24. Access control: the owner pattern

Vyper has no modifiers, so access control is expressed as an explicit `assert` at the top of each sensitive function — slightly more verbose than Solidity, but nothing is hidden behind a name you have to look up.

owner: public(address)

@deploy
def __init__():
    self.owner = msg.sender

@external
def set_rate(new_rate: uint256):
    assert msg.sender == self.owner, "not owner"
    self.rate = new_rate

For multi-role systems, a `HashMap[address, bool]` of admins (or a role-based `HashMap[address, HashMap[bytes32, bool]]`) generalises this pattern beyond a single owner.

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