It takes a mapping of fields to values, used to initialize the record. For any field which has a default value and is not set through the mapping argument, the default value will be used.
Warning
while most value types are what would be expected (integer for Integer, a string for Char or Text),
Date, Datetime and Binary fields use string values
One2many and Many2many use a special command protocol detailed in the documentation for the write method.
POST /restapi/1.0/object/{object_name}?vals={values_for_the_object's_fields}
Request:
POST /restapi/1.0/object/res.partner?vals={'name':'Peter Mitchell','street':'31 Hong Kong street','city':'Taipei','zip':'106','country_id':482} HTTP/1.1
Host: <your Odoo server url>