Cyclic references

As described in the cyclic references section, Pydantic detects cyclic references in validation inputs and raises a ValidationError with the recursion_loop error type, rather than a RecursionError. The same applies during serialization, where a ValueError is raised.

This example shows how these errors can be caught in validators and serializers to drop the cyclic references and still produce a result.

Because this ValidationError is raised without actually exceeding the maximum recursion depth, you can catch and handle it without needing to worry about the limited remaining recursion depth:

from __future__ import annotations

from collections.abc import Generator
from contextlib import contextmanager
from dataclasses import field

from pydantic import BaseModel, ValidationError, field_validator


def is_recursion_validation_error(exc: ValidationError) -> bool:
    errors = exc.errors()
    return len(errors) == 1 and errors[0]['type'] == 'recursion_loop'


@contextmanager
def suppress_recursion_validation_error() -> Generator[None]:
    try:
        yield
    except ValidationError as exc:
        if not is_recursion_validation_error(exc):
            raise exc


class Node(BaseModel):
    id: int
    children: list[Node] = field(default_factory=list)

    @field_validator('children', mode='wrap')
    @classmethod
    def drop_cyclic_references(cls, children, h):
        try:
            return h(children)
        except ValidationError as exc:
            if not (
                is_recursion_validation_error(exc)
                and isinstance(children, list)
            ):
                raise exc

            value_without_cyclic_refs = []
            for child in children:
                with suppress_recursion_validation_error():
                    value_without_cyclic_refs.extend(h([child]))
            return h(value_without_cyclic_refs)


# Create data with cyclic references representing the graph 1 -> 2 -> 3 -> 1
node_data = {'id': 1, 'children': [{'id': 2, 'children': [{'id': 3}]}]}
node_data['children'][0]['children'][0]['children'] = [node_data]

print(Node.model_validate(node_data))
#> id=1 children=[Node(id=2, children=[Node(id=3, children=[])])]

Similarly, if Pydantic encounters a recursive reference during serialization, rather than waiting for the maximum recursion depth to be exceeded, a ValueError is raised immediately:

from pydantic import TypeAdapter

# Create data with cyclic references representing the graph 1 -> 2 -> 3 -> 1
node_data = {'id': 1, 'children': [{'id': 2, 'children': [{'id': 3}]}]}
node_data['children'][0]['children'][0]['children'] = [node_data]

try:
    # Try serializing the circular reference as JSON
    TypeAdapter(dict).dump_json(node_data)
except ValueError as exc:
    print(exc)
    """
    Error serializing to JSON: ValueError: Circular reference detected (id repeated)
    """

This can also be handled if desired:

from dataclasses import field
from typing import Any

from pydantic import (
    SerializerFunctionWrapHandler,
    TypeAdapter,
    field_serializer,
)
from pydantic.dataclasses import dataclass


@dataclass
class NodeReference:
    id: int


@dataclass
class Node(NodeReference):
    children: list['Node'] = field(default_factory=list)

    @field_serializer('children', mode='wrap')
    def serialize(
        self, children: list['Node'], handler: SerializerFunctionWrapHandler
    ) -> Any:
        """
        Serialize a list of nodes, handling circular references by excluding the children.
        """
        try:
            return handler(children)
        except ValueError as exc:
            if not str(exc).startswith('Circular reference'):
                raise exc

            result = []
            for node in children:
                try:
                    serialized = handler([node])
                except ValueError as exc:
                    if not str(exc).startswith('Circular reference'):
                        raise exc
                    result.append({'id': node.id})
                else:
                    result.append(serialized)
            return result


# Create a cyclic graph:
nodes = [Node(id=1), Node(id=2), Node(id=3)]
nodes[0].children.append(nodes[1])
nodes[1].children.append(nodes[2])
nodes[2].children.append(nodes[0])

print(nodes[0])
#> Node(id=1, children=[Node(id=2, children=[Node(id=3, children=[...])])])

# Serialize the cyclic graph:
print(TypeAdapter(Node).dump_python(nodes[0]))
"""
{
    'id': 1,
    'children': [{'id': 2, 'children': [{'id': 3, 'children': [{'id': 1}]}]}],
}
"""