I've found the solution by observing the View Field item in the Debug menu.

I've discovered a city_id hide parameter. So:

<xpath expr="//div//field[@name='street2']" position="after">

                    

<

field name="city_id" invisible="1" /

>



               

<

/xpath

>



               

<

field name="city" position="attributes"

>



                    

<

attribute name="attrs"

>

{'readonly': ['|','&', ('type', '=', 'contact'), ('parent_id', '!=', False), ('x_some_field', '!=', 0)]}

<

/attribute

>



               

<

/field

>