tutorial.pod

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=head1 NAME

Test::Tutorial - A tutorial about writing really basic tests

=head1 DESCRIPTION


I<AHHHHHHH!!!!  NOT TESTING!  Anything but testing!  
Beat me, whip me, send me to Detroit, but don't make 
me write tests!>

I<*sob*>

I<Besides, I don't know how to write the damned things.>


Is this you?  Is writing tests right up there with writing
documentation and having your fingernails pulled out?  Did you open up
a test and read 

    ######## We start with some black magic

and decide that's quite enough for you?

It's ok.  That's all gone now.  We've done all the black magic for
you.  And here are the tricks...


=head2 Nuts and bolts of testing.

Here's the most basic test program.

    #!/usr/bin/perl -w

    print "1..1\n";

    print 1 + 1 == 2 ? "ok 1\n" : "not ok 1\n";

since 1 + 1 is 2, it prints:

    1..1
    ok 1

What this says is: C<1..1> "I'm going to run one test." [1] C<ok 1>
"The first test passed".  And that's about all magic there is to
testing.  Your basic unit of testing is the I<ok>.  For each thing you
test, an C<ok> is printed.  Simple.  B<Test::Harness> interprets your test
results to determine if you succeeded or failed (more on that later).

Writing all these print statements rapidly gets tedious.  Fortunately,
there's B<Test::Simple>.  It has one function, C<ok()>.

    #!/usr/bin/perl -w

    use Test::Simple tests => 1;

    ok( 1 + 1 == 2 );

and that does the same thing as the code above.  C<ok()> is the backbone
of Perl testing, and we'll be using it instead of roll-your-own from
here on.  If C<ok()> gets a true value, the test passes.  False, it
fails.

    #!/usr/bin/perl -w

    use Test::Simple tests => 2;
    ok( 1 + 1 == 2 );
    ok( 2 + 2 == 5 );

from that comes

    1..2
    ok 1
    not ok 2
    #     Failed test (test.pl at line 5)
    # Looks like you failed 1 tests of 2.

C<1..2> "I'm going to run two tests."  This number is used to ensure
your test program ran all the way through and didn't die or skip some
tests.  C<ok 1> "The first test passed."  C<not ok 2> "The second test
failed".  Test::Simple helpfully prints out some extra commentary about
your tests.

It's not scary.  Come, hold my hand.  We're going to give an example
of testing a module.  For our example, we'll be testing a date
library, B<Date::ICal>.  It's on CPAN, so download a copy and follow
along. [2]


=head2 Where to start?

This is the hardest part of testing, where do you start?  People often
get overwhelmed at the apparent enormity of the task of testing a
whole module.  Best place to start is at the beginning.  Date::ICal is
an object-oriented module, and that means you start by making an
object.  So we test C<new()>.

    #!/usr/bin/perl -w

    use Test::Simple tests => 2;

    use Date::ICal;

    my $ical = Date::ICal->new;         # create an object
    ok( defined $ical );                # check that we got something
    ok( $ical->isa('Date::ICal') );     # and it's the right class

run that and you should get:

    1..2
    ok 1
    ok 2

congratulations, you've written your first useful test.


=head2 Names

That output isn't terribly descriptive, is it?  When you have two
tests you can figure out which one is #2, but what if you have 102?

Each test can be given a little descriptive name as the second
argument to C<ok()>.

    use Test::Simple tests => 2;

    ok( defined $ical,              'new() returned something' );
    ok( $ical->isa('Date::ICal'),   "  and it's the right class" );

So now you'd see...

    1..2
    ok 1 - new() returned something
    ok 2 -   and it's the right class


=head2 Test the manual

Simplest way to build up a decent testing suite is to just test what
the manual says it does. [3] Let's pull something out of the 
L<Date::ICal/SYNOPSIS> and test that all its bits work.

    #!/usr/bin/perl -w

    use Test::Simple tests => 8;

    use Date::ICal;

    $ical = Date::ICal->new( year => 1964, month => 10, day => 16, 
                             hour => 16, min => 12, sec => 47, 
                             tz => '0530' );

    ok( defined $ical,            'new() returned something' );
    ok( $ical->isa('Date::ICal'), "  and it's the right class" );
    ok( $ical->sec   == 47,       '  sec()'   );
    ok( $ical->min   == 12,       '  min()'   );    
    ok( $ical->hour  == 16,       '  hour()'  );
    ok( $ical->day   == 17,       '  day()'   );
    ok( $ical->month == 10,       '  month()' );
    ok( $ical->year  == 1964,     '  year()'  );

run that and you get:

    1..8
    ok 1 - new() returned something
    ok 2 -   and it's the right class
    ok 3 -   sec()
    ok 4 -   min()
    ok 5 -   hour()
    not ok 6 -   day()
    #     Failed test (- at line 16)
    ok 7 -   month()
    ok 8 -   year()
    # Looks like you failed 1 tests of 8.

