Function fields in OpenERP:

Parameters:

fnct, arg=None, fnct_inv=None, fnct_inv_arg=None, type="float",
 fnct_search=None, obj=None, method=False, store=False, multi=False

Here

1. fnct is the function or method that will compute the field value. It should be declared before declaration of the functional field.
2. fnct_inv is the function or method which will allow writing values in that field.
3. type is the field type name returned by the function. It can be any field type name except function.
4. fnct_search allows you to define the searching behavior on that field.
5. method whether the field is computed by a method (of an object) or a global function
6. store If you want to store field in database or not. Default is False.
7. multi is a group name. All fields with the same multi parameter will be calculated in a single function call.

Example

 'testrest':fields.function(get_reste, store=True, string='Restant',type='integer'),
 def get_reste(self, cr, uid, ids, field_name, arg, context):
    x = {}
    for record in self.browse(cr, uid, ids ,context):
            if record.statut !=  'entree':
                x[record.id]= record.testrest + record.entree_nbr
    return x