BaseStreamEvent Class — langchain Architecture

Source Code

libs/core/langchain_core/runnables/schema.py lines 56–161

class BaseStreamEvent(TypedDict):
    """Streaming event.

    Schema of a streaming event which is produced from the `astream_events` method.

    Example:
        ```python
        from langchain_core.runnables import RunnableLambda


        async def reverse(s: str) -> str:
            return s[::-1]


        chain = RunnableLambda(func=reverse)

        events = [event async for event in chain.astream_events("hello")]

        # Will produce the following events
        # (where some fields have been omitted for brevity):
        [
            {
                "data": {"input": "hello"},
                "event": "on_chain_start",
                "metadata": {},
                "name": "reverse",
                "tags": [],
            },
            {
                "data": {"chunk": "olleh"},
                "event": "on_chain_stream",
                "metadata": {},
                "name": "reverse",
                "tags": [],
            },
            {
                "data": {"output": "olleh"},
                "event": "on_chain_end",
                "metadata": {},
                "name": "reverse",
                "tags": [],
            },
        ]
        ```
    """

    event: str
    """Event names are of the format: `on_[runnable_type]_(start|stream|end)`.

    Runnable types are one of:

    - **llm** - used by non chat models
    - **chat_model** - used by chat models
    - **prompt** --  e.g., `ChatPromptTemplate`
    - **tool** -- from tools defined via `@tool` decorator or inheriting
        from `Tool`/`BaseTool`
    - **chain** - most `Runnable` objects are of this type

    Further, the events are categorized as one of:

    - **start** - when the `Runnable` starts
    - **stream** - when the `Runnable` is streaming
    - **end* - when the `Runnable` ends

    start, stream and end are associated with slightly different `data` payload.

    Please see the documentation for `EventData` for more details.
    """
    run_id: str
    """An randomly generated ID to keep track of the execution of the given `Runnable`.

    Each child `Runnable` that gets invoked as part of the execution of a parent
    `Runnable` is assigned its own unique ID.
    """
    tags: NotRequired[list[str]]
    """Tags associated with the `Runnable` that generated this event.

    Tags are always inherited from parent `Runnable` objects.

    Tags can either be bound to a `Runnable` using `.with_config({"tags":  ["hello"]})`
    or passed at run time using `.astream_events(..., {"tags": ["hello"]})`.