How Can I use my own new field here?:

I have this:

class hr_timesheet_sheet(osv.osv):
    _inherit = "hr_timesheet_sheet.sheet"
	def test(self):
		iid = self.task_field
    _columns = {
        'task_field': fields.many2one('project.task', 'task_id', required=True)
    }

How Can I use my own field = "task_field" in a simple variable or in a function that I want to write?

Or how can I use my own field in other modules?