Filtering data with a Python class helper

class DataFilter:
    """A beginner-friendly helper to filter lists of dictionaries."""
    
    def __init__(self, data):
        self.data = data
    
    def filter_by(self, key, value):
        """Return items where data[key] == value."""
        return [item for item in self.data if item.get(key) == value]
    
    def filter_gt(self, key, threshold):
        """Return items where data[key] > threshold."""
        return [item for item in self.data if item.get(key, 0) > threshold]
    
    def filter_contains(self, key, substring):
        """Return items where substring is in str(data[key])."""
        return [item for item in self.data 
                if substring.lower() in str(item.get(key, "")).lower()]


if __name__ == "__main__":
    people = [
        {"name": "Alice", "age": 30, "city": "New York"},
        {"name": "Bob", "age": 25, "city": "Boston"},
        {"name": "Charlie", "age": 35, "city": "New York"},
        {"name": "Diana", "age": 28, "city": "Chicago"}
    ]
    
    helper = DataFilter(people)
    print(helper.filter_by("city", "New York"))
    print(helper.filter_gt("age", 27))
    print(helper.filter_contains("name", "li"))