Python Identifiers & Reserved Keywords (Detailed Guide)

🔹 Identifiers in Python

What is an Identifier?

An identifier is the name used to identify variables, functions, classes, modules, and objects in Python.

✅ Examples of Identifiers:

name = "Alice"      # Variable identifier
def add_numbers():  # Function identifier
    pass

class Student:      # Class identifier
    pass

Rules for Naming Identifiers in Python

1. Can contain letters (A-Z, a-z), digits (0-9), and underscores (_)
   • ✅ my_variable, Class1, num_2 are valid.
2. Must start with a letter (A-Z, a-z) or an underscore (_) but NOT a digit (0-9)
   • ✅ _temp, var_name are valid.
   • ❌ 2variable is invalid.
3. Cannot be a Python reserved keyword (e.g., if, else, while)
   • ❌ class = 10 is invalid because class is a reserved word.
4. Identifiers are case-sensitive
   • MyVar, myvar, and MYVAR are different.
5. Can be of any length but should be meaningful for readability.
6. Cannot contain special characters (@, #, $, %, &, etc.).
   • ❌ user@name is invalid.

✅ Valid Identifiers:

age = 25
_name = "Alice"
class1 = "Python"  # Not a reserved keyword

❌ Invalid Identifiers:

2name = "Bob"     # Starts with a digit ❌
user@name = "Tom" # Special character ❌
class = "Math"    # Reserved keyword ❌

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🔹 Python Reserved Keywords

Reserved keywords (also called keywords) are special words in Python that cannot be used as identifiers because they have a predefined meaning.

List of Python Keywords (Python 3.x)

['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 
 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 
 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 
 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 
 'try', 'while', 'with', 'yield']

Using keyword Module to Get Keywords

You can check all Python keywords using the keyword module:

import keyword
print(keyword.kwlist)  # Prints a list of all reserved keywords

Examples of Reserved Keywords Usage

✅ Correct Usage (As Keywords)

if True:
    print("This is valid")  # 'if' is used correctly as a keyword

def my_function():          # 'def' is correctly used to define a function
    pass

❌ Incorrect Usage (As Identifiers)

class = "Python"  # ❌ Invalid: 'class' is a keyword

def = 10          # ❌ Invalid: 'def' is a keyword

✅ Corrected Version

class_name = "Python"  # ✅ Valid identifier
function_name = 10     # ✅ Valid identifier

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🔹 Summary

Feature	Identifiers	Keywords
Definition	User-defined names for variables, functions, etc.	Reserved words with predefined meanings
Rules	Letters, digits, and _, cannot start with digits or special characters	Cannot be used as variable or function names
Examples	my_var, Student, _temp123	if, else, def, class, import
Case-Sensitive?	✅ Yes (MyVar ≠ myvar)	✅ Yes (True is different from true)
Customizable?	✅ Yes	❌ No

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✅ Conclusion

• Identifiers are names given to variables, functions, and classes, but they must follow certain rules.
• Reserved Keywords are built-in words in Python that cannot be used as identifiers.
• Use meaningful identifiers to make code readable.

Would you like examples of best practices for naming variables and functions? 😊