Perl – How to monkey-patch an instance method in Perl

monkeypatchingperl

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

sub _user_agent_get_basic_credentials_patch {
  return ($username, $password);
}

my $agent = LWP::UserAgent->new();
$agent->get_basic_credentials = _user_agent_get_basic_credentials_patch;

This isn't the right syntax — it yields:

Can't modify non-lvalue subroutine
call at [module] line [lineno].

As I recall (from Programming Perl), dispatch lookup is performed dynamically based on the blessed package (ref($agent), I believe), so I'm not sure how instance monkey patching would even work without affecting the blessed package.

I know that I can subclass the UserAgent, but I would prefer the more concise monkey-patched approach. Consenting adults and what have you. 😉

Best Answer

As answered by Fayland Lam, the correct syntax is:

    local *LWP::UserAgent::get_basic_credentials = sub {
        return ( $username, $password );
    };

But this is patching (dynamically scoped) the whole class and not just the instance. You can probably get away with this in your case.

If you really want to affect just the instance, use the subclassing you described. This can be done 'on the fly' like this:

{
   package My::LWP::UserAgent;
   our @ISA = qw/LWP::UserAgent/;
   sub get_basic_credentials {
      return ( $username, $password );
   };

   # ... and rebless $agent into current package
   $agent = bless $agent;
}

Related Solutions

Python – Adding a Method to an Existing Object Instance

In Python, there is a difference between functions and bound methods.

>>> def foo():
...     print "foo"
...
>>> class A:
...     def bar( self ):
...         print "bar"
...
>>> a = A()
>>> foo
<function foo at 0x00A98D70>
>>> a.bar
<bound method A.bar of <__main__.A instance at 0x00A9BC88>>
>>>

Bound methods have been "bound" (how descriptive) to an instance, and that instance will be passed as the first argument whenever the method is called.

Callables that are attributes of a class (as opposed to an instance) are still unbound, though, so you can modify the class definition whenever you want:

>>> def fooFighters( self ):
...     print "fooFighters"
...
>>> A.fooFighters = fooFighters
>>> a2 = A()
>>> a2.fooFighters
<bound method A.fooFighters of <__main__.A instance at 0x00A9BEB8>>
>>> a2.fooFighters()
fooFighters

Previously defined instances are updated as well (as long as they haven't overridden the attribute themselves):

>>> a.fooFighters()
fooFighters

The problem comes when you want to attach a method to a single instance:

>>> def barFighters( self ):
...     print "barFighters"
...
>>> a.barFighters = barFighters
>>> a.barFighters()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: barFighters() takes exactly 1 argument (0 given)

The function is not automatically bound when it's attached directly to an instance:

>>> a.barFighters
<function barFighters at 0x00A98EF0>

To bind it, we can use the MethodType function in the types module:

>>> import types
>>> a.barFighters = types.MethodType( barFighters, a )
>>> a.barFighters
<bound method ?.barFighters of <__main__.A instance at 0x00A9BC88>>
>>> a.barFighters()
barFighters

This time other instances of the class have not been affected:

>>> a2.barFighters()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'barFighters'

More information can be found by reading about descriptors and metaclass programming.

Perl – How to start an interactive console for Perl

You can use the perl debugger on a trivial program, like so:

perl -de1

Alternatively there's Alexis Sukrieh's Perl Console application, but I haven't used it.

Best Answer

Related Solutions

Python – Adding a Method to an Existing Object Instance

Perl – How to start an interactive console for Perl

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