Graph nodes and edges in LangChain - Step-by-Step Execution

Concept Flow - Graph nodes and edges

Create Graph Object

↓

Add Node

→Add Node

↓

Add Edge

→Add Edge

↓

Graph Ready for Use

Start by creating a graph, add nodes one by one, then connect nodes with edges to build relationships.

Execution Sample

LangChain

graph = Graph()
graph.add_node('A')
graph.add_node('B')
graph.add_edge('A', 'B')

This code creates a graph, adds two nodes 'A' and 'B', then connects them with an edge.

Execution Table

Step	Operation	Nodes in Graph	Edges in Graph	Visual State
1	Create Graph	{}	{}	Graph initialized empty
2	Add node 'A'	{'A'}	{}	Nodes: A; Edges: none
3	Add node 'B'	{'A', 'B'}	{}	Nodes: A, B; Edges: none
4	Add edge from 'A' to 'B'	{'A', 'B'}	{('A', 'B')}	Nodes: A, B; Edges: A→B
5	End	{'A', 'B'}	{('A', 'B')}	Graph ready with 2 nodes and 1 edge

💡 All nodes and edges added, graph construction complete

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
graph.nodes	{}	{'A'}	{'A', 'B'}	{'A', 'B'}	{'A', 'B'}
graph.edges	{}	{}	{}	{('A', 'B')}	{('A', 'B')}

Key Moments - 2 Insights

Why do we add nodes before edges?

Edges connect existing nodes, so nodes must exist first as shown in steps 2 and 3 before step 4 adds an edge.

What happens if we add an edge between nodes not in the graph?

The graph will not recognize the edge because nodes must be added first; see execution_table step 4 where nodes exist before edge.

Visual Quiz - 3 Questions

Look at the execution table, how many nodes are in the graph after step 3?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'Nodes in Graph' column at step 3 in the execution_table

At which step is the first edge added to the graph?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Operation' and 'Edges in Graph' columns in the execution_table

If we add a node 'C' after step 4, how would the 'Nodes in Graph' change?

A {'A', 'B'}

B {'A', 'B', 'C'}

C {'C'}

D {}

💡 Hint

Refer to variable_tracker for how nodes are added step by step

Concept Snapshot

Graph basics:
- Create a graph object
- Add nodes first
- Then add edges connecting nodes
- Nodes store points, edges store connections
- Graph is ready when nodes and edges are set

Full Transcript

This visual trace shows how to build a graph in Langchain by creating a graph object, adding nodes one by one, and then connecting them with edges. Each step updates the graph's nodes and edges collections. Nodes must exist before edges connect them. The execution table tracks these changes step-by-step, and the variable tracker shows how the graph's internal state evolves. This helps beginners see the order and structure needed to build graphs correctly.