Programming Testing: Isolation

This page is about writing code in a way that it can be tested in isolation (without collaborators).

When designing for testing, you want to flatten out your call-stack.
Each method ideally can be used at the toplevel, with a fake, avoiding functions that call functions that call functions etc.
Most of the time, this is achieved with interfaces.

Overrides

If possible in your language, you can override methods with a fake, or a mock.
In some strongly typed languages like java, you can subclass your object and override even private methods.

Separating Produce/Modify

Separate creating/fetching info from mutating it.
This lets you test your mutations independently of collaborators with a variety of inputs.

func GetHtml() string {
    // ...
}

func ModifyHtml(html string) string {
    // ...
}

Within parent callers, you can override these methods.

Abstract Side-Effects

The easiest way to do this is probably dependency inversion.
Bind a runner, executor, or library-interface you can sub in fakes for.