Managing Recursive Models

Explore how to effectively define and work with recursive models in Pydantic, enabling sophisticated data structures like trees and nested objects.

By Kevin McAleer,    2 Minutes

Understanding Recursive Models

Recursive models allow for the definition of complex, nested data structures within Pydantic. These are particularly useful for representing hierarchical data, such as organizational charts, file systems, or any nested relationships.

Defining Recursive Models

To define a recursive model in Pydantic, a model may reference itself within its fields. Pydantic supports this natively, but it requires forward declaration for self-referencing models, using a string to indicate the model’s name before it’s fully defined.

from typing import List, Optional
from pydantic import BaseModel

class TreeNode(BaseModel):
    name: str
    children: Optional[List['TreeNode']] = None

In this example, TreeNode is a model representing a node in a tree. Each node can have multiple children, which are also instances of TreeNode, demonstrating the recursive nature of the model.

Working with Recursive Models

Recursive models are instantiated and used in the same way as any other Pydantic model. However, when working with deeply nested data, consider the complexity and potential performance implications.

tree = TreeNode(
    name="root",
    children=[
        TreeNode(name="child1"),
        TreeNode(name="child2", children=[TreeNode(name="grandchild1")]),
    ],
)

print(tree)

This code creates a simple tree structure with a root node, two child nodes, and one grandchild node, showcasing the recursive model in action.

Handling Circular References

Pydantic’s parse_obj method can be used to handle circular references or deeply nested structures by providing a dictionary representation of the data. This method ensures that instances are correctly created without exceeding Python’s recursion limit.

Lesson Assignment

Define a recursive model Category for representing a category tree where each category can have multiple subcategories. Each Category should have fields for id, name, and subcategories (optional, a list of Category).

Create an instance of your Category model representing a simple category structure with at least one subcategory and test its instantiation and serialization capabilities.

Additional Resources

• Pydantic Documentation on Recursive Models
• Python’s Type Hints for Recursive Type