> For the complete documentation index, see [llms.txt](https://docs.seismic.systems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.seismic.systems/clients/python/src20/event-watching/src20-event-watcher.md).

# SRC20EventWatcher

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

Typically created via [`watch_src20_events()`](/clients/python/src20/event-watching/watch-src20-events.md) or [`watch_src20_events_with_key()`](/clients/python/src20/intelligence-providers/watch-src20-events-with-key.md) rather than instantiated directly.

## Constructor

```python
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`](/clients/python/api-reference/types/bytes32.md) |            | 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

```python
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](/clients/python/src20/event-watching/async-src20-event-watcher.md) — Async variant
* [watch\_src20\_events](/clients/python/src20/event-watching/watch-src20-events.md) — Factory with auto key fetch
* [watch\_src20\_events\_with\_key](/clients/python/src20/intelligence-providers/watch-src20-events-with-key.md) — Factory with explicit key
* [DecryptedTransferLog](/clients/python/src20/types/decrypted-transfer-log.md) — Transfer callback data
* [DecryptedApprovalLog](/clients/python/src20/types/decrypted-approval-log.md) — Approval callback data


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