Unused Variable

Property	Value
Rule Code	NTX8
Severity	Warning

Background

Unused variables can indicate mistakes in the code (forgotten logic) or unnecessary code that increases complexity and gas costs.

Purpose

Detects variables that are declared but never used within a function.

Example

@external
def process_data(_value: uint256) -> uint256:
    # Non-compliant - variable declared but never used
    temp: uint256 = _value * 2

    return _value

@external
def process_data_fixed(_value: uint256) -> uint256:
    # Compliant - all declared variables are used
    temp: uint256 = _value * 2

    return temp

The process_data function would be flagged by this rule because the temp variable is declared but never used.

Intentionally Unused Variables in For Loops

When you need to iterate a specific number of times but don't actually use the loop variable, you can use the underscore (_) convention to indicate that the variable is intentionally unused. This prevents the linter from reporting it as an unused variable.

@external
def repeat_operation(count: uint256):
    # Non-compliant - 'i' is declared but never used
    for i: uint256 in range(count):
        # Some operation that doesn't use 'i'
        pass

@external
def repeat_operation_fixed(count: uint256):
    # Compliant - using '_' indicates intentionally unused variable
    for _: uint256 in range(count):
        # Some operation that doesn't use the loop variable
        pass

By naming the loop variable _, you signal to both the linter and other developers that the variable is intentionally unused, preventing false positive warnings.