ML Prep

Implement value equality for a mutable class

Problem

Implement Point(x, y) with public coordinates, a mutating translate(dx, dy) method, and value equality between Point instances. Equality with an unrelated type must not raise an exception. Then explain the result of point_report().

Reveal answer or reference solution

__eq__ compares both coordinates when the other object is a Point and returns NotImplemented otherwise. Two distinct points initially have equal values but different identities. Translating p also changes its alias, not q, so p != q afterward while p is alias remains true.

class Point:
    def __init__(self, x: int, y: int) -> None:
        self.x = x
        self.y = y
 
    def translate(self, dx: int, dy: int) -> None:
        self.x += dx
        self.y += dy
 
    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Point):
            return NotImplemented
        return self.x == other.x and self.y == other.y
 
def point_report() -> tuple[object, ...]:
    point = Point(1, 2)
    equal_point = Point(1, 2)
    alias = point
    before = (point == equal_point, point is equal_point, point == alias)
    point.translate(1, -1)
    after = (point == equal_point, point is alias)
    return before, after, (point.x, point.y), (equal_point.x, equal_point.y)

Public tests

  • point_report() → ((True, False, True), (False, True), (2, 1), (1, 2))
  • Point(0, 0) == Point(0, 0) → True
  • Point(0, 0) == (0, 0) → False
Outcome

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