Python Basics — Keywords & Identifiers

Keywords & Identifiers

Every programming language has reserved words that have special meaning. Python calls these keywords. Meanwhile, the names you give to your variables, functions, and classes are called identifiers.

Python Keywords

Keywords are reserved — you cannot use them as variable names, function names, or any other identifiers.

Full List of Python Keywords (3.12)

import keyword
print(keyword.kwlist)

Category	Keywords
Logic	True, False, None
Boolean Ops	and, or, not
Control Flow	if, elif, else
Loops	for, while, break, continue, pass
Functions	def, return, lambda, yield
Classes	class
Exceptions	try, except, finally, raise
Imports	import, from, as
Variable Scope	global, nonlocal
Membership/Identity	in, is
Context Manager	with
Assertion	assert
Deletion	del
Async	async, await
Pattern Matching	match, case (3.10+, soft keywords)

What Happens If You Use a Keyword as a Name?

class = "Math"  # SyntaxError!

Error:

SyntaxError: invalid syntax

Python will refuse to run your code if you try to use a keyword as an identifier.

Checking If a Word Is a Keyword

import keyword

print(keyword.iskeyword("for"))      # True
print(keyword.iskeyword("name"))     # False
print(keyword.iskeyword("class"))    # True
print(len(keyword.kwlist))           # 35

Output:

True
False
True
35

Identifiers

An identifier is any name you create in Python — variables, functions, classes, modules. Think of them as labels you stick on things.

Naming Rules

Rule	Valid	Invalid
Must start with a letter or underscore	name, _count, _	1name, @value
Can contain letters, digits, underscores	total_1, my_var2	my-var, my var
Cannot be a keyword	score, result	class, for
Case-sensitive	name ≠ Name ≠ NAME	—
No spaces allowed	first_name	first name
No special characters	hello_world	hello@world

Naming Conventions (snake_case)

Python uses snake_case for most names — lowercase words separated by underscores:

# Good — snake_case for variables and functions
user_name = "Alice"
total_score = 95
max_retry_count = 3

def calculate_area(length, width):
    return length * width

# Bad — these work but violate convention
userName = "Alice"        # camelCase (used in Java/JavaScript)
TotalScore = 95           # PascalCase (reserved for classes)
MAXRETRYCOUNT = 3         # Hard to read without underscores

Convention Summary

Type	Convention	Example
Variables	snake_case	user_age, total_price
Functions	snake_case	get_name(), calculate_tax()
Constants	UPPER_SNAKE_CASE	MAX_SIZE, PI, BASE_URL
Classes	PascalCase	MyClass, UserProfile
Modules	snake_case	my_module.py, utils.py
Packages	lowercase	mypackage, requests
Private	_leading_underscore	_internal_value
Name Mangling	__double_leading	__secret
Magic/Dunder	__double_both__	__init__, __str__

Good vs Bad Identifier Names

# Good — descriptive and clear
student_count = 42
temperature_celsius = 36.6
is_logged_in = True

# Bad — vague and unclear
x = 42
t = 36.6
flag = True

Always choose names that describe what the variable holds. Your future self (and your teammates) will thank you.

Case Sensitivity

Python treats uppercase and lowercase letters as different:

name = "Alice"
Name = "Bob"
NAME = "Charlie"

print(name)   # Alice
print(Name)   # Bob
print(NAME)   # Charlie

Output:

Alice
Bob
Charlie

These are three completely separate variables. Be careful with capitalization.

Summary

• Python has 35 keywords that cannot be used as identifiers
• Use keyword.iskeyword() to check if a word is reserved
• Identifiers must start with a letter or underscore
• Python is case-sensitive — name and Name are different
• Use snake_case for variables and functions
• Use PascalCase for classes
• Use UPPER_SNAKE_CASE for constants
• Choose descriptive names — clarity over brevity

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