How can I unit test Perl functions that print to the screen?

Question

I'm trying to use Test::More to unit test Perl functions that print to the screen.

I understand that this output may interfere with tools such as prove.

How can I capture this output so I can print it with diag(), and also run tests on the output itself?

Answer 1

UPDATE: IMHO, the correct answer to this question ought to be to use Test::Output:

#!/usr/bin/perl  use strict; use warnings;  use Test::More tests => 1; use Test::Output;  sub myfunc { print "This is a test\n" }  stdout_is(\&myfunc, "This is a test\n", 'myfunc() returns test output'); 

Output:

 C:\Temp> tm 1..1 ok 1 - myfunc() returns test output 

I am leaving the original answer for reference as, I believe, it still illustrates a useful technique.

You can localize STDOUT and reopen to a scalar before calling the function, restore afterward:

#!/usr/bin/perl  use strict; use warnings;  use Test::More tests => 1;  sub myfunc { print "This is a test\n" }  sub invoke {     my $sub = shift;     my $stdout;     {         local *STDOUT;         open STDOUT, '>', \$stdout             or die "Cannot open STDOUT to a scalar: $!";         $sub->(@_);         close STDOUT             or die "Cannot close redirected STDOUT: $!";     }     return $stdout; }  chomp(my $ret =  invoke(\&myfunc));  ok($ret eq "This is a test", "myfunc() prints test string" ); diag("myfunc() printed '$ret'"); 

Output:

 C:\Temp> tm 1..1 ok 1 - myfunc() prints test string # myfunc() printed 'This is a test' 

For versions of perl older than 5.8, you probably need to use IO::Scalar, but I do not know much about how things worked before 5.8.