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v7apionchangeoverridev8.0
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Temur

Need to override onchange function from old v7 API, like this one:

Python (v7 API):

...
def onchange_field_a(self, cr, uid, ids, field_a, field_b, field_c, context=None):
    vals = {'value':{} }
    #
    # A huge code here to be executed on field_a changes...
    #
    return vals
...

XML (v7 API):

...
    <field name="field_a" on_change="onchange_field_a(field_a, field_b, field_c)"
...

v7 onchange function said to be incompatible with the new v8 api. here is related posts on this forum that I've found for now:

https://www.odoo.com/forum/help-1/question/75299 

https://www.odoo.com/forum/help-1/question/73893 

- however, we'd like to override onchange_field_a function using new v8.0 API and 1) we do not want to rewrite a huge code written in v7 onchange function, which is well tested and working in the parent model and 2) we do not want to copy-paste it either. Beside that it's bad practice to have copy of a same code in several places, it should be harder to maintain such a copy, as it's likely to receive further bugfixes and improvements (if any) in the same v7 implementation and the same place where it's defined. So, we need to reuse existing v7 onchange_field_a function code defined in the superclass by calling it from overridden v8 onchange_field_a function and add our additional code afterwards (or before,depending on case). 

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Temur
Author Best Answer

right question to ask here is what it makes incompatible onchange v7 vs v8? and can we overcome this incompatibility in order to reuse existing code? Actually it's possible by applying quite simple workaround to the onchange v7 vs v8 incompatibility:

Python (v8 API):

...
@api.onchange("field_a")
@api.depends("field_b", "field_c", "custom_field_in_my_class")
def onchange_field_a(self):

    ### section 1. get result of original onchange_field_a from superclass:
    orig_result = super(my_class,self).onchange_field_a(self.field_a, self.field_b, self.field_c)

    ### section 2. workaround for api incompatibility:
    if type(orig_result) is dict and orig_result.has_key('value'):
        for field, value in orig_result.get('value').items():
            if hasattr(self, field):
                setattr(self, field, value)

    ### section 3. your additional code, for example:
    if self.field_c:
        self.custom_field_in_my_class = True
...

Old api way was to return value dictionary for changed fields, whereas new api way is to assign changes directly to fields of virtual record inside the onchange function, so in the "section 2" we turn value return through dictionary into direct assignments to fields. that's it, api incompatibility is handled by the "section 2".

Note:
field's "on_change" property in XML is no more necessary in v8 

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Temur
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XML part: we can just ignore XML part in this case. it'll work with or without removing on_change property from corresponding XML record, this record just become redundant, but do not interfere normal execution of this overridden function.
Temur
Author
Note for Relational Fields: if any field from @api.onchange or @api.depends lists are relational fields, then in "section1" you have to use self.field_name.id or self.field_name.ids respectively for many2one and x2many fields. for example, if field_a has a type Many2one and field_b has a type one2many or many2many, then instead of:
orig_result = super(my_class,self).onchange_field_a(self.field_a, self.field_b, self.field_c)
we'll have: 
orig_result = super(my_class,self).onchange_field_a(self.field_a.id, self.field_b.ids, self.field_c)
Temur
Author
Return Value: as it's v8.0 style onchange function (@api.onchange), we do NOT need to return anything. the "orig_result" result of call parent function is just used in the "section 2", where it's turned into direct assignments. No need to return anything, "section 3" is just normal v8.0 @api.onchange code with direct assignments to fields of "self"
Gavin Yap

Hi Temur, I've tried to over the onchange methood fo hr.payslip 

 
    @api.onchange("contract_id")
    @api.depends( "date_from", "date_to", "employee_id", "contract_id")
    def onchange_field_a(self):
        print "do something"
it doesn't work for contract_id. Can you advise?
Temur
Author

if there is already a XML entry with on_change in a corresponding form view, then it may require to be replaced with on_change="1", Try to add attribute directly: <field name="contract_id" position="attributes"> <atribute name="on_change" >1</attribute></field>, if it does not work then you can simply replace entire contract_id field with itself, but with on_change="1" (pay attention to other properties in field definition as well, if there is any, in order to preserve them by the way)

Temur
Author

Here is original definition of contract_id in it's view: 

<field name="contract_id" domain="[('employee_id','=',employee_id),('date_start','&lt;=',date_to),'|',('date_end','&gt;=',date_from),('date_end','=',False)]" on_change="onchange_contract_id(date_from, date_to, employee_id, contract_id)" context="{'default_employee_id': employee_id}"/>
There is "domain" and "context" attributes as well, most probably you'll need to preserve them, but change the existing "on_change" property, in order to have the on_change="1" record in it's place. There is a few ways to do so. Here is one of the options, inherit the view and replace on_change property of contract_id (Note: please try the "attribute" way described in the previous comment first, it's interesting if that one will work), by replacing an entire field with itself, like:
<field name="contract_id" position="replace" >
<field name="contract_id" domain="[('employee_id','=',employee_id),('date_start','&lt;=',date_to),'|',('date_end','&gt;=',date_from),('date_end','=',False)]" on_change="1" context="{'default_employee_id': employee_id}" />
</field>