Object "dumping"

Derek Thomson dthomson at users.sf.net
Mon Feb 3 02:47:38 CET 2003

Hi all,

I'm debugging a particularly nasty problem in a moderately complex bit 
of Python, and I really need something that I can use to just print out 
the state of an object.

I also do a lot of Perl, and that's available in Perl with the 
Data::Dumper module.

I know Python has pprint, but that stops when it encounters an object 
ie. it only dumps sequences and hashes, which is less than what I want.

As I like to illustrate my examples, here's a simple class in Python:

#!/usr/bin/env python

class RequestHeader:

     def __init__(self,
                  _service_context, _request_id, _response_expected,
                  _object_key, _operation, _requesting_principal):

         self.service_context      = _service_context
         self.request_id           = _request_id
         self.response_expected    = _response_expected
         self.object_key           = _object_key
         self.operation            = _operation
         self.requesting_principal = _requesting_principal


     def get_request_id(self):
         return self.request_id

     # ... other methods ...

Now, if I attempt to use pprint to "dump" the object to a stream as 
follows ...

#!/usr/bin/env python

from requestheader import RequestHeader
import pprint

rh1 = RequestHeader('some context', 13, 1, 'an object key',
                     'op1name', 'principal1')

rh2 = RequestHeader('some other context', 14, 2, 'different object key',
                     'opname2', 'principal2')

rh_list = [ rh1, rh2 ]


... I get:

[<requestheader.RequestHeader instance at 0x81150d4>,
  <requestheader.RequestHeader instance at 0x8127b8c>]

Now, to show you what I actually want, here it is in Perl. First the 
class definition:

#!/usr/bin/env perl

use warnings;
use strict;

package RequestHeader;

use English;

# (Perl uses packages for class namespaces)

sub new
     # Get the constructor arguments
     my ($class,
	$service_context, $request_id, $response_expected,
         $object_key, $operation, $requesting_principal)    = @ARG;

     # We will represent instances of this class with a hash table
     my $self = {
	service_context      => $service_context,
	request_id           => $request_id,
	response_expected    => $response_expected,
	object_key           => $object_key,
	operation            => $operation,
	requesting_principal => $requesting_principal };
     # Bless the reference to the hash so that method calls know which
     # class (or package) this object belongs to
     bless $self, $class;

     # Return the new object reference
     return $self;

sub get_request_id
     my ($self) = @ARG;

     return $self->{request_id};

# ... other methods ...

return 1; # Return module load success

Now, here's the code to "dump" some of those objects ...

#!/usr/bin/env perl

use warnings;
use strict;

use RequestHeader;

use Data::Dumper;

my $rh1 = RequestHeader->new('some context', 13, 1,
			     'an object key', 'op1name',

my $rh2 = RequestHeader->new('some other context', 14, 2,
			     'different object key', 'op2name',

my $rh_list = [ $rh1, $rh2 ];

print Dumper($rh_list);

When I run this, I get:

$VAR1 = [
           bless( {
                    'requesting_principal' => 'principal2',
                    'object_key' => 'an object key',
                    'service_context' => 'some context',
                    'response_expected' => 1,
                    'operation' => 'op1name',
                    'request_id' => 13
                  }, 'RequestHeader' ),
           bless( {
                    'requesting_principal' => 'principal2',
                    'object_key' => 'different object key',
                    'service_context' => 'some other context',
                    'response_expected' => 2,
                    'operation' => 'op2name',
                    'request_id' => 14
                  }, 'RequestHeader' )

... which is what I need for debugging purposes. Now, I know I can add 
methods to the RequestHeader class for printing, but I don't want to do 
that for every single class I want to dump. Now, as I'm constantly 
harping on, the Python and Perl object models are quite similar, so a 
generic "object dumper" should be possible. Does one exist?


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