Whoops, a failure! [4] Test::Simple helpfully lets us know on what line
the failure occurred, but not much else.  We were supposed to get 17,
but we didn't.  What did we get??  Dunno.  We'll have to re-run the
test in the debugger or throw in some print statements to find out.

Instead, we'll switch from B<Test::Simple> to B<Test::More>.  B<Test::More>
does everything B<Test::Simple> does, and more!  In fact, Test::More does
things I<exactly> the way Test::Simple does.  You can literally swap
Test::Simple out and put Test::More in its place.  That's just what
we're going to do.

Test::More does more than Test::Simple.  The most important difference
at this point is it provides more informative ways to say "ok".
Although you can write almost any test with a generic C<ok()>, it
can't tell you what went wrong.  Instead, we'll use the C<is()>
function, which lets us declare that something is supposed to be the
same as something else:

    #!/usr/bin/perl -w

    use Test::More tests => 8;

    use Date::ICal;

    $ical = Date::ICal->new( year => 1964, month => 10, day => 16, 
                             hour => 16, min => 12, sec => 47, 
                             tz => '0530' );

    ok( defined $ical,            'new() returned something' );
    ok( $ical->isa('Date::ICal'), "  and it's the right class" );
    is( $ical->sec,     47,       '  sec()'   );
    is( $ical->min,     12,       '  min()'   );    
    is( $ical->hour,    16,       '  hour()'  );
    is( $ical->day,     17,       '  day()'   );
    is( $ical->month,   10,       '  month()' );
    is( $ical->year,    1964,     '  year()'  );

"Is C<$ical-E<gt>sec> 47?"  "Is C<$ical-E<gt>min> 12?"  With C<is()> in place,
you get some more information

    1..8
    ok 1 - new() returned something
    ok 2 -   and it's the right class
    ok 3 -   sec()
    ok 4 -   min()
    ok 5 -   hour()
    not ok 6 -   day()
    #     Failed test (- at line 16)
    #          got: '16'
    #     expected: '17'
    ok 7 -   month()
    ok 8 -   year()
    # Looks like you failed 1 tests of 8.

letting us know that C<$ical-E<gt>day> returned 16, but we expected 17.  A
quick check shows that the code is working fine, we made a mistake
when writing up the tests.  Just change it to:

    is( $ical->day,     16,       '  day()'   );

and everything works.

So any time you're doing a "this equals that" sort of test, use C<is()>.
It even works on arrays.  The test is always in scalar context, so you
can test how many elements are in a list this way. [5]

    is( @foo, 5, 'foo has 5 elements' );


=head2 Sometimes the tests are wrong

Which brings us to a very important lesson.  Code has bugs.  Tests are
code.  Ergo, tests have bugs.  A failing test could mean a bug in the
code, but don't discount the possibility that the test is wrong.

On the flip side, don't be tempted to prematurely declare a test
incorrect just because you're having trouble finding the bug.
Invalidating a test isn't something to be taken lightly, and don't use
it as a cop out to avoid work.


=head2 Testing lots of values

We're going to be wanting to test a lot of dates here, trying to trick
the code with lots of different edge cases.  Does it work before 1970?
After 2038?  Before 1904?  Do years after 10,000 give it trouble?
Does it get leap years right?  We could keep repeating the code above,
or we could set up a little try/expect loop.

    use Test::More tests => 32;
    use Date::ICal;

    my %ICal_Dates = (
            # An ICal string     And the year, month, date
            #                    hour, minute and second we expect.
            '19971024T120000' =>    # from the docs.
                                [ 1997, 10, 24, 12,  0,  0 ],
            '20390123T232832' =>    # after the Unix epoch
                                [ 2039,  1, 23, 23, 28, 32 ],
            '19671225T000000' =>    # before the Unix epoch
                                [ 1967, 12, 25,  0,  0,  0 ],
            '18990505T232323' =>    # before the MacOS epoch
                                [ 1899,  5,  5, 23, 23, 23 ],
    );


    while( my($ical_str, $expect) = each %ICal_Dates ) {
        my $ical = Date::ICal->new( ical => $ical_str );

        ok( defined $ical,            "new(ical => '$ical_str')" );
        ok( $ical->isa('Date::ICal'), "  and it's the right class" );

        is( $ical->year,    $expect->[0],     '  year()'  );
        is( $ical->month,   $expect->[1],     '  month()' );
        is( $ical->day,     $expect->[2],     '  day()'   );
        is( $ical->hour,    $expect->[3],     '  hour()'  );
        is( $ical->min,     $expect->[4],     '  min()'   );    
        is( $ical->sec,     $expect->[5],     '  sec()'   );
    }

So now we can test bunches of dates by just adding them to
C<%ICal_Dates>.  Now that it's less work to test with more dates, you'll
be inclined to just throw more in as you think of them.
Only problem is, every time we add to that we have to keep adjusting
the C<use Test::More tests =E<gt> ##> line.  That can rapidly get
annoying.  There's two ways to make this work better.