How to Mock DynamoDB with a Simple Dict Store in Python

import json
from typing import Any, Dict, Optional


class MockDynamoDB:
    def __init__(self) -> None:
        self._store: Dict[str, Dict[str, Any]] = {}

    def put_item(self, table_name: str, item: Dict[str, Any]) -> None:
        key = str(item.get("id"))
        if table_name not in self._store:
            self._store[table_name] = {}
        self._store[table_name][key] = item

    def get_item(self, table_name: str, key: str) -> Optional[Dict[str, Any]]:
        return self._store.get(table_name, {}).get(key)

    def query_by_value(self, table_name: str, field: str, value: Any) -> list:
        return [
            item for item in self._store.get(table_name, {}).values()
            if item.get(field) == value
        ]


if __name__ == "__main__":
    db = MockDynamoDB()

    db.put_item("users", {"id": 1, "name": "Alice", "age": 30})
    db.put_item("users", {"id": 2, "name": "Bob", "age": 25})
    db.put_item("users", {"id": 3, "name": "Alice", "age": 40})

    print(json.dumps(db.get_item("users", "1"), indent=2))
    print(json.dumps(db.query_by_value("users", "name", "Alice"), indent=2))