_first_node() — langchain Function Reference

Architecture documentation for the _first_node() function in graph.py from the langchain codebase.

Function python LangChainCore Runnables called by 2

Entity Profile

LangChainCore→ Runnables→ _first_node() — langchain Function Reference

Dependency Diagram

graph TD
  c2fe47ba_c1ff_8826_55de_cd87be044850["_first_node()"]
  81a6280b_4752_84db_f1fa_1a3b8040d481["graph.py"]
  c2fe47ba_c1ff_8826_55de_cd87be044850 -->|defined in| 81a6280b_4752_84db_f1fa_1a3b8040d481
  202074c8_de09_73dd_415c_edea3bae12f1["first_node()"]
  202074c8_de09_73dd_415c_edea3bae12f1 -->|calls| c2fe47ba_c1ff_8826_55de_cd87be044850
  600fa640_4465_baee_3ef3_0c1fda5bf94b["trim_first_node()"]
  600fa640_4465_baee_3ef3_0c1fda5bf94b -->|calls| c2fe47ba_c1ff_8826_55de_cd87be044850
  style c2fe47ba_c1ff_8826_55de_cd87be044850 fill:#6366f1,stroke:#818cf8,color:#fff

Relationship Graph

Source Code

libs/core/langchain_core/runnables/graph.py lines 706–721

def _first_node(graph: Graph, exclude: Sequence[str] = ()) -> Node | None:
    """Find the single node that is not a target of any edge.

    Exclude nodes/sources with IDs in the exclude list.

    If there is no such node, or there are multiple, return `None`.

    When drawing the graph, this node would be the origin.
    """
    targets = {edge.target for edge in graph.edges if edge.source not in exclude}
    found: list[Node] = [
        node
        for node in graph.nodes.values()
        if node.id not in exclude and node.id not in targets
    ]
    return found[0] if len(found) == 1 else None