Python Keywords & Identifiers

Keywords

Keywords are words the Python interpreter reserves for its own syntax — if, class, for, def, and about 30 others — and using one as a variable name raises a SyntaxError because the interpreter can't tell your intent from its own grammar.

Example: Keywords

# 'class' is a reserved keyword
# class = 5  # SyntaxError: invalid syntax
print("keywords cannot be used as variable names")

Identifiers

An identifier is any name you choose for a variable, function, or class. Picking names that describe what they hold (user_count rather than x) means the code explains itself, cutting down on comments you'd otherwise need to write.

Example: Identifiers

user_count = 42
print(user_count)

Naming Rules

Identifiers must start with a letter or underscore and can only contain letters, digits, and underscores after that — no spaces, hyphens, or symbols like $ or @, which Python reserves for other syntax entirely.

Example: Naming Rules

_valid_name = 1  # starts with an underscore, which is allowed
name2 = 2  # letters followed by a digit, also allowed
print(_valid_name, name2)

Case Sensitivity in Names

Python treats page and Page as two completely separate names, since identifier matching is case-sensitive throughout the language.

A single accidental capitalization mismatch is a common source of NameError bugs that can be confusing to spot by eye.

Example: Case Sensitivity in Names

page = "lowercase"  # lowercase variable name
Page = "uppercase"  # capitalized name is a completely different variable
print(page)
print(Page)

Best Practices for Names

Favor full, descriptive words over cryptic abbreviations or single letters (except for very short-lived loop counters like i), and follow Python's snake_case convention for variables and functions so your code reads consistently with the rest of the ecosystem.

Example: Best Practices for Names

total_price = 19.99  # descriptive snake_case name for a price value
for i in range(3):  # short name "i" is fine for a throwaway loop counter
    print(i)
print(total_price)