How can I monkey-patch an instance method in Perl?

Question

I\'m trying to monkey-patch (duck-punch :-) a LWP::UserAgent instance, like so:

sub _user_agent_get_basic_credentials_patch {
  return ($usernam         


        

Answer 1

Building upon John Siracusa's answer… I found that I still wanted a reference to the original function. So I did this:

MONKEY_PATCH_INSTANCE:
{
  my $counter = 1; # could use a state var in perl 5.10

  sub monkey_patch_instance
  {
    my($instance, $method, $code) = @_;
    my $package = ref($instance) . '::MonkeyPatch' . $counter++;
    no strict 'refs';
    my $oldFunction = \&{ref($instance).'::'.$method};
    @{$package . '::ISA'} = (ref($instance));
    *{$package . '::' . $method} = sub {
        my ($self, @args) = @_;
        $code->($self, $oldFunction, @args);
    };
    bless $_[0], $package; # sneaky re-bless of aliased argument
  }
}

# let's say you have a database handle, $dbh
# but you want to add code before and after $dbh->prepare("SELECT 1");

monkey_patch_instance($dbh, prepare => sub {
    my ($self, $oldFunction, @args) = @_;

    print "Monkey patch (before)\n";
    my $output = $oldFunction->(($self, @args));
    print "Monkey patch (after)\n";

    return $output;
    });

It's the same as in the original answer, except I pass through some parameters $self and $oldFunction.

This lets us invoke $self's $oldFunction as usual, but decorate additional code around it.