I like to add some fields in the warehouse so I added the class with columns in the stock.py. But it shows error. I can not find what the error is.

The error message is

Traceback (most recent call last):
  File "/home/rck/Desktop/odoo-8.0/openerp/http.py", line 539, in _handle_exception    return super(JsonRequest, self)._handle_exception(exception)  File "/home/rck/Desktop/odoo-8.0/openerp/http.py", line 590, in dispatch    return self._json_response(result)  File "/home/rck/Desktop/odoo-8.0/openerp/http.py", line 528, in _json_response    body = simplejson.dumps(response)  File "/usr/local/lib/python2.7/dist-packages/simplejson/__init__.py", line 366, in dumps    return _default_encoder.encode(obj)  File "/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 269, in encode    chunks = self.iterencode(o, _one_shot=True)  File "/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 348, in iterencode    return _iterencode(o, 0)  File "/usr/local/lib/python2.7/dist-packages/simplejson/encoder.py", line 246, in default   

raise TypeError(repr(o) + " is not JSON serializable")

TypeError: product.step1() is not JSON serializable

My py code in stock.py is 

class product_step1(osv.osv):
    _name = "product.step1"   

_columns = {       

'name':fields.float('Name'),       

'b':fields.float('B'),       

'c':fields.float('C'),       

'd':fields.float('D'),       

'e':fields.float('E'),       

'f':fields.float('F'),       

'g':fields.float('G'),       

'h':fields.float('H'),       

'i':fields.float('I'),       

'j':fields.float('J'),           

}

my xml code in stock_view.xml is

<record model="ir.ui.view" id="step1_tree_view">       

<field name="name">step1.tree</field>       

<field name="model">product.step1</field>       

<field name="type">tree</field>       

<field name="arch" type="xml">       

<tree string="Product Details">       

<field name="name"/>       

<field name="b"/>       

<field name="c"/>       

</tree>        </field>        </record>

 <record model="ir.ui.view" id="step1_form_view">       

<field name="name">step1.form</field>       

<field name="model">product.step1</field>       

<field name="type">form</field>       

<field name="arch" type="xml">       

<form string="Product Details">       

<sheet>           

<group>               

<group>       

<field name="name"/>       

<field name="b"/>       

<field name="c"/>       

<field name="d"/>       

<field name="e"/>                   

</group>               

<group>                   

<field name="f"/>                   

<field name="g"/>                   

<field name="h"/>                   

<field name="i"/>                   

<field name="j"/>               

</group>           

</group>       

</sheet>       

</form>        </field>        </record>
       

<record model="ir.actions.act_window" id="action_step1_form">       

<field name="name">step1</field>       

<field name="res_model">product.step1</field>       

</record>
        <menuitem name="Product Details" parent="menu_stock_root" id="step1_menu_mainform"/>       

<menuitem name="Product Description" parent="step1_menu_mainform" id="step1_menu1_mainform" action="action_step1_form"/>  

When I click the Quants and stock Moves tab in warehouse this error has been shown.

Notify me where I'm doing the mistake.

Thank you.