To display a debug log, you can use the standard python logging module.

Basically, you need to get a logger and you can use it in your methods to output log messages.

Usually, in OpenERP, a _logger is get at the top of the python module, its name being the name of the python module.

import logging
from openerp.osv import orm

_logger = logging.getLogger(__name__)


class project_task_work(orm.Model):
    _inherit = 'project.task.work'

    def create(self, cr, uid, vals, context=None):
        _logger.debug('Create a %s with vals %s', self._name, vals)
        return super(project_task_work, self).create(cr, uid, vals, context=context)

Note that you will need to start your server using the --debug option to show debug logs.

Other options for logging at a higher level (that doesn't require the --debug option) are:

_logger.info('FYI: This is happening')

_logger.warning('WARNING: I don't think you want this to happen!')

_logger.error('ERROR: Something really bad happened!')