Writing tests for code is just as important as writing the code itself. Time spent writing tests is less time spent tracking down and fixing bugs, making it a great investment. Despite knowing this it can sometimes be hard to force yourself to stop coding and write tests. Fortunately, Perl has some modules to make it pretty simple. Here’s the module we’re going to be testing:

Perl
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#!/usr/bin/perl
 
package MyMaths;
use strict;
 
sub new
{
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self = {};
 
        bless($self, $class);
        return $self;
}
 
sub add
{
        my $self = shift;
        my $total = 0;
        foreach my $factor (@_)
        {
                $total += $factor;
        }
        return $total;
}
 
1;
#!/usr/bin/perl
 
package MyMaths;
use strict;
 
sub new
{
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self = {};
 
        bless($self, $class);
        return $self;
}
 
sub add
{
        my $self = shift;
        my $total = 0;
        foreach my $factor (@_)
        {
                $total += $factor;
        }
        return $total;
}
 
1;

As you can see, we have a simple class with one method, add, which adds together all the arguments passed to it. We now need to define a test file for this module, called mymaths.t:

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#!/usr/bin/perl
 
use Test::More tests=>6;
use MyMaths;
 
$mymaths = new MyMaths;
is( $mymaths->add(1,2,3), 6, "1 + 2 + 3 = 6");
is( $mymaths->add(6,2), 8,"6 + 2 = 8");
is( $mymaths->add(1,2,3,4), 10, "1 + 2 + 3 + 4 = 10");
is( $mymaths->add(1,2), 3, "1 + 2 = 3");
is( $mymaths->add(2), 2, "2 = 2");
is( $mymaths->add(2,-1), 1, "2 + -1 = 1");
#!/usr/bin/perl
 
use Test::More tests=>6;
use MyMaths;
 
$mymaths = new MyMaths;
is( $mymaths->add(1,2,3), 6, "1 + 2 + 3 = 6");
is( $mymaths->add(6,2), 8,"6 + 2 = 8");
is( $mymaths->add(1,2,3,4), 10, "1 + 2 + 3 + 4 = 10");
is( $mymaths->add(1,2), 3, "1 + 2 = 3");
is( $mymaths->add(2), 2, "2 = 2");
is( $mymaths->add(2,-1), 1, "2 + -1 = 1");

This test file uses the is() method of the builtin Test::More module. This method takes 3 arguments: the test, the expected result and a description. If a test fails, is() will give some feedback on the test.

edit: To run the tests use the `prove` utility which is part of the Test::Harness package:

prove -v mymaths.t

Run all tests in the current directory with:
prove -v .

To run the tests we use another builtin module, Test::Harness. Create tests.pl:

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#!/usr/bin/perl
 
use Test::Harness qw(&runtests);
 
@tests = @ARGV ? @ARGV : <*.t>;
 
runtests @tests;
#!/usr/bin/perl
 
use Test::Harness qw(&runtests);
 
@tests = @ARGV ? @ARGV : <*.t>;
 
runtests @tests;

This script can either be run with a list of test files as arguments, or with no arguments to run all test files. Running the script results in:

./mymaths.t .. 
1..6
ok 1 - 1 + 2 + 3 = 6
ok 2 - 6 + 2 = 8
ok 3 - 1 + 2 + 3 + 4 = 10
ok 4 - 1 + 2 = 3
ok 5 - 2 = 2
ok 6 - 2 + -1 = 1
ok
All tests successful.
Files=1, Tests=6,  0 wallclock secs ( 0.01 usr  0.02 sys +  0.02 cusr  0.01 csys =  0.06 CPU)
Result: PASS

Everything went fine, as expected. Now go back to the original class definition and make a typo on line 22, so it reads = instead of += and rerun the tests:

./mymaths.t .. 
1..6
not ok 1 - 1 + 2 + 3 = 6
not ok 2 - 6 + 2 = 8
not ok 3 - 1 + 2 + 3 + 4 = 10
not ok 4 - 1 + 2 = 3
ok 5 - 2 = 2
not ok 6 - 2 + -1 = 1

#   Failed test '1 + 2 + 3 = 6'
#   at ./mymaths.t line 7.
#          got: '3'
#     expected: '6'

#   Failed test '6 + 2 = 8'
#   at ./mymaths.t line 8.
#          got: '2'
#     expected: '8'

#   Failed test '1 + 2 + 3 + 4 = 10'
#   at ./mymaths.t line 9.
#          got: '4'
#     expected: '10'

#   Failed test '1 + 2 = 3'
#   at ./mymaths.t line 10.
#          got: '2'
#     expected: '3'

#   Failed test '2 + -1 = 1'
#   at ./mymaths.t line 12.
#          got: '-1'
#     expected: '1'
# Looks like you failed 5 tests of 6.
Dubious, test returned 5 (wstat 1280, 0x500)
Failed 5/6 subtests 

Test Summary Report
-------------------
./mymaths.t (Wstat: 1280 Tests: 6 Failed: 5)
  Failed tests:  1-4, 6
  Non-zero exit status: 5
Files=1, Tests=6,  0 wallclock secs ( 0.03 usr  0.00 sys +  0.03 cusr  0.00 csys =  0.06 CPU)
Result: FAIL

Most of the tests failed and gave good feedback to help find the problem.

Summary

Although writing tests can be a chore it is worth doing. If you write tests a little at a time:

  • When you design a class.
  • When you add a method to a class.
  • When you find a bug.

You’ll develop a good library of tests without too much effort.

Further reading: Test::More perldoc