HI Priya,

I think you have to overwrite the fields_view_get function

Here I given example:

Try this below code its may helps to you.

    def fields_view_get(self, cr, uid, view_id=None, view_type='form', context=None, toolbar=False, submenu=False):
        res = super(class_name, self).fields_view_get(cr, uid, view_id=view_id, view_type=view_type, context=context, toolbar=toolbar, submenu=submenu)
          if view_type == 'form':
            doc = etree.XML(res['arch'])
            for node in doc.xpath("//field[@name='user_id']"):
              node.set('readonly', "1")
              setup_modifiers(node, res['fields']['user_id'])
            res['arch'] = etree.tostring(doc)

        return res