In Odoo 9, i am using on_change function and i need to hide some custom fields in view sale order

in models.py i have:

...

#Selecction Field

b = fields.Selection([('hide', 'Hide'),('nohide', 'Not Hide'),('choiceoption', 'Choice Option')], default='choiceoption')

b_value = fields.Char('Value', default='Hello')

b_unit = fields.Char('Unit', readonly='True', default='World')

#Function

def on_change_b(self,cr,user,ids,b,b_value,b_unit,context=None):

if b=='choiceoption':

v = {'b_value': 'Choice Option', 'b_unit': 'Choice Option'}

return {'value': v}

elif b=='nohide':

v = {'b_value': 'No Hide Field b_value', 'b_unit': 'No Hide Field b_unit'}

return {'value': v}

else:

v = {'b_value': 'Yes Hide Field b_value', 'b_unit': 'Yes Hide Field b_unit'}

return {'value': v}

...

in templates.xml i have:

...

<field name="b" on_change="on_change_b(b, b_value, b_unit)" />

<field name="b_value" />

<field name="b_unit" />

...

1.- How i can hide the field "b_value" and "b_unit" when i choice "Hide" in the selection field, i think this must be writed in the function "on_change_b" but i do not know how doit the code, maybe with "attrs"

2.- How to initialize a field with invisible = yes for default like:

fieldxyz = fields.Char('This is Field XYZ', default=This is the value of fieldxyz', attrs="{'invisible':true}") <---This is correct?