Cache Storage Variable

Property	Value
Rule Code	NTX7
Severity	Optimization

Experimental Rule

This rule is currently experimental and not enabled by default. It is available in the codebase for testing purposes but is not yet production-ready.

Background

Reading from storage is one of the most gas-expensive operations in Vyper (and Ethereum in general). Caching storage variables in memory when they are accessed multiple times can significantly reduce gas costs.

Purpose

Detects when a storage variable is accessed multiple times in a function and suggests caching it to save gas.

Example

# Storage variable
user_balance: public(uint256)

# Non-compliant - accessing storage variable multiple times
@external
def process_balance() -> uint256:
    if self.user_balance > 100:
        return self.user_balance - 100
    else:
        return self.user_balance

# Compliant - caching storage variable
@external
def process_balance_cached() -> uint256:
    balance_cache: uint256 = self.user_balance
    if balance_cache > 100:
        return balance_cache - 100
    else:
        return balance_cache

The process_balance function would be flagged by this rule because it accesses self.user_balance multiple times. The process_balance_cached function is compliant because it caches the storage variable.