Notes on TDD - chapter 3

• Related pages
  • Test driven development

We can use objects as values (similar to how we are using Dollar). This is called *Value Object*. The values of the instance variables of the object *never* changes once they have been set my the constructor. This is exactly the case with our current =Dollar implementation, where once we set the amount, it will stay as is. If we call times, it will return a new Value Object.

Using Value Object prevents us from worrying about aliasing problems. Aliasing is when changing the first value of an object inadvertently changes the value of another object.

In computing, aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in the program.

When dealing with Value Object (and to prevent aliasing), all operations must returns a new object. Another implication, is that Value Objects need to implement equals(). Say if we use Dollar as the key for a dictionary/hash table, then we need to implement the hash code function if we implement equals().

How can we test equality with our Dollar object?

class DollarV3:
    amount: int

    def __init__(self, amount: int):
        self.amount = amount

    def times(self, multipler: int) -> "DollarV3":
        return DollarV3(self.amount * multipler)

    def __eq__(self, value: "DollarV3") -> bool:
        return True

def test_equality():
    a = DollarV3(5)
    b = DollarV3(5)
    assert a == b

Running test_equality
test_equality passed!

Now the bar is green again, but we hardcoded True in our __eq__ method.

DONE Triangulation #

Triangualation is a weird concept that took me some minutes to understand the reason behind it. The idea behind is simple: we only generalize the code when we have two examples or more (i.e. assertions). When we then demand a more general solution, then we generalize.

def test_equality():
    assert DollarV3(5) == DollarV3(5)
    assert DollarV3(5) != DollarV3(6)

run_tests()

Our last assertion failed because we’ve hardcoded True as the return type. The second assertion is our triangulation. At this point, we can move forward with generalization.

class DollarV4(DollarV3):
    def __eq__(self, value: "DollarV3") -> bool:
        return self.amount == value.amount

def test_equality():
    a = DollarV4(5)
    b = DollarV4(6)
    assert DollarV4(5) == DollarV4(5)
    assert DollarV4(5) != DollarV4(6)

run_tests()

And voila!

In practice, what we are actually doing is using two assertions to drive the generalization of the code. Triangulation provides us a way to think about the problem from a different direction.

References #

1. Aliasing
2. Value Object