Naming Variables: Rules and Best Practices
Keywords are reserved words used by Python to define program structure and cannot be used as variable names.
Try it: Assignment, Statement by Statement
How an assignment statement evaluates its right side first (using the names' current values) and only then binds the name on the left: updating x = x + 1, augmented operators like += and //=, chained a = b = 0, simultaneous a, b = b, a + b versus one-at-a-time assignment, rebinding a name to a value of another type, the NameError for a name that was never assigned, and the SyntaxError for a name that breaks the naming rules.
How it works
- Before anything runs, Python compiles the whole program; a name that breaks the rules (starts with a digit, contains a space or a dash, or is a keyword) is a SyntaxError and no line runs at all.
- For each assignment, evaluate the complete right side first, looking every name up in the variable table as it is now.
- Then bind the name(s) on the left to that value: a new name is added to the table, an existing one is replaced (even by a value of a different type).
- x += 5 reads x, applies the operator (+, -, *, //, %, **) and binds the result back to x; a = b = 0 evaluates 0 once and binds a, then b.
- a, b = b, a + b builds the whole right-hand tuple from the OLD values before binding a and b; two separate statements would use the new a in the second line.
- Reading a name that is not in the table raises NameError and the program stops at that line.
Default run (10 steps): A 4-line program. Python first compiles all of it, then runs it top to bottom. The variable table on the right starts empty. … Program finished. Final table: x = 16.
Simplified: The programs are fixed templates: you choose the template, its numbers, the operator and the variable name, never the code. The lab's own small Python tokenizer and parser (written for the statements these programs use: assignments, print calls, if / elif / else, for, while, def and return) checks the text; nothing is executed as real Python. Every program the lab can show was compiled by CPython 3.12 and the lab reproduces its exception type, message, line, caret columns and the printed report. A small interpreter written for the same statements then runs them (integers of any size, the str/float results these templates produce, and print); each program was also run in CPython 3.12 under sys.settrace, and the lab reproduces the variables after every line, the output and the error.
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A Name Has a Role
When you name a variable, you are choosing a user-defined identifier: a name for a value in your program. Python also has its own reserved words. These keywords are used to define program structure, so they are not available for variable naming. The central question is therefore not only whether a name looks readable, but also whether Python has already assigned that word a special structural role.
How Python Interprets Names
Python must interpret each name according to the role it plays in a statement. A user-defined variable name is available for the programmer to choose, while a reserved keyword already belongs to Python's program-structure vocabulary. If the same word is placed where a variable name is expected, Python cannot treat it as an ordinary user-defined name. The statement is therefore syntactically invalid and produces a SyntaxError.
Reserved keyword: a word reserved by Python to define program structure. It cannot be used as a variable name.
The Reserved Set
Python has 35 specific reserved keywords that are off-limits for variable names. The important recognition skill is to separate this fixed reserved set from names that a programmer is allowed to define. A word may look short, familiar, or readable and still be unavailable if Python reserves it for program structure.
| Name category | Python's treatment | Can it be a variable name? |
|---|---|---|
| User-defined identifier | Chosen by the programmer | Yes, when it follows the naming rules |
| Reserved keyword | Used to define program structure | No |
The key distinction is whether Python has already reserved the word for program structure.
Shape Rules for Names
A variable name can fail before keyword checking if its shape violates Python's naming rules. Variable names cannot start with a number. They also cannot contain most special characters, including characters such as @ or $. These restrictions are separate from the reserved-keyword rule: a name can be invalid because of its characters, or because the entire word is reserved for Python's program structure.
Classifying Three Candidate Names
Decide why each generated candidate should not be used as a Python variable name.
- 1Candidate A
A candidate that begins with a number violates the rule that variable names cannot start with a number.
- 2Candidate B
A candidate containing @ violates the restriction on most special characters.
- 3Candidate C
A candidate that is one of Python's reserved keywords cannot be used as a variable name because the word already defines program structure.
The three candidates fail for two different kinds of reasons: invalid name shape and reserved-word conflict.
Mistakes That Cause SyntaxError
Common mistake Using a reserved keyword as a variable name
What it looks likeGenerated example: choosing a word from Python's reserved program-structure vocabulary as the name on the left side of an assignment.
Why it failsPython already uses that word for program structure, so it cannot also be interpreted as an ordinary user-defined variable name.
Do this insteadChoose a different user-defined identifier that is not in the reserved set.
Common mistake Starting a variable name with a number
What it looks likeGenerated example: a candidate whose first character is a digit.
Why it failsVariable names cannot start with a number.
Do this insteadBegin the name with an allowed non-numeric character instead.
Common mistake Putting @ or $ inside a variable name
What it looks likeGenerated example: a candidate containing @ or $.
Why it failsVariable names cannot contain most special characters, including these examples.
Do this insteadRemove the restricted special character and choose a name that follows the naming rules.
When Python reports a SyntaxError near a proposed variable name, inspect the name in two passes. First, check its shape: does it begin with a number, or does it contain a character such as @ or $? Second, check its role: is the complete word one of Python's 35 reserved keywords? This process distinguishes a malformed name from a name that conflicts with Python's program structure.
A Naming Decision Routine
- 1Start with the complete candidate name rather than judging only part of it.
- 2Check that the name does not start with a number.
- 3Check that it does not contain restricted special characters such as @ or $.
- 4Check whether the complete word is one of Python's 35 reserved keywords.
- 5If any check fails, select another user-defined identifier before continuing.
For each of the following generated descriptions, identify the rule that must be checked first: a name beginning with a digit, a name containing $, a word used by Python for program structure, and a name that passes those three checks. Explain whether the issue is name shape, reserved-word status, or no issue identified by the rules in this article.
Hints
- A leading digit is a shape problem.
- The dollar sign is a special-character problem.
- A word used for program structure must be checked against the reserved set.
- Passing these checks does not require inventing additional restrictions not covered here.
What to Remember
- 1User-defined identifiers are names chosen for variables, while reserved keywords are reserved by Python for program structure.
- 2Python has 35 reserved keywords, and none of them can be used as variable names.
- 3Using a reserved keyword where Python expects a variable name creates a syntax conflict and causes a
SyntaxError. - 4Variable names cannot start with a number or contain most special characters, including @ and $.
- 5When diagnosing an invalid name, check both its character pattern and its reserved-word status.
At a glance
- A variable name must be available for programmer definition and must not be a reserved Python keyword.
- The reserved keyword set contains 35 words used to define program structure.
- A reserved keyword used as a variable name creates a
SyntaxErrorbecause Python cannot assign it both roles. - Names beginning with numbers or containing characters such as @ or $ violate variable-naming rules.