Reserved Keywords

A reserved keyword is a word that Python keeps for its own syntax.

Programming

Why Names Need Rules

When you write score = 42, score is an identifier: a name given to a variable or another Python entity. The name labels the value 42 so that you can retrieve it, modify it, or use it in calculations. Python needs strict rules for names so the interpreter can reliably parse your code and distinguish one name from another.

A reserved keyword is a word that Python keeps for its own syntax. Names such as class, for, and if are not ordinary names available for variables because Python uses them for language constructs. An identifier must also obey character and capitalization rules. Therefore, choosing a variable name involves two checks: the spelling must have a valid identifier form, and the name must not be reserved by Python.

Checking an Identifier

Python's identifier rules can be checked from left to right. The first character must be a letter or an underscore. After that first character, the name may contain letters, underscores, or digits. Spaces and hyphens are not allowed. A digit may therefore appear later in a name, but not at the beginning.

Which identifier names are accepted by Python, and which are rejected because they start with a digit, contain invalid characters, or use spaces?

Worked example

Classifying identifier names

Decide whether _count, 3_count, count_3, count-3, count 3, and __init__ follow Python's identifier rules.

1
_count
Valid: an underscore is allowed as the first character, and the remaining characters are allowed.
2
3_count
Invalid: the name starts with a digit. Digits can appear after the first character, not before it.
3
count_3
Valid: the name starts with a letter, and the digit appears later.
4
count-3
Invalid: a hyphen is not one of the allowed identifier characters.
5
count 3
Invalid: a space is not allowed inside an identifier.
6
__init__
Valid: it starts with an underscore and uses only letters and underscores afterward.

Result

The valid names are _count, count_3, and __init__. The invalid names are 3_count, count-3, and count 3.

Reading Names Exactly

Python identifiers are case-sensitive. Uppercase and lowercase letters are treated as different characters, so myname and myName are completely different identifiers. The same distinction applies to names such as count, Count, and COUNT. They may look related to a human reader, but Python treats each spelling as a separate name.

How does Python treat count, Count, and COUNT? Each capitalization pattern is a different identifier.

Worked example

Following capitalization precisely

A name is created with the spelling myname. What happens when the spelling myName is used later?

1
Compare the spellings
myname uses a lowercase n, while myName uses an uppercase N.
2
Apply case-sensitivity
Python treats these spellings as different identifiers rather than as two ways to write the same name.
3
Interpret the later reference
The name myName has not been created merely because myname exists. Python reports that myName is not defined.

Result

Capitalization must match exactly whenever an identifier is used.

Reserved Words and Naming Choices

Definition

Reserved keywords are words Python reserves for its syntax instead of allowing them to be used as ordinary variable names. The source examples include class, for, and if. These words must retain their language meaning, so they cannot be chosen as variable identifiers.

This restriction is separate from the character rules. A name can have the right shape and still be unavailable because Python has reserved it. Conversely, a name that is not reserved can still be invalid if it begins with a digit or contains a space or hyphen.

Name choice	Identifier character rules	Reserved-word issue	Result
score	Follows the character rules	Not identified as reserved in the source examples	Valid form
class	Has an identifier-like character form	Reserved for Python syntax	Cannot be used as an ordinary variable name
3_count	Starts with a digit	Reserved-word issue is not the problem	Invalid form
count-3	Contains a hyphen	Reserved-word issue is not the problem	Invalid form

A name must satisfy both the character rules and the restriction on reserved keywords.

Mistakes Beginners Make

Common mistake
Starting an identifier with a digit
What it looks like
```
3_count
```
Why it fails
The first character must be a letter or underscore. Python uses this restriction to distinguish identifiers from numbers.
Do this instead
Rewrite the name so that it begins with a letter or underscore, such as count_3.
Common mistake
Using spaces between words
What it looks like
count 3
Why it fails
Spaces are not allowed inside an identifier.
Do this instead
Use underscores to separate words, such as count_3.
Common mistake
Using a hyphen between words
What it looks like
```
count-3
```
Why it fails
Hyphens are not allowed identifier characters.
Do this instead
Replace the hyphen with an underscore, such as count_3.
Common mistake
Changing capitalization accidentally
What it looks like
myname followed by myName
Why it fails
Python is case-sensitive, so these are different identifiers.
Do this instead
Use exactly the same capitalization each time the identifier is referenced.
Common mistake
Using a reserved keyword as a variable name
What it looks like
class, for, or if
Why it fails
These words are reserved for Python syntax rather than ordinary variable names.
Do this instead
Choose a different identifier that follows the character rules and is not reserved.

Before

3_count count-3 count 3 myname and myName

The first three names contain invalid characters or positions; the last pair differs in capitalization.

After

count_3 count_3 count_3 myname and myname

The rewritten names use an allowed structure, and the repeated name uses matching capitalization.

Best practice

Use snake_case for variable names: lowercase words separated by underscores, such as player_score. This convention is not enforced by Python, but it is the style experienced Python programmers expect to see. It helps readers distinguish separate words without using spaces or hyphens.

Name-Checking Practice

MEDIUM

Classify each name as valid or invalid, and give the rule that decides your answer: player_score, PlayerScore, 4players, _players, players-3, and COUNT.

Hints

Check the first character before checking the rest.
Look for spaces or hyphens.
Compare capitalization exactly.
Remember that an underscore may be the first character and a digit may appear later.

Your attempt (not saved)

1
To check a Python identifier, first verify that its first character is a letter or underscore. Then verify that every later character is a letter, underscore, or digit. Reject names containing spaces or hyphens, reject reserved keywords such as class, for, and if, and compare capitalization exactly because Python is case-sensitive.

Key Takeaways

An identifier is a name given to a variable or another Python entity.
A valid identifier starts with a letter or underscore and then uses only letters, underscores, or digits.
Spaces, hyphens, and leading digits make identifier names invalid.
Python is case-sensitive, so count, Count, and COUNT are different identifiers.
Reserved keywords such as class, for, and if cannot be used as ordinary variable names.

Related concepts

More concepts starting with R