use and BEGIN

tall_man has asked for the wisdom of the Perl Monks concerning the following question:

A person I work with refuses to use "use". He says that he read somewhere that if "use" is called twice on the same module, then the BEGIN blocks of that module are executed twice; for example, doing "use" twice on a database module will create two socket connections.

I have tried to convince him there is no essential difference between a "use Module qw(a b);" and:

BEGIN {
   require Module;
   Module->import(qw(a b));
}
[download]

I don't believe that either require or use will repeat BEGIN blocks of a module. Is there a definitive reference that would help to convince him?

Comment on use and BEGIN Download Code

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Re: use and BEGIN by blokhead (Monsignor) on Jul 19, 2003 at 20:04 UTC
A simple test of his assumption should be enough to convince: foo.pm: `package foo; BEGIN { print "foo's BEGIN\n"; } print "foo's body\n"; sub import { shift @_; print "foo's import: @_\n"; } 1;` [download] bar.pm: `package bar; use foo qw(from bar); 1;` [download] test.pl: `use lib '.'; use foo qw(from test.pl); use foo qw(from test.pl again); use bar;` [download] Output: $ perl test.pl foo's BEGIN foo's body foo's import: from test.pl foo's import: from test.pl again foo's import: from bar $ [download] BEGIN blocks are executed at compilation. `use` and `require` don't compile the same code twice. Look in perlvar under `%INC` for more info: The "require" operator uses this hash to determine whether a particular file has already been included. blokhead	[reply] [d/l] [select]
Re: use and BEGIN by tilly (Archbishop) on Jul 19, 2003 at 20:58 UTC
In addition to the experiment already suggested, `perldoc -f use` and `perldoc -f require` (or referring to various htmlized versions of the core documentation) will confirm your understanding.	[reply] [d/l] [select]
Re: use and BEGIN by broquaint (Abbot) on Jul 20, 2003 at 12:55 UTC
He says that he read somewhere that if "use" is called twice on the same module, then the BEGIN blocks of that module are executed twice Nope, just the `import` sub is called. The actual file being included isn't even touched on the second `use` as its existence has already been noted in `%INC` so that further proves that the `BEGIN` blocks are not touched. If written in pure perl `use` would look something like this `use File::Spec::Functions; sub use { my($pkg, @args) = @_; my(@dirs, $file) = split '::', $pkg; my $path = catfile @dirs, "$file.pm"; return if exists $INC{$path}; my $fullpath = catfile( (grep { -f cafile($_, $path) } @INC)[0], $path ); my $code = IO::File->new($fullpath) or die "$!"; $INC{$path} = $fullpath; eval $code->getlines; $file->import(@args); }` [download] Of course it's missing the code references in `%INC` implementation, and it's totally untested, but it should give you some idea of how `use` actually works. HTH `_________ broquaint`	[reply] [d/l]