RunnableBranch Class — langchain Architecture

Source Code

libs/core/langchain_core/runnables/branch.py lines 42–461

class RunnableBranch(RunnableSerializable[Input, Output]):
    """`Runnable` that selects which branch to run based on a condition.

    The `Runnable` is initialized with a list of `(condition, Runnable)` pairs and
    a default branch.

    When operating on an input, the first condition that evaluates to True is
    selected, and the corresponding `Runnable` is run on the input.

    If no condition evaluates to `True`, the default branch is run on the input.

    Examples:
        ```python
        from langchain_core.runnables import RunnableBranch

        branch = RunnableBranch(
            (lambda x: isinstance(x, str), lambda x: x.upper()),
            (lambda x: isinstance(x, int), lambda x: x + 1),
            (lambda x: isinstance(x, float), lambda x: x * 2),
            lambda x: "goodbye",
        )

        branch.invoke("hello")  # "HELLO"
        branch.invoke(None)  # "goodbye"
        ```
    """

    branches: Sequence[tuple[Runnable[Input, bool], Runnable[Input, Output]]]
    """A list of `(condition, Runnable)` pairs."""
    default: Runnable[Input, Output]
    """A `Runnable` to run if no condition is met."""

    def __init__(
        self,
        *branches: tuple[
            Runnable[Input, bool]
            | Callable[[Input], bool]
            | Callable[[Input], Awaitable[bool]],
            RunnableLike,
        ]
        | RunnableLike,
    ) -> None:
        """A `Runnable` that runs one of two branches based on a condition.

        Args:
            *branches: A list of `(condition, Runnable)` pairs.
                Defaults a `Runnable` to run if no condition is met.

        Raises:
            ValueError: If the number of branches is less than `2`.
            TypeError: If the default branch is not `Runnable`, `Callable` or `Mapping`.
            TypeError: If a branch is not a `tuple` or `list`.
            ValueError: If a branch is not of length `2`.
        """
        if len(branches) < _MIN_BRANCHES:
            msg = "RunnableBranch requires at least two branches"
            raise ValueError(msg)

        default = branches[-1]

        if not isinstance(
            default,
            (Runnable, Callable, Mapping),  # type: ignore[arg-type]
        ):
            msg = "RunnableBranch default must be Runnable, callable or mapping."
            raise TypeError(msg)

        default_ = cast(
            "Runnable[Input, Output]", coerce_to_runnable(cast("RunnableLike", default))
        )

        branches_ = []

        for branch in branches[:-1]:
            if not isinstance(branch, (tuple, list)):
                msg = (
                    f"RunnableBranch branches must be "
                    f"tuples or lists, not {type(branch)}"
                )
                raise TypeError(msg)