langgraph/docs/docs/how-tos/command.ipynb

How to combine control flow and state updates with Command

!!! info "Prerequisites" This guide assumes familiarity with the following:

- [State](../../concepts/low_level#state)
- [Nodes](../../concepts/low_level#nodes)
- [Edges](../../concepts/low_level#edges)
- [Command](../../concepts/low_level#command)

It can be useful to combine control flow (edges) and state updates (nodes). For example, you might want to BOTH perform state updates AND decide which node to go to next in the SAME node. LangGraph provides a way to do so by returning a Command object from node functions:

def my_node(state: State) -> Command[Literal["my_other_node"]]:
    return Command(
        # state update
        update={"foo": "bar"},
        # control flow
        goto="my_other_node"
    )

If you are using subgraphs, you might want to navigate from a node within a subgraph to a different subgraph (i.e. a different node in the parent graph). To do so, you can specify graph=Command.PARENT in Command:

def my_node(state: State) -> Command[Literal["my_other_node"]]:
    return Command(
        update={"foo": "bar"},
        goto="other_subgraph",  # where `other_subgraph` is a node in the parent graph
        graph=Command.PARENT
    )

!!! important "State updates with Command.PARENT"

When you send updates from a subgraph node to a parent graph node for a key that's shared by both parent and subgraph [state schemas](../../concepts/low_level#schema), you **must** define a [reducer](../../concepts/low_level#reducers) for the key you're updating in the parent graph state. See this [example](#navigating-to-a-node-in-a-parent-graph) below.

This guide shows how you can do use Command to add dynamic control flow in your LangGraph app.

Setup

First, let's install the required packages

In [1]:

%%capture --no-stderr
%pip install -U langgraph

Set up LangSmith for LangGraph development

Sign up for LangSmith to quickly spot issues and improve the performance of your LangGraph projects. LangSmith lets you use trace data to debug, test, and monitor your LLM apps built with LangGraph — read more about how to get started here.

Let's create a simple graph with 3 nodes: A, B and C. We will first execute node A, and then decide whether to go to Node B or Node C next based on the output of node A.

Basic usage

In [2]:

import random
from typing_extensions import TypedDict, Literal

from langgraph.graph import StateGraph, START
from langgraph.types import Command


# Define graph state
class State(TypedDict):
    foo: str


# Define the nodes


def node_a(state: State) -> Command[Literal["node_b", "node_c"]]:
    print("Called A")
    value = random.choice(["a", "b"])
    # this is a replacement for a conditional edge function
    if value == "a":
        goto = "node_b"
    else:
        goto = "node_c"

    # note how Command allows you to BOTH update the graph state AND route to the next node
    return Command(
        # this is the state update
        update={"foo": value},
        # this is a replacement for an edge
        goto=goto,
    )


def node_b(state: State):
    print("Called B")
    return {"foo": state["foo"] + "b"}


def node_c(state: State):
    print("Called C")
    return {"foo": state["foo"] + "c"}

We can now create the StateGraph with the above nodes. Notice that the graph doesn't have conditional edges for routing! This is because control flow is defined with Command inside node_a.

In [3]:

builder = StateGraph(State)
builder.add_edge(START, "node_a")
builder.add_node(node_a)
builder.add_node(node_b)
builder.add_node(node_c)
# NOTE: there are no edges between nodes A, B and C!

graph = builder.compile()

!!! important

You might have noticed that we used `Command` as a return type annotation, e.g. `Command[Literal["node_b", "node_c"]]`. This is necessary for the graph rendering and tells LangGraph that `node_a` can navigate to `node_b` and `node_c`.

In [4]:

from IPython.display import display, Image

display(Image(graph.get_graph().draw_mermaid_png()))

If we run the graph multiple times, we'd see it take different paths (A -> B or A -> C) based on the random choice in node A.

In [5]:

graph.invoke({"foo": ""})

Out [5]:

Called A
Called C

{'foo': 'bc'}

Navigating to a node in a parent graph

Now let's demonstrate how you can navigate from inside a subgraph to a different node in a parent graph. We'll do so by changing node_a in the above example into a single-node graph that we'll add as a subgraph to our parent graph.

!!! important "State updates with Command.PARENT"

When you send updates from a subgraph node to a parent graph node for a key that's shared by both parent and subgraph [state schemas](../../concepts/low_level#schema), you **must** define a [reducer](../../concepts/low_level#reducers) for the key you're updating in the parent graph state.

In [6]:

import operator
from typing_extensions import Annotated


class State(TypedDict):
    # NOTE: we define a reducer here
    # highlight-next-line
    foo: Annotated[str, operator.add]


def node_a(state: State):
    print("Called A")
    value = random.choice(["a", "b"])
    # this is a replacement for a conditional edge function
    if value == "a":
        goto = "node_b"
    else:
        goto = "node_c"

    # note how Command allows you to BOTH update the graph state AND route to the next node
    return Command(
        update={"foo": value},
        goto=goto,
        # this tells LangGraph to navigate to node_b or node_c in the parent graph
        # NOTE: this will navigate to the closest parent graph relative to the subgraph
        # highlight-next-line
        graph=Command.PARENT,
    )


subgraph = StateGraph(State).add_node(node_a).add_edge(START, "node_a").compile()


def node_b(state: State):
    print("Called B")
    # NOTE: since we've defined a reducer, we don't need to manually append
    # new characters to existing 'foo' value. instead, reducer will append these
    # automatically (via operator.add)
    # highlight-next-line
    return {"foo": "b"}


def node_c(state: State):
    print("Called C")
    # highlight-next-line
    return {"foo": "c"}

In [7]:

builder = StateGraph(State)
builder.add_edge(START, "subgraph")
builder.add_node("subgraph", subgraph)
builder.add_node(node_b)
builder.add_node(node_c)

graph = builder.compile()

In [8]:

graph.invoke({"foo": ""})

Out [8]:

Called A
Called C

{'foo': 'bc'}