How to Implement Sparse Fieldsets in Python

from dataclasses import dataclass, field
from typing import Dict, List, Optional


@dataclass
class MockResponse:
    data: Dict[str, object] = field(default_factory=dict)
    included: List[Dict[str, object]] = field(default_factory=list)


def select_fields(
    data: Dict[str, object],
    sparse_fields: Optional[Dict[str, List[str]]] = None
) -> MockResponse:
    """
    Mock sparse fieldset selection from a full data object.
    Filters fields per resource type based on requested fields.
    """
    if not sparse_fields:
        return MockResponse(data=data)

    selected_data = {}
    included_records = []

    # Process primary data
    resource_type = data.get("type", "")
    requested_fields = sparse_fields.get(resource_type)

    if requested_fields is not None:
        selected_data = {
            field_name: data[field_name]
            for field_name in data
            if field_name in requested_fields or field_name == "id"
        }
    else:
        selected_data = dict(data)

    # Process related included records
    for related in data.get("relationships", {}).values():
        link_data = related.get("data")
        if isinstance(link_data, list):
            for record in link_data:
                record_type = record["type"]
                record_fields = sparse_fields.get(record_type)
                if record_fields is not None:
                    filtered = {
                        k: v for k, v in record.items()
                        if k in record_fields or k == "id"
                    }
                    included_records.append(filtered)
                else:
                    included_records.append(record)

    return MockResponse(data=selected_data, included=included_records)


if __name__ == "__main__":
    full_data = {
        "type": "articles",
        "id": "1",
        "title": "JSON:API paints my bikeshed!",
        "body": "Shortest article ever.",
        "author": {"data": {"type": "people", "id": "42"}}
    }

    sparse = {"articles": ["title"], "people": ["name"]}

    result = select_fields(full_data, sparse)
    print("Selected data:", result.data)
    print("Included records:", result.included)