)

# 3. Process: Define a constant value (uppercase convention)
TICKET_PRICE = 75

# 4. Process: Calculate the total cost
total = ticket_count * TICKET_PRICE

# 5. Output: Display the final result
print("Total cost:", total)  # Example output: Total cost: 225

Explanation:

• input() returns a string, requiring conversion for math.
• int() converts the string to an integer for arithmetic.
• TICKET_PRICE uses uppercase, the naming convention for constants.
• Multiplication calculates the total cost, and the result is printed.

3. Short Practice Exercises

3.1. Value Prediction

What is the final value and data type of total?

variable2 = 18.4
variable3 = 2.6
total = variable2 + variable3

Show solution

total = 21.0.

Note: The total variable is of data type float.

3.2. Naming Conventions

Identify which names follow Python best practices:

• total_amount
• Value1
• client.id
• CustomerName

Show solution

total_amount

3.3. Input Conversion

Write one line of code asking for a grade and storing it as a float in final_grade.

Show solution

final_grade = float( input("Enter your grade: ") )

3.4. Error Identification

What error occurs with "Count" + 7?

Show solution

TypeError

Explanation: Python cannot add a string ("Count") and an integer (7).

4. Google Colab: Try It Yourself

Practice variable assignment, manipulation, and naming conventions with this Colab notebook:

👉 Open the Variables & Constants Colab notebook ↗

First time using Google Colab? Read the quick beginner guide ↗

5. Mini-Quiz

5.1. What is the primary function of a variable?

A) Execute repetitive actions.
B) Print text.
C) Store a value in memory.
D) Convert strings to integers.

Show answer

C) Store a value in memory.

5.2. What is the data type result of 10 + 4.1?

A) Integer
B) Boolean
C) String
D) Float

Show answer

D) Float

5.3. Which function always returns input as a string?

A) print()
B) int()
C) input()
D) float()

Show answer

C) input()

5.4. What describes a constant?

A) Changes many times.
B) Stays fixed during execution.
C) Stores only names.
D) Stores decimals.

Show answer

B) Stays fixed during execution.

5.5. What happens after the second line is executed?

id = "Ana"
id = 12

A) The variable id is updated to hold the value 12, and its data type becomes an integer.
B) The program execution stops because a syntax error is raised at that line of code.
C) The variable id continues to store the string value "Ana" without being changed.
D) The variable id is removed entirely from memory and no longer exists in the program.

Show answer

A) The variable id is updated to hold the value 12, and its data type becomes an integer.

6. Common Mistakes

• Starting variable names with numbers or invalid characters.
• Trying to add strings and numbers directly ("hello" + 3).
• Using ALL_CAPS for regular variables (reserve for constants).
• Confusing = (assignment) with == (comparison).

7. Summary Diagram

mindmap
  root((Variables & Constants))
    Variables
      Changeable
      Store data
      Created with =
    Constants
      Fixed value
    Input
      Always string
      Convert with int or float
    Naming Conventions
      Consistency is key
      Variables
        snake_case
        camelCase
      Constants     
        ALL_CAPS

8. Optional Extensions

• Write three valid and three invalid variable names.
• Identify constants in a real-world situation.
• Create a small program that multiplies two input values.