Python Identifiers

Identifiers are names given to variables, functions, classes, modules, and other objects in Python. They help us reference these objects in our code.

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Rules for Python Identifiers

• Must start with a letter (a-z, A-Z) or an underscore (_).
• Can contain letters, numbers (0-9), and underscores.
• Cannot be a Python keyword (like if, for, while, etc.).
• Are case-sensitive (myVar and myvar are different).

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Valid Identifiers

name = "Alice"       # Starts with a letter  
_age = 25            # Starts with an underscore  
MAX_VALUE = 100      # Constants are usually UPPER_CASE  
first_name = "Dave"  # Uses underscore (hyphens are invalid) 
user1 = "Bob"        # Contains a number (but doesn't start with it)   

# Function name with underscore  
def greet_user(username):  
    print(f"Hello, {username}!")  

# Class name in PascalCase
class Car:
    def __init__(self, model):  
        self.model = model  # Instance variable  
  

Explanation:

• name - A simple variable name that starts with a letter.
• _age - Valid because it starts with an underscore.
• MAX_VALUE - Constants are usually written in uppercase.
• first_name - Uses underscores for readability.
• user1 - Contains a number but does not start with one.
• greet_user - Function names typically use underscores.
• Car - Follows PascalCase convention for class names.

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Invalid Identifiers

2name = "Charlie"  
# Starts with a number (SyntaxError)  

class = "Math"  
# Uses a Python keyword "class" (SyntaxError)  

first-name = "Dave"  
# Hyphen is not allowed (SyntaxError)  

$price = 10.99  
# Special characters not allowed (SyntaxError)  

user@name = "Eve"  
# @ symbol is invalid (SyntaxError)  

def 1st_func():  
    pass  
# Function name starts with a number (SyntaxError)  
  

Explanation:

• 2name - Starts with a number, which is not allowed.
• class - Cannot use a reserved keyword as an identifier.
• first-name - Hyphens are not allowed; use underscores instead.
• $price - Special characters like $ are not allowed.
• user@name - @ is not a valid identifier character.
• 1st_func - Cannot start with a number.

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