# What Is Graph Data Structure in Python?

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Scott Campbell

A graph is a non-linear data structure that is used to represent relationships between different objects. It consists of a set of vertices (also called nodes) and a set of edges that connect these vertices. In Python, graphs can be implemented using various data structures such as dictionaries or lists.

## Vertices and Edges

In a graph, each vertex represents an object, and each edge represents the relationship between two objects. For example, consider a social network where each person is represented by a vertex, and the friendship between two people is represented by an edge connecting their respective vertices.

### Different Types of Graphs

There are several types of graphs based on their properties:

• Undirected Graph: In this type of graph, the edges do not have any direction. The relationship between two vertices is symmetric.
• Directed Graph: In this type of graph, the edges have a direction.

The relationship between two vertices may not be symmetric.

• Weighted Graph: In this type of graph, each edge has an associated weight or cost. It is used to represent the strength or distance between two vertices.

### Graph Representation

In Python, there are different ways to represent a graph. One commonly used approach is using a dictionary where the keys represent the vertices, and the values represent the edges connected to each vertex.

To illustrate this, let’s create an undirected graph using a dictionary:

``````
graph = {
'A': ['B', 'C'],
'B': ['A', 'C', 'D'],
'C': ['A', 'B'],
'D': ['B']
}
``````

In this example, the keys ‘A’, ‘B’, ‘C’, and ‘D’ represent the vertices, and the corresponding values represent the edges connected to each vertex. For instance, the vertex ‘A’ is connected to vertices ‘B’ and ‘C’.

### Graph Traversal

Graph traversal is the process of visiting all the vertices in a graph. There are two commonly used methods for graph traversal:

• Breadth-First Search (BFS): It explores all the vertices at the same level before moving to the next level.
• Depth-First Search (DFS): It explores as far as possible along each branch before backtracking.

Both BFS and DFS can be implemented using various algorithms, such as recursion or iteration with the help of a stack or queue.

## Conclusion

The graph data structure is a powerful tool for representing relationships between objects. It allows us to model complex networks and solve various problems efficiently. Python provides several ways to implement graphs, and understanding their properties and traversal methods is essential for working with graphs effectively.