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Asked: 2026-05-14T00:05:37+00:00

I am a bit new to the Devel::Cover module, but have found it very

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I am a bit new to the Devel::Cover module, but have found it very useful in making sure I am not missing tests.

A problem I am running into is understanding the report from Devel::Cover. I’ve looked at the documentation, but can’t figure out what I need to test to get 100% coverage.

Edit – I should make it clear that I am not saying I need 100% coverage,
because as multiple people point out,
100% coverage is a loose term, does
not mean that my code is bug free, and
may not always be completely
necessary. Since I am new at
Devel::Cover, I am interested to know
why my code is not 100% coverage, in case I am missing some important
tests.

Here is the output from the cover report:

line  err   stmt   bran   cond    sub    pod   time   code
...
36                                                    sub connect_database {
37             3                    3      1   1126       my $self = shift;
38             3    100                          24       if ( !$self->{dsn} ) {
39             1                                  7           croak 'dsn not supplied - cannot connect';
40                                                        }
41    ***      2            33                   21       $self->{dbh} = DBI->connect( $self->{dsn}, q{}, q{} )
42                                                          || croak "$DBI::errstr";
43             1                                 11       return $self;
44                                                    }
...
line  err      %      l  !l&&r !l&&!r   expr
----- --- ------ ------ ------ ------   ----
41    ***     33      1      0      0   'DBI'->connect($$self{'dsn'}, '', '') || croak("$DBI::errstr")

And here is and example of my code that tests this specific line:

my $database = MyModule::Database->new( { dsn => 'Invalid DSN' });
throws_ok( sub { $database->connect_database() }, 
   qr/Can't connect to data source/, 
   'Test connection exception (invalid dsn)' );

This test passes – the connect does throw an error and fulfills my “throws_ok” test.

I do have some tests that test for a successful connection, which is why I think I have 33% coverage, but if I’m reading it correctly, cover thinks I am not testing the “|| croak” part of the statement. I thought I was, with the “throws_ok” test, but obviously I am missing something.

Does anyone have advice on how I can test my DBI->connect line successfully?

Thanks!

Edit:

brian tipped me off to the HTML report and the truth table that explains why line #41 is not passing. The only problem is that I can’t figure out what it is telling me. I guess the real core of my question is why is this specific line not passing coverage.

Here is the truth table:

LINE # %  # coverage    # condition
41   # 33 # A | B | dec # 'DBI'->connect($$self{'dsn'}, '', '') || croak("$DBI::errstr")
     #    # 0 | 0 | 0   #
     #    # 0 | 1 | 1   #
     #    # 1 | X | 1   # (THIS LINE IS Green - the rest are red)

If anyone could help explain this truth table, I’d appreciate it. It has also been mentioned that to pass the coverage I need to have a mock database object, but I don’t quite see how anything in the coverage results that would clue me in to this.

Thanks again!

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1 Answer

  1. Editorial Team

    Also, don’t get too hung up on 100% test coverage. The goal is to fully test your application, not get perfect scores in Devel::Cover. See Ovid’s posts on the subject:

    In your case, it looks like you don’t cover all the branches, so you don’t get perfect scores. You need to tests that exercise both sides of that ||. You’re getting 33% percent coverage because you only handle one-third of the cases for that line. The HTML report from Devel::Cover shows you the truth table and which cases you are missing.

    The truth table shows you the possible states that you have to cover for the branching. A 1 shows you a condition which is true, 0 shows you a condition that is false, and X shows you a condition you won’t reach. You have to test all combinations that can execute. Since the || is a short-circuit operator, you don’t have to test conditions once one of them passes:

     0 || 1     connect to database fails and croak succeeds
 0 || 0     connect to database fails and croak fails (unlikely)
 1 || X     connect to database succeeds, so short circuit

    This is a bit unrelated to your particular problem, but I find it comes up often in these problems. Although Effective Perl Programming is a month of so away from hitting the shelves, Josh McAdams spent quite a bit of time talking about dependency injection in Perl. If you’re having a tough time testing your code, you usually have a design problem. If you’re internally generating database objects in subroutines, for instance, you are painting yourself into a corner. That’s why it might be hard to test. This may not be the problem in your case, but it’s something to think about.

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