Variables and Naming

Exercise — Give five valid and five invalid variable names, with the reason for each.

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The Rules

	Rule
✅	Can start with a letter (a-z, A-Z) or an underscore (_)
✅	Can contain letters, digits (0-9), and underscores
✅	Case-sensitive — age, Age, AGE are three different variables
❌	Cannot start with a digit
❌	Cannot contain spaces
❌	Cannot use special characters (!, @, #, $, %, -, …)
❌	Cannot be a Python keyword (if, for, while, return, …)

Case-sensitivity in action:

age = 1
Age = 2
AGE = 3
print(age, Age, AGE)   # → 1 2 3   three separate variables

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5 Valid Names

1. student_name — snake_case

student_name = "Vettri"

✅ Starts with a letter, uses an underscore to separate words. This is the Python standard.

2. _age — leading underscore

_age = 28

✅ An underscore is a legal first character. By convention it signals "internal / private — don't touch from outside".

3. studentAge — camelCase

studentAge = 25

✅ Legal — letters only. But camelCase is the Java/JavaScript convention; Python prefers student_age.

4. total_annual_salary — multiple underscores

total_annual_salary = 500000

✅ All letters and underscores. Long but self-documenting, which is better than a cryptic tas.

5. department1 — digits after the first character

department1 = "Engineering"

✅ Digits are allowed anywhere except the first character.

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5 Invalid Names

❌ 1. Starts with a digit

1student_name = "Vettri"

Error

SyntaxError: invalid decimal literal

Why: names must begin with a letter or underscore. Python starts reading 1 as a number literal and then hits letters, which makes no sense.

Fix: student_name1 or _1student_name

❌ 2. Contains a space

student name = "Vettri"

Error

SyntaxError: invalid syntax

Why: Python uses whitespace to separate tokens, so it sees two separate names.

Fix: student_name or studentName

❌ 3. Contains a special character

student@name = "Vettri"

Error

SyntaxError: cannot assign to expression here. Maybe you meant '==' instead of '='?

Why: @, #, $, %, & etc. all have their own meanings. @ in particular is the matrix-multiplication operator, so Python reads student@name as student matmul name — an expression, and you cannot assign to an expression.

Different special characters fail in genuinely different ways, which is worth seeing:

You write	Actual error	Why
student@name = "V"	SyntaxError: cannot assign to expression here	@ is the matmul operator
student%name = "V"	SyntaxError: cannot assign to expression here	% is the modulo operator
student$name = "V"	SyntaxError: invalid syntax	$ isn't an operator at all
student#name = "V"	NameError: name 'student' is not defined	# starts a comment — the line becomes just student

That last one is the sneakiest: it isn't a syntax error at all. Python silently discards everything after #, leaving a line that merely evaluates student.

Fix: student_name

❌ 4. Uses a Python keyword

if = 5

Error

SyntaxError: invalid syntax

Why: keywords are reserved — they're part of the language grammar.

Fix: pick a different word — condition = 5, loop_var = "Hello"

❌ 5. Contains a hyphen

student-name = "Vettri"

Error

SyntaxError: cannot assign to expression here. Maybe you meant '==' instead of '='?

Why: - is the subtraction operator. Python reads student-name as student minus name, which is an expression, and you can't assign to an expression.

Fix: student_name

tip

This is the single most common slip for people coming from CSS, HTML, or shell scripting, where kebab-case is normal. In Python, - is always arithmetic.

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Python Keywords

Python 3.11 has 35 reserved keywords. None can be used as a variable name.

import keyword
print(len(keyword.kwlist))   # → 35
print(keyword.kwlist)

Grouped by purpose:

Group	Keywords
Control flow	if, elif, else, for, while, break, continue, pass
Functions	def, return, lambda, yield
Classes	class
Imports	import, from, as
Exceptions	try, except, finally, raise, assert
Logic & membership	and, or, not, in, is
Constants	True, False, None
Scope	global, nonlocal
Context & async	with, async, await
Other	del

Check any name yourself:

import keyword
print(keyword.iskeyword("class"))   # → True
print(keyword.iskeyword("klass"))   # → False

Soft keywords

match, case, type and _ are contextual. They're legal variable names, but avoid them — using match as a variable will confuse anyone reading your code.

Built-ins are not keywords, but don't shadow them either

list = [1,2,3] is legal — no error — but now list() is broken for the rest of your program. Same for str, dict, sum, type, id, input. This is worse than a SyntaxError because it fails silently, much later.

list = [1, 2, 3]
list((4, 5))     # TypeError: 'list' object is not callable

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Naming Conventions

Legal is not the same as good. These are conventions from PEP 8, Python's style guide.

