29. self and module-level state

Any reference to a state variable must be prefixed with `self.` — this makes it visually unmistakable whether a line touches persistent storage (expensive, permanent) or a local/memory variable (cheap, temporary).

total: public(uint256)

@external
def add(amount: uint256):
    local_copy: uint256 = self.total   # read storage into memory
    local_copy += amount
    self.total = local_copy            # write back to storage

This explicitness is one of Vyper's clearest departures from Solidity, where storage vs memory can sometimes be ambiguous at a glance (especially with struct/array references).

Check your understanding