> 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/async-src20-event-watcher.md).

# AsyncSRC20EventWatcher

Asynchronous SRC20 event watcher class

Polling-based SRC20 event watcher that runs as an `asyncio.Task`.

Typically created via [`async_watch_src20_events()`](/clients/python/src20/event-watching/watch-src20-events.md) or [`async_watch_src20_events_with_key()`](/clients/python/src20/intelligence-providers/watch-src20-events-with-key.md) rather than instantiated directly.

## Constructor

```python
class AsyncSRC20EventWatcher:
    def __init__(
        self,
        w3: AsyncWeb3,
        aes_key: Bytes32,
        *,
        token_address: ChecksumAddress | None = None,
        on_transfer: AsyncTransferCallback | TransferCallback | None = None,
        on_approval: AsyncApprovalCallback | ApprovalCallback | None = None,
        on_error: AsyncErrorCallback | ErrorCallback | None = None,
        poll_interval: float = 2.0,
        from_block: int | str = "latest",
    ) -> None
```

## Parameters

| Parameter       | Type                                                        | Default    | Description                                    |
| --------------- | ----------------------------------------------------------- | ---------- | ---------------------------------------------- |
| `w3`            | `AsyncWeb3`                                                 |            | Async 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`   | Callback                                                    | `None`     | Async or sync callback for Transfer events     |
| `on_approval`   | Callback                                                    | `None`     | Async or sync callback for Approval events     |
| `on_error`      | Callback                                                    | `None`     | Async or sync callback for errors              |
| `poll_interval` | `float`                                                     | `2.0`      | Seconds between polls                          |
| `from_block`    | `int \| "latest"`                                           | `"latest"` | Starting block                                 |

## Methods

| Method          | Description                          |
| --------------- | ------------------------------------ |
| `await start()` | Start the async polling task         |
| `await stop()`  | Cancel the task and wait for cleanup |
| `is_running`    | Property — `True` if task is active  |

## Async Context Manager

```python
async with AsyncSRC20EventWatcher(w3, key, on_transfer=cb) as watcher:
    await asyncio.sleep(60)
# Automatically stopped
```

## Notes

* Callbacks can be sync or async — the watcher detects and awaits coroutines automatically
* `CancelledError` is suppressed during shutdown
* If `on_error` is not provided, errors are logged at DEBUG level

## See Also

* [SRC20EventWatcher](/clients/python/src20/event-watching/src20-event-watcher.md) — Sync 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


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.seismic.systems/clients/python/src20/event-watching/async-src20-event-watcher.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.