Style	Use for	Example
snake_case	variables, functions	student_name, total_amount, is_valid
camelCase	avoid in Python	studentName — works, but non-standard
PascalCase	classes	StudentProfile, PersonRecord
UPPER_CASE	constants	MAX_SIZE = 100, PI = 3.14159
_leading	internal use	_internal_value
__dunder__	special methods — don't invent your own	__init__, __str__

# Variables — snake_case
user_age = 25
is_student = True
total_price = 100.50
student_list = ["Alice", "Bob", "Charlie"]

# Class — PascalCase
class StudentProfile:
    pass

# Constants — UPPER_CASE
MAX_STUDENTS = 100
DATABASE_URL = "localhost:5432"

note

Python has no real constants — MAX_STUDENTS = 100 can be reassigned. UPPER_CASE is a message to other programmers, not a guarantee to the interpreter.

Naming well matters more than naming legally:

Poor	Better	Why
x	student_count	says what it holds
data	exam_scores	says what kind of data
flag	is_enrolled	is_/has_ prefix reads as a question
l, O, I	anything else	look like 1 and 0 in most fonts
temp2	celsius_reading	numbered names hide meaning

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Quick Test

Are these valid?

#	Name	Verdict	Reason
1	myVariable	✅ valid	starts with a letter, alphanumeric
2	_private	✅ valid	starts with an underscore
3	2fast	❌ invalid	starts with a digit
4	user-name	❌ invalid	contains a hyphen
5	user_name_123	✅ valid	letters, underscores, digits (not first)
6	class	❌ invalid	reserved keyword
7	first-name	❌ invalid	contains a hyphen
8	__init__	✅ valid	double underscores are legal (Python uses them)
9	MAX SIZE	❌ invalid	contains a space
10	firstName	✅ valid	camelCase works, but isn't Python standard

You can check any of these programmatically — see Run It Yourself below.

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Summary

Rule	Status
Starts with a letter	✅ allowed
Starts with an underscore	✅ allowed
Starts with a digit	❌ not allowed
Contains letters, digits, underscores	✅ allowed
Contains spaces	❌ not allowed
Contains special chars (@, #, $, %)	❌ not allowed
Contains a hyphen -	❌ not allowed
Is a Python keyword	❌ not allowed
Shadows a built-in (list, str, sum)	⚠️ legal but harmful
Case-sensitive	✅ age ≠ Age ≠ AGE

Key takeaways

• Letter or underscore to start; letters, digits, underscores thereafter
• - is subtraction — never use it in a name
• 35 keywords are reserved; keyword.iskeyword() will tell you
• Shadowing built-ins compiles fine and breaks things silently — the dangerous case
• snake_case for variables, PascalCase for classes, UPPER_CASE for constants
• Descriptive beats short: student_count over x

See also: Values and Types for what the variables can hold · Data Types for the full type list

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Run It Yourself

Python's own checker for whether a name is legal:

check_names.py

import keyword

candidates = [
    "myVariable", "_private", "2fast", "user-name", "user_name_123",
    "class", "first-name", "__init__", "MAX SIZE", "firstName",
]

print(f"{'Name':<16} {'Valid':<7} Reason")
print("-" * 50)
for name in candidates:
    if keyword.iskeyword(name):
        valid, reason = False, "reserved keyword"
    elif not name.isidentifier():
        valid, reason = False, "not a legal identifier"
    else:
        valid, reason = True, "OK"
    print(f"{name:<16} {'✅' if valid else '❌':<7} {reason}")

Output

Name             Valid   Reason
--------------------------------------------------
myVariable       ✅       OK
_private         ✅       OK
2fast            ❌       not a legal identifier
user-name        ❌       not a legal identifier
user_name_123    ✅       OK
class            ❌       reserved keyword
first-name       ❌       not a legal identifier
__init__         ✅       OK
MAX SIZE         ❌       not a legal identifier
firstName        ✅       OK

str.isidentifier() and keyword.iskeyword() together are the exact test Python itself applies — no need to guess.

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Practice Questions

From the Unit 1 question bank. Tags and marks are explained on the Python index.

Unit 1 § A — Variables, Naming & Data Types

A1. [THEORY] State all the rules for a valid Python variable name. Then give 5 valid and 5 invalid names, explaining why each invalid one is rejected. [5]

A2. [OUT] [2]

age = 20
Age = 30
AGE = 40
print(age, Age, AGE)
print(age + Age + AGE)

A3. [THEORY] For each name below, say valid or invalid. If invalid, give the reason. [5]

2nd_place    _total    class    my-var    Total$
firstName    for       data 1   __init__   Σ

(One of these looks invalid but is actually legal. Find it.)