20. Events and log

Events are declared with the `event` keyword and emitted with `log`. Indexed fields let off-chain listeners filter efficiently by topic, exactly as in Solidity.

event Deposited:
    sender: indexed(address)
    amount: uint256

@external
@payable
def deposit():
    self.balances[msg.sender] += msg.value
    log Deposited(sender=msg.sender, amount=msg.value)

Events cost far less gas than storage and are the standard way for a front-end or indexer to reconstruct contract history without re-reading the whole state.

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