<field name="readonly_partner_name" invisible="1" />                       

 <xpath expr="//field[@name='partner_name']" position="attributes">                            

<attribute name="attrs">{'readonly':[('readonly_partner_name','!=', False)]}</attribute>                       

 </xpath>

@api.onchange('applicant_credential_id') 

 def _onchnage_applicant_credential_id(self): 

 for rec in self: 

 if rec.applicant_credential_id: 

 rec.partner_name = rec.applicant_credential_id.full_name 

 rec.readonly_partner_name = True

 else:

 rec.readonly_partner_name = False

not giving me result where i'm wrong