How can we test an application that makes calles to some external system, for example needs to access a website or a web-base API? We can hit the external system for every test-run, but that will probably slow down our testing, might get us banned from the web site, but maybe most importantly (for the tester), the test will be unreliable. In addition, it will be impossible to test cases when the external web site returns some error condition.

Let's try a simple example in which we fetch a web page and count specific strings.

Download: mp4 webm

YouTube

More specifically we have a file called MyWebAPI.pm with the following code in it:

package MyWebAPI;
use strict;
use warnings;

use LWP::Simple qw(get);

my $URL = 'http://www.dailymail.co.uk/';

sub new {
    return bless {}, shift;
}

sub count_strings {
    my ($self, @strings) = @_;

    my $content = get $URL;

    my %data;
    foreach my $str (@strings) {
        $data{$str} = () = $content =~ /$str/ig;
    }
    return \%data;
}

1;

This code fetches the content of British Daily Mail that provides reliable information on the status of the world. Once the page is fetched we count how many times given specific strings appear and return the numbers as a reference to a hash containing "string" => count pairs.

Test life web server

We can write a test script call webapi.t with the following content:

use strict;
use warnings;

use FindBin qw($Bin);
use lib $Bin;

use Test::More;
plan tests => 1;

use MyWebAPI;

my $w = MyWebAPI->new;

is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        'Beyonce'     => 26,
        'Miley Cyrus' => 3,
    };

We check the relative popularity of Beyonce and Miley Cyrus and as we can see Beyonce is winnint 26 to 3. If we run this script perl webapi.t the output will indicate that everything is ok:

1..1
ok 1

Unfortunately the actual content of the website changes and thus the numbers will change. That means our test will soon break eve though thet actual "application" is still working correctly.

We can solve this by either disregarding the actual number in the result and check only if there was a number. This will make our test more universal, but weaker. Or, we can replace the content of the web site as returned by the get function of LWP::Simple.

We have other issues as well. For example how can we test the behaviour of our application in the case when the Daily Maily web site is down or returns garbage? We could build our fake version of Daily Maily, but it is probably more simple to fake the get function.

That's what we are going to do. We are going to use Test::Mock::Simple to fake (or mock) the get function.

Mocking the get function

We load the module using use Test::Mock::Simple; and then instead of use MyWebAPI we load our module to be tested using my $mock = Test::Mock::Simple->new(module => 'MyWebAPI');

then we replace the get function imported from LWP::Simple, but an anonymous function we provide, which will return a simple string:

$mock->add(get => sub {
    return 'Beyonce Beyonce Miley Cyrus';
});

We also adjusted the values in the expected hash to reflect the string we return. This is the new version of the test script:

use strict;
use warnings;

use FindBin qw($Bin);
use lib $Bin;

use Test::More;
plan tests => 1;

use Test::Mock::Simple;
my $mock = Test::Mock::Simple->new(module => 'MyWebAPI');

my $w = MyWebAPI->new;

$mock->add(get => sub {
    return 'Beyonce Beyonce Miley Cyrus';
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        'Beyonce'     => 2,
        'Miley Cyrus' => 1,
    };

More test cases

We can now add more test cases, including one when the get function returns the empty string. We just have to remember to update the plan as well.

use strict;
use warnings;

use FindBin qw($Bin);
use lib $Bin;

use Test::More;
plan tests => 3;

use Test::Mock::Simple;
my $mock = Test::Mock::Simple->new(module => 'MyWebAPI');

my $w = MyWebAPI->new;

$mock->add(get => sub {
    return 'Beyonce Beyonce Miley Cyrus';
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        'Beyonce'     => 2,
        'Miley Cyrus' => 1,
    };

$mock->add(get => sub {
    return 'Beyonce';
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        Beyonce => 1,
        'Miley Cyrus' => 0,
    };

$mock->add(get => sub {
    return '';
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        Beyonce => 0,
        'Miley Cyrus' => 0,
    };

Testing exception

As a tester I don't know if the get function of LWP::Simple would ever throw an exception, but I wonder how would the MyWebAPI module handle it?

I can add the following test case:

$mock->add(get => sub {
    die 'Something went wrong'; 
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        Beyonce => 0,
        'Miley Cyrus' => 0,
    };

As it can be seen from the result, the MyWebAPI module does not handle such cases. Is this a bug, or is this the correct behavior? That's beyond the scope of this article, but now at least we know how to check what happens in this extreme case.

A bug

What if the web site contains the name "Cyrus Miley" broken in two lines like in the following example?

$mock->add(get => sub {
    return 'Beyonce Miley Cyrus Miley
Cyrus';
});
is_deeply $w->count_strings('Beyonce', 'Miley Cyrus'), 
    {
        Beyonce => 1,
        'Miley Cyrus' => 2,
    };

Indeed this test fails now:

1..4
ok 1
ok 2
ok 3
not ok 4
#   Failed test at ../training/pm/examples/mock/webapi.t line 61.
#     Structures begin differing at:
#          $got->{Miley Cyrus} = '1'
#     $expected->{Miley Cyrus} = '2'
# Looks like you failed 1 test of 4.

We can now update the module fixing this case and we can test it in a controlled environment.