Is there a Perl equivalent to Python's `if __name__ == '__main__'`?

Question

Is there a way to determine if the current file is the one being executed in Perl source? In Python we do this with the following construct:

if __name__ == '__main__':
    # This file is being executed.
    raise NotImplementedError

I can hack something together using FindBin and __FILE__, but I'm hoping there's a canonical way of doing this. Thanks!

Poser

Answer 1

unless (caller) {
  print "This is the script being executed\n";
}

See caller. It returns undef in the main script. Note that that doesn't work inside a subroutine, only in top-level code.

Shearer

Answer 2

See the "Subclasses for Applications (Chapter 18)" portion of brian d foy's article Five Ways to Improve Your Perl Programming.

Threecornered

Answer 3

unless caller is good, but a more direct parallel, as well as a more explicit check, is:

use English qw<$PROGRAM_NAME>;

if ( $PROGRAM_NAME eq __FILE__ ) { 
    ...
}

Just thought I'd put that out there.

EDIT

Keep in mind that $PROGRAM_NAME (or '$0') is writable, so this is not absolute. But, in most practice--except on accident, or rampaging modules--this likely won't be changed, or changed at most locally within another scope.

Rainout

That's not guaranteed to work. For one thing, $0 (the real name of $PROGRAM_NAME) is actually a writable variable in Perl. But FILE isn't affected by changing $0. – Shearer

Not only is $0 mutable, so is __FILE__. – Monachism