SRC20EventWatcher

Synchronous SRC20 event watcher class

Polling-based SRC20 event watcher that runs in a background daemon thread.

Typically created via watch_src20_events() or watch_src20_events_with_key() rather than instantiated directly.

Constructor

class SRC20EventWatcher:
    def __init__(
        self,
        w3: Web3,
        aes_key: Bytes32,
        *,
        token_address: ChecksumAddress | None = None,
        on_transfer: TransferCallback | None = None,
        on_approval: ApprovalCallback | None = None,
        on_error: ErrorCallback | None = None,
        poll_interval: float = 2.0,
        from_block: int | str = "latest",
    ) -> None

Parameters

Parameter	Type	Default	Description
w3	Web3		Web3 instance
aes_key	Bytes32		32-byte AES-256 viewing key
token_address	ChecksumAddress | None	None	SRC20 contract to filter (None = all tokens)
on_transfer	TransferCallback | None	None	Callback for Transfer events
on_approval	ApprovalCallback | None	None	Callback for Approval events
on_error	ErrorCallback | None	None	Callback for errors
poll_interval	float	2.0	Seconds between polls
from_block	int | "latest"	"latest"	Starting block

Methods

Method	Description
start()	Start the background polling thread
stop()	Signal stop and wait for thread exit
is_running	Property — True if thread is alive

Context Manager

with watch_src20_events_with_key(w3, viewing_key=key, on_transfer=cb) as watcher:
    time.sleep(60)
# Automatically stopped

Notes

• Runs as a daemon thread named "src20-watcher" — exits when main thread exits

• Block tracking is automatic; the watcher remembers the last processed block

• If on_error is not provided, errors are logged at DEBUG level

• Computes keccak256(aes_key) once at initialization for event filtering

See Also

• AsyncSRC20EventWatcher — Async variant

• watch_src20_events — Factory with auto key fetch

• watch_src20_events_with_key — Factory with explicit key

• DecryptedTransferLog — Transfer callback data

• DecryptedApprovalLog — Approval callback data