How to Implement a Data Helper Class in Python

from __future__ import annotations
from dataclasses import dataclass, field
from typing import Any, Dict, List, Optional


@dataclass
class DataHelper:
    """A beginner-friendly data utility with common system design patterns."""
    data: List[Dict[str, Any]] = field(default_factory=list)

    def add_record(self, record: Dict[str, Any]) -> None:
        """Add a single record using the Command pattern."""
        self.data.append(record)

    def filter_by(self, key: str, value: Any) -> List[Dict[str, Any]]:
        """Filter records using a simple Strategy pattern."""
        return [r for r in self.data if r.get(key) == value]

    def transform(self, key: str, func) -> List[Any]:
        """Apply a transformation to a key across all records (Map pattern)."""
        return [func(r[key]) for r in self.data if key in r]

    def summarize(self, key: str, agg: str = "sum") -> Optional[Any]:
        """Aggregate a numeric key using different strategies."""
        values = [r[key] for r in self.data if key in r]
        if not values:
            return None
        if agg == "sum":
            return sum(values)
        if agg == "avg":
            return sum(values) / len(values)
        if agg == "max":
            return max(values)
        return None

    def __len__(self) -> int:
        """Support len() for counting records."""
        return len(self.data)


if __name__ == "__main__":
    helper = DataHelper()
    helper.add_record({"name": "Alice", "score": 90})
    helper.add_record({"name": "Bob", "score": 75})
    helper.add_record({"name": "Alice", "score": 85})

    print("All records:", helper.data)
    print("Filter by Alice:", helper.filter_by("name", "Alice"))
    print("Scores doubled:", helper.transform("score", lambda x: x * 2))
    print("Sum of scores:", helper.summarize("score", "sum"))
    print("Average score:", helper.summarize("score", "avg"))
    print("Number of records:", len(helper))