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Python Variable Names

A good name is documentation you get for free. Python only enforces a few rules — the rest is habit that makes your code readable to the next person (often future you).

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The rules Python enforces

Only four rules are actually checked by the interpreter. Break one of these and you get a SyntaxErrorbefore the program even runs:

  • A name must start with a letter (a–z, A–Z) or an underscore _.
  • The rest of the name may contain letters, digits, and underscores — nothing else. No spaces, no dashes.
  • Names are case-sensitive: age, Age, and AGE are three different variables.
  • A name cannot be one of Python's reserved keywords (if, class, return, …).

Example — all of these are legal

myvar   = 1
my_var  = 2
_my_var = 3
myVar   = 4
MYVAR   = 5
myvar2  = 6

Example — each of these is a SyntaxError

2myvar = 7    # cannot start with a digit
my-var = 8    # a dash is read as "my minus var"
my var = 9    # spaces separate tokens
class  = 10   # class is a reserved keyword

Legal, but a bad idea

Some names Python accepts happily yet you should still avoid, because they quietly reassign something you'll want later. The most common trap is shadowing a built-in — using a name likelist, str, sum, id, or type for your own value.

Example — shadowing list breaks it

list = [1, 2, 3]      # legal: 'list' now points at your list
nums = list(range(5)) # TypeError: 'list' object is not callable

Nothing is wrong on the first line, so the error shows up far away and looks mysterious. Pickitems or numbers instead and the built-in stays intact.

Not sure whether a word is a keyword? Ask Python: import keyword; keyword.iskeyword("class")returns True. Editors also colour keywords differently — a name that lights up likereturn is a warning sign.

Choose names that describe the value

Single letters are fine for a throwaway loop counter or a coordinate, but for anything that lives more than a couple of lines, spell it out. Code is read far more often than it is written.

PreferAvoidWhy
total_pricetpThe reader shouldn't decode abbreviations.
user_countn2Numbers on names say nothing about meaning.
is_activeflagBooleans read well as is_/has_ questions.
i, x, ythe_indexShort is fine for tiny, obvious scopes.

Multi-word names: pick a style and keep it

When a name needs more than one word, you have to join them somehow. Three conventions are common; Python's official style guide, PEP 8, has a clear preference.

my_variable_name = "snake_case"   # PEP 8: use this for variables & functions
myVariableName   = "camelCase"    # common in JavaScript/Java, not Python
MyVariableName   = "PascalCase"   # reserve this for class names
MAX_RETRIES      = 5              # ALL_CAPS for constants that never change

The rule of thumb: snake_case for variables and functions, PascalCase for classes, and UPPER_SNAKE_CASE for constants. Consistency matters more than the choice itself.

A note on underscores

Leading and trailing underscores carry convention, not enforcement — Python won't stop you, but other programmers read them as signals:

NameConvention it signals
_temp"Internal — don't rely on this from outside."
_"A value I'm deliberately ignoring," e.g. for _ in range(3):
__dunder__Reserved by Python (e.g. __init__) — don't invent your own.

Try It Yourself

Exercise: Three of these five names are problematic. Which, and why?

total = 0
2nd_place = "silver"
str = "hello"
first_name = "Ada"
user-id = 7
Show answer

2nd_place is a SyntaxError (starts with a digit). user-id is aSyntaxError (the dash is a minus sign). str = "hello" is legal but shadows the built-in str() — rename it to something like greeting. total andfirst_name are both fine.

Key Takeaways

  • Start with a letter or underscore; letters, digits, and underscores only; case matters; no keywords.
  • Avoid names that shadow built-ins like list, str, or sum.
  • Use snake_case for variables, PascalCase for classes, ALL_CAPS for constants.
  • Descriptive names are free documentation — spell things out beyond tiny scopes.

📘 Real-World Deep Dive

Names are how you communicate intent to the next reader. PEP 8 conventions plus a few defensive rules (no built-in shadows, prefer long-but-clear names) eliminate dozens of confusing bugs every project hits.

Real-Life Scenario

Reviewing a small ETL pipeline: rename variables so each conveys its type, its role in the pipeline, and its scope.

Real-Life Example

from dataclasses import dataclass

# BAD
xs = [r for r in raw if r["s"] == 1]
y = sum(float(x["amt"]) for x in xs)

# GOOD
STATUS_ACTIVE = 1
active_rows = [row for row in raw_rows if row["status"] == STATUS_ACTIVE]
total_amount = sum(float(row["amount"]) for row in active_rows)

@dataclass(frozen=True)
class LoadedEtl:
    source: str
    rows: int
    total_amount: float

report = LoadedEtl(
    source="orders.csv",
    rows=len(active_rows),
    total_amount=total_amount,
)
print(report)

Expected Output

LoadedEtl(source='orders.csv', rows=187, total_amount=12450.75)

Common mistakes

  • Shadowing builtins (list, dict, id, type) silently breaks later code in the file.
  • Names that differ only by case (User vs. user) cause cross-module confusion.
  • Single-letter names (l, I, O) look like 1/0 — restrict them to short-lived loops.

🚀 Performance & Best Practices

  • Long descriptive names cost roughly zero at runtime; pick clarity first.
  • Use SCREAMING_SNAKE_CASE for module-level constants so they grep out as obviously non-mutables.
  • Hybrid prefixes (is_*, _private) communicate intent without needing a docstring.

🧪 Try It Yourself

  1. Install ruff check --select N and clean up every pep8-naming violation.
  2. Rename the xs/y/data variables in a real script to self-explanatory names.
  3. Add a verify step that no Python builtin name is shadowed at module level.

FAQ: Python Variable Names

Common questions about this page.

What is Python Variable Names?

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