Replace Derived Variable with Query

Replace Derived Variable with Query calculates a value from its authoritative inputs instead of storing it separately. The class no longer has to keep two representations of the same fact synchronized. Here query means a read operation, not necessarily a database request. 1

Original version: width and area must be updated together

A rectangle stores width, height, and their product. The original code updates the product correctly; the concern is the maintenance obligation, not an existing arithmetic defect.

public sealed class Rectangle
{
    private int width;
    private readonly int height;
    public int Area { get; private set; }
    public Rectangle(int width, int height)
    {
        this.width = width;
        this.height = height;
        Area = width * height;
    }
    public void SetWidth(int width)
    {
        this.width = width;
        Area = width * height;
    }
}

class Rectangle:
    def __init__(self, width, height):
        self._width = width
        self._height = height
        self.area = width * height

    def set_width(self, width):
        self._width = width
        self.area = width * self._height

Every operation that changes a dimension must also update area. Forgetting the second assignment in a future method would leave contradictory state.

Refactored version: derive area when asked

public sealed class Rectangle
{
    private int width;
    private readonly int height;
    public Rectangle(int width, int height)
    {
        this.width = width;
        this.height = height;
    }
    public int Area { get { return width * height; } }
    public void SetWidth(int width) { this.width = width; }
}

class Rectangle:
    def __init__(self, width, height):
        self._width = width
        self._height = height

    @property
    def area(self):
        return self._width * self._height

    def set_width(self, width):
        self._width = width

A three-by-four rectangle still reports twelve. After changing width to five, it reports twenty without an area-update statement. There is no stored area to become stale.

A cached value and a query make different promises

Multiplication is cheap, and the example uses small sequentially accessed integers. An expensive calculation may justify caching, but then cache invalidation is an explicit responsibility rather than an accidental duplicate field.

A saved historical result is also different from a derived current value. Replacing a recorded invoice total with a query against today’s prices would change meaning. Preserve snapshots when they represent what happened at a particular time.

The Python revision also removes ordinary assignment to the public area attribute. Callers that previously set area directly must migrate; preserving the shown dimension-based operations does not preserve every possible mutation of the old object.

The useful question is whether this value is an independent fact or merely an inexpensive consequence of facts already stored. Derive the second kind when doing so makes consistency easier to maintain.

Sources

1. Martin Fowler — Replace Derived Variable with Query.