Warning: lots of magic ahead!

My colleague came up with a question regarding a very special case of unit testing and I'm not sure how to answer it.

• there are loads of legacy routines which produce complicated data-structure and write them to file.
• he doesn't want to test the very complicated output but the intermediate datastructures without changing the code much

My response was the clean approach is to refactor the code into additional subs called from the "printers" returning the data structures and test those nested "units".

Obviously the code would be easier to maintain and reusable.

• He declined it, cause he is very critical about performance and doesn't want the overhead of extra sub calls.*

So I suggested a "dirtier" approach to use a constant TEST to flag that tests are running and to safe the desired code into package variables named "$PKG::TEST_something" (or probably $PKG::TST::Something)

The advantage is that every code conditioned by ... if TEST won't show up in production code.

Here a condensed demo in a single file, the output of B::Deparse shows that the TEST code disappears. (In a real test.t file the "PKG::TEST" flag has to be set before requiring the module PKG to be effective)

Question Is there a better way to do this?

#  set TEST flag before compiling tested module
use constant 'PKG::TEST' => 1; 

# ###  here normally use PKG
{
    package PKG;

    use strict;
    use warnings;
    use Data::Dump qw/pp dd/;
    use v5.16;





    sub create_output {
        my $count =shift;
    
        my $string;
        my $fh;

        my %hash;

        # create (mildly) complex demo HoA
        $hash{$_} = [reverse 1.. $count--]
            for "a".."f";

        # memorize datastructure when testing
        $PKG::TEST_result = \%hash if TEST;

        my $target = "c:/tmp/output.txt";

        #  redirect filehande to string when testing
        $target = \$PKG::TEST_content if TEST;
    
        open $fh, '>', $target;


        #  Output to "file"
        print $fh pp \%hash;
    }
}


# ### my tests
package main;

use B::Deparse;
use Data::Dump qw/pp dd/;


# prove that TEST code disappears unless TEST is true 
print B::Deparse->new->coderef2text(\&PKG::create_output);

PKG::create_output(5);

if ( PKG::TEST ) {
    use Test::More;

    my $expected ={
        a => [5, 4, 3, 2, 1],
        b => [4, 3, 2, 1],
        c => [3, 2, 1],
        d => [2, 1],
        e => [1],
        f => [],
    };
    
    is_deeply(
        $PKG::TEST_result,
        $expected,
        "internal result"
    );

    my $expected_str = pp $expected;
    
    is(
        $PKG::TEST_content,
        $expected_str,
        "file output"
    );

    done_testing();
}
[download]

The problems I see:

• still litering production code with TEST statements
• when testing multiple subs which call each other they may confuse the results
• there doesn't seem to be a way to check the subs name at compiletime
• __SUB__ gives me only the subref which isn't known at test start.
• I can only use __SUB__ as a hash key to store different results...

*) a case were inlining would be nice, more about that in a future post

update

added comments to code

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