title: AttributeError: 'NoneType' object has no attribute '_columns'
body: <p>I am writing a module in OpenERP that would inherit from the Accounts module. Below is my simplified code</p>
  
  <pre><code>from osv import fields, osv
  import decimal_precision as dp
  
  class account_molly(osv.osv):
  
      def __computeOB(self, cr, uid, ids, context=None):
          res = []
          return 0
  
      def __computeCB(self, cr, uid, ids, context=None):
          res = []
          return 0
  
      _name = "account.molly"
      _description = "This is Molly Special Account View"
      _inherit = "account.account"
      _columns = {
          'opening_balance': fields.function(__computeOB, digits_compute=dp.get_precision('Account'), method=True, string='Opening Balance'),
          'end_balance': fields.function(__computeCB, digits_compute=dp.get_precision('Account'), method=True, string='End Balance'),
      }   
  
  account_molly()
  
  # vim:expandtab:smartindent:tabstop=4:softtabstop=4:shiftwidth=4:
  
  </code></pre>
  
  <p>When I try to install the module, I get 
  <pre><code>AttributeError: 'NoneType' object has no attribute '_columns'</code></pre> Please what could be the problem</p>