Writing unit tests for methods which have a degree of randomization

Question

I'm using PHPUnit and my traditional approach is to use mock objects and methods that get called by the methods I'm testing. The mock objects are told what to expect as input by the unit test. The problem is that part of the input supplied to the mock objects are being randomly generated by the methods being tested (and the unit test has no way of knowing what those are).

Just wondering if anyone could suggest a solution.

Answer 1

It's hard to tell without knowing your specific case, but maybe you could refactor to inject some random number provider into your object under test. During your test you could use a hardcoded random number seed to get reproducible results.

Answer 2

You don't have to use mocks that are strict about their inputs - only do that when it adds value to the tests.

In other scenarios, use stubs, which don't care about their input, or have some methods be strict and some loose as necessary.

Answer 3

If you have a map of expected results, and you draw randomly from the key set, you can have a degree of randomness and still be able to predict what the expected result is.

Answer 4

Can the "random part" be injected into the method (or is the randomness the core feature)?

E.g. (maybe a oversimplified example taking "random" literally) instead of

function foo($x, $y) {
  return $x * rand(1, $y);
}

use something like

function foo($x, $r) {
  return $x * $r->getNext();
}

This way you eliminate as much "randomness" as possible in your tests, since you can pass an object for $r that doesn't really return random values but e.g. edge cases.

Answer 5

I always think of a unit test as something repeatable. (I mean that each run behaves the same and gives the same result).
There are other preconditions for a unit test, but for me, it is the most important one. (Have a look at this very good definition)

Given this, it could looks impossible to test you specific case.
I would try to separate the random thing of your code into a specific part. Then, I would think about it as a data source, and therefore I would try to mock it as well.

I hope you can apply this to your context.