Variables and identifiers are fundamental to programming in Python. In this guide, you’ll learn how to name variables correctly, avoid syntax errors, and follow Python’s naming conventions. We’ll also explore reserved keywords and practice with hands-on exercises.
Variables and Identifiers in Python
What Are Variables?
Variables are containers for storing data. They are created using an identifier (name) and the assignment operator =
.
age = 25
name = "Mourad"
user_name = 'admin'
server_ip = '192.168.1.1'
Rules for Naming Identifiers
- Can include letters, numbers, and underscores.
- Cannot start with a number.
- Cannot use reserved keywords (e.g.,
def
,class
). - Case-sensitive (
Age
≠age
).
Invalid Identifiers Example
#def = 5 # SyntaxError: "def" is a reserved keyword
Reserved Keywords
Python has predefined keywords that cannot be used as identifiers. View them using the keyword
module:
import keyword
print(keyword.kwlist)
Practice Work
Exercise 1: Fix Invalid Identifiers
Correct the invalid identifiers in the code below:
2nd_name = "Ali"
class = "Python101"
user-email = "test@example.com"
second_name = "Ali"
course_class = "Python101"
user_email = "test@example.com"
Exercise 2: Create Valid Identifiers
Write variables for the following data:
- A constant for maximum login attempts (value: 3).
- A string storing a server’s domain name (“api.example.com”).
- A boolean indicating whether a user is active (True/False).
MAX_LOGIN_ATTEMPTS = 3
server_domain = "api.example.com"
is_user_active = True
Exercise 3: Reserved Keywords Check
Write a script to check if the word "async"
is a reserved keyword in Python.
import keyword
print("async" in keyword.kwlist) # Output: True (in Python 3.7+)
Workshop: Real-World Practice
Workshop 1: User Registration Script
Create a script that asks for a user’s first name, last name, and age. Store the data in variables and print a summary. Follow these rules:
- Use snake_case for variable names.
- Avoid reserved keywords.
first_name = input("Enter your first name: ")
last_name = input("Enter your last name: ")
user_age = int(input("Enter your age: "))
print(f"User: {first_name} {last_name}, Age: {user_age}")
Workshop 2: Data Processing
Calculate the total price of items in a shopping cart using variables:
item1_price = 20.5
item2_price = 15.75
item3_price = 5.0
total = item1_price + item2_price + item3_price
print(f"Total: ${total:.2f}")
Best Practices
- Use Descriptive Names:
user_age
instead ofa
. - Follow Case Conventions:
- snake_case for variables (e.g.,
server_ip
). - UPPER_CASE for constants (e.g.,
MAX_USERS
).
- snake_case for variables (e.g.,
- Avoid Abbreviations:
first_name
is clearer thanfn
.
Conclusion
Mastering variables and identifiers is the first step to writing clean, maintainable Python code. Practice the exercises and workshops to solidify your understanding, and always follow Python’s naming conventions.
Want more? Check out our post on Python Data Types or Python Functions.