Hello Michele, 

Default value is not working because this field is read only type and while creating any record this field is not updating inside the database.

So you can update your code like that,

code = fields.Char ('Code', size = 32, required = True, readonly = True, default = lambda self: _ ('New'), track_visibility = 'onchange')
then inherit the create method

    @ api.model

    def create (self, waltz):

        if waltz:

            waltz ['code'] = self.env ['ir.sequence']. next_by_code ('sprogroup.purchase.request') or _ ('New')

        return super (YourClass, self) .create (waltz)

I hope your query is resolved by now. 

Thanks

Anisha Bahukhandi

Technical Content Writer

Webkul