Part four of a series of posts about automated testing.

Tests can be used to document behaviour, and to ensure that APIs you are writing are easy to use for the caller. If implementation details of your code change, but the interface is stable, the tests themselves should not need to change (except for fixture setup code). To get the most benefit from tests, they should in general be written against reusable APIs, not against implementation details.

For example, as part of our work subcontracting for Santa’s elves, we have built this Sleigh Wash service:

package Local::Application::SleighWash;

use Moo;
use Carp;

has max_height => (is => 'ro');

sub wash_sleigh {
    my ($self, $sleigh, %options) = @_;

    $self->_check_sleigh_height($sleigh); # throws if too tall

    ... # confidential sleigh wash details redacted

sub _check_sleigh_height {
    my ($self, $sleigh) = @_;

    if ($sleigh->height() > $self->max_height()) {
        croak "Sleigh too tall!";


Rather than test the “_check_sleigh_height” method directly, it is better to exercise “wash_sleigh”. This ensures that:

  • if we later want to refactor how the internal checks are arranged, the tests will enable us to do that, and
  • we make sure wash_sleigh hasn’t forgotten to call the private method.

The same principle can apply to entire classes which are just implementation details called from a single place, where all the behaviour can be exercised via the public API.

There can be exceptions to this rule, where specific implementation details need to be tested (e.g. confirming security or performance properties), but these should be more unusual.

Further reading