Python Variable Names (Naming Rules and Conventions)

You already know how to create a variable. Now comes the part that makes your code readable or confusing: choosing good Python variable names. Some naming rules are strict, and breaking them causes an error. Others are conventions that every Python developer follows. In this lesson, you'll learn both, plus a sneaky naming bug that catches almost every beginner.

What are the rules for Python variable names?

These rules are enforced by Python itself. Break one, and your program won't run.

1. A name can contain letters (a–z, A–Z), digits (0–9) and the underscore _.
2. A name cannot start with a digit. player1 is fine; 1player is not.
3. A name cannot contain spaces, hyphens or symbols like $, @ or !.
4. Names are case-sensitive: city, City and CITY are three different variables.
5. A name cannot be a keyword, a word Python reserves for its own grammar, like if or class.

user_name = "maya_k"
total2 = 150
_count = 0
print(user_name, total2, _count)

maya_k 150 0

What happens if you use an invalid name?

Python refuses to run the file and shows a SyntaxError. Here's what each broken rule looks like.

Starting with a digit

2nd_place = "Leo"

SyntaxError: invalid decimal literal

Python sees the 2 and expects a number, then gets confused by the letters after it.

Using a hyphen

first-name = "Ana"

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

A hyphen is the minus sign, so Python reads this as "first minus name", which you can't assign to.

Using a space

my score = 10

SyntaxError: invalid syntax

Using a keyword

class = "Math 101"

SyntaxError: invalid syntax

How do you check if a word is a Python keyword?

Python 3.12 has 35 keywords. You don't need to memorize them. The built-in keyword module can check for you:

import keyword

print(keyword.iskeyword("class"))
print(keyword.iskeyword("score"))
print(len(keyword.kwlist))
print(keyword.kwlist[:5])

True
False
35
['False', 'None', 'True', 'and', 'as']

The full list includes words like if, else, for, while, def, return, import, in, is, not, or, pass and with. Run print(keyword.kwlist) to see them all. Most editors also color keywords differently, which is a quick visual warning.

Is Python case-sensitive with variable names?

Yes. Every difference in capitalization creates a separate variable:

city = "Rome"
City = "Oslo"
print(city, City)

Rome Oslo

This is legal, but please don't do it on purpose. Two names that differ only in capitals are an easy way to confuse yourself.

Naming styles: snake_case, camelCase and more

When a name has several words, you need a way to join them, since spaces aren't allowed. There are a few common styles:

The official style guide, PEP 8, recommends snake_case for variable names: all lowercase, with underscores between words. Python won't complain about camelCase, but your code will look out of place next to everyone else's.

MAX_TICKETS = 6          # constant
tickets_requested = 4    # variable in snake_case

print(tickets_requested <= MAX_TICKETS)

True

How do you choose good variable names?

A good name tells the reader what the value means, without needing a comment. Compare these:

Long names aren't bad, but don't go overboard. total_price is better than both tp and the_total_price_of_all_items_in_the_cart. Also avoid single letters that look like numbers: lowercase l, uppercase O and uppercase I.

Why you shouldn't name a variable list, str or sum

Python has built-in functions such as print, input, list, str, int, sum, max and type. They are not keywords, so Python lets you reuse their names. But if you do, your variable shadows (hides) the built-in function, and the original stops working.

sum = 10 + 20
scores = [8, 9, 10]
print(sum(scores))

TypeError: 'int' object is not callable

On line 1, sum became the number 30. On line 3, you tried to use it as the built-in sum() function, but now it's just a number, and you can't "call" a number. Here's the same bug with list:

list = ["eggs", "milk"]
numbers = list(range(3))

TypeError: 'list' object is not callable

The fix is to pick a more specific name, like total or groceries. It's also a better name anyway.

What does a single underscore _ mean?

By convention, a lone underscore _ is used for a value you have to receive but don't plan to use. It tells readers "ignore this one".

for _ in range(3):
    print("Hip hip hooray!")

Hip hip hooray!
Hip hip hooray!
Hip hip hooray!

The loop needs a variable, but we never use it, so _ is perfect. It's also handy when unpacking values:

name, _, city = "Ava", 29, "Boston"
print(name, city)

Ava Boston

Names that start with an underscore, like _cache, have their own meaning in larger programs ("for internal use"). You'll see that when you learn about modules and classes.

Common mistakes with variable names

• Starting with a number, like 3d_model. Use model_3d instead.
• Using hyphens as in zip-code. Use zip_code.
• Shadowing built-ins such as list, str, input, sum or max.
• Typos in capitalization: creating Total and later using total gives a NameError.
• Meaningless names like a, temp2 or stuff that force readers to guess.

Practice exercise

1st-deposit = 200
b = 50
sum = 1st-deposit + b
print(sum)

first_deposit = 200
second_deposit = 50
account_balance = first_deposit + second_deposit
print(account_balance)

250

What's next?

Your variables now have clear, valid names. Next, explore the different kinds of values they can hold in Python data types.