code: def get_uninvoiced_hours(entries, billable=None):
      
      statuses = ('invoiced', 'not-invoiced')
      if billable is not None:
          billable = (billable.lower() == u'billable')
          entries = [e for e in entries if e.activity.billable == billable]
      hours = sum([e.hours for e in entries if e.status not in statuses])
      return '{0:.2f}'.format(hours)
docs: Given an iterable of entries, return the total hours that have
      not been invoiced. If billable is passed as 'billable' or 'nonbillable',
      limit to the corresponding entries.