attrs={'invisible'} on field tags doesn't work in fields_view_get method!

[heba]

I am working on openerp v.7. I override fields_view_get function and create in it many2one field as following:
result['arch']= '''<field name= "hw_sub_category" attrs="{'invisible': [('state','=','initial')]}"/> '''

The field is added successfully but it always visible.

I guessed that because may be attrs invisible doesn't work on the field tag in fields_view_get function so I set the field under group and add the attrs to the group as following:

_arch_moves += '''<group attrs="{'invisible': [('state','=','initial')]}">'''
_arch_moves += '''<field name= "hw_sub_category"/> '''
_arch_moves += '''</group>'''
result['arch']= _arch_moves

but also the same thing the field is always visible.

I tried also to set the invisible in the fields.update itself as following:
result['fields'].update({'hw_sub_category': {'string': 'Hardware subcategory', 'type': 'many2one','relation':'dynamic.fields', 'invisible':True}})

By searching I found the following way:
 doc = etree.XML(result['arch'])
  for node in doc.xpath("//field[@name='hw_sub_category']"):
      node.set('invisible', '1')
  result['arch'] = etree.tostring(doc)

Also The field always visible.

So why the invisible attribute doesn't work in the fields_view_get method and how can I overcome this?

Thanks in advance.