i have try to add custom filed in sale order line

Here is my .PY

class CustomSaleOrderLine(models.Model):

    _inherit = 'sale.order.line'

 _totalweight = fields.Float(string="Total Weight",compute='_totalweight')

 @api.multi

    def _totalweight(self):

  self._totalweight = 15.00

What i am trying to achieve is that i take the value order of product qty and multiply it by 5

I am doing it right way ? but how ever i expected to see this 

  File "/odoo/odoo-server/addons/sale_order_line_custom/models/models.py", line 8
    _totalweight = fields.Float(string="Total Weight",compute='_totalweight')
    ^
IndentationError: unexpected indent

But i think in my code i dont think there is anything wrong with indentation 

Please help