What is the best practice when writing Perl tests that involve randomness?

Question!

While working on some updates to my module List::Gen, I decided to add a ->pick(num) method, which will return a num sized list of random elements from its source. To test this, I used srand to seed the random number generator, and made several tests of the form:

srand 1234;
is $src->pick(5)->str, '3 6 1 7 9';

And this all worked well on the Windows machine I was on at the time. However, when I moved the project over to a Mac workstation, all of the randomness tests failed, since despite having the same random seed, rand was producing different results. I gather this is from different underlying C implementations of rand().

So the question is, what is the best cross platform way to test these functions? Should I overload the rand function with my own? Should I build in hooks to the functions that use rand to enable a "testing" mode that produces predicable output? Are there other methods?

I would prefer answers that include core Perl techniques, as I am trying to keep the module's dependency tree small.

Test::Random and Test::MockRandom seem to be CPAN's suggestions, does anyone have experience with these modules?



Answers

You could run a few picks and make sure they don't all return the same thing. That's the purpose of the function, after all.

By : ikegami


I prefer to simply encapsulate the environmental dependency and override it for testing purposes, a test pattern called Test Stub. Test Stub also covers other indirect inputs like the system time and file handles. These should all be construed as different forms of the same problem, which is why I think the CPAN solutions to this problem less-than-great.

Applied to the random number domain, we have something like:

use strict;
use warnings;

package Foo;

sub new {
    my ($class) = @_;

    return bless {} =
By : darch


Maybe the random part does not matter for your tests?

The passing tests could check for the following:

  1. did ->pick( X ) return X elements?
  2. Are all X elements part of the $src list?
  3. Test for 0, 1, etc...
  4. (maybe?) Test that two different srand seeds return different lists

This is essentially what you are already doing, since you are trying to take rand() out of the equation. May as well go all the way and test that your function does what it says on the tin.



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By: admin