> ## Documentation Index
> Fetch the complete documentation index at: https://docs.coreweave.com/llms.txt
> Use this file to discover all available pages before exploring further.

# StreamWriter

> Sync and async writer for streaming input to a process.

Source: [src/cwsandbox/\_types.py:384](https://github.com/coreweave/cwsandbox-client/blob/v0.22.0/src/cwsandbox/_types.py#L384)

```python theme={"system"}
class StreamWriter(queue: asyncio.Queue[bytes | None], loop_manager: _LoopManager)
```

Sync and async writer for streaming input to a process.

StreamWriter wraps a bounded `asyncio.Queue` and provides both synchronous and
asynchronous write interfaces, so you can send streaming input in
both sync and async contexts.

The stream uses `None` as a sentinel value to signal end-of-stream (EOF).
The queue is bounded (\~16 items for \~1MB with 64KB chunks) to provide
backpressure.

**Attributes**

* `QUEUE_SIZE` : Default: `16`.

## Properties

### closed

```python theme={"system"}
@property
def closed(self) -> bool
```

True if `close()` has been called.

## Methods

### write

```python theme={"system"}
write(data: bytes) -> OperationRef[None]
```

Write raw bytes to the stream.

Queues the data for sending to the process stdin. Blocks (via `OperationRef.result()`)
if the queue is full, providing backpressure.

**Parameters**

* `data` (`bytes`): The bytes to write.

**Returns**

* `OperationRef[None]`: An `OperationRef` that completes when the data is queued.

**Raises**

* `SandboxExecutionError`: If the stream is closed or has failed.

### writeline

```python theme={"system"}
writeline(text: str, encoding: str = 'utf-8') -> OperationRef[None]
```

Write a line of text to the stream.

Encodes the text, appends a newline, and queues it for sending.

**Parameters**

* `text` (`str`): The text to write.
* `encoding` (`str`): The text encoding to use. Defaults to "utf-8".

**Returns**

* `OperationRef[None]`: An `OperationRef` that completes when the data is queued.

**Raises**

* `SandboxExecutionError`: If the stream is closed or has failed.

### close

```python theme={"system"}
close() -> OperationRef[None]
```

Close the stream, sending EOF sentinel.

The EOF sentinel is queued at the end, so pending writes complete first.
Multiple calls to `close()` are idempotent and return immediately.

**Returns**

* `OperationRef[None]`: An `OperationRef` that completes when EOF is queued.

### set\_exception

```python theme={"system"}
set_exception(exception: BaseException) -> None
```

Store an exception to raise on subsequent writes.

Called internally when the stream fails (for example, when the process exits).

**Parameters**

* `exception` (`BaseException`): The exception to store.