I have a field email and a field starting_day. I would like starting_day to be required if email has been selected. My model:

class sales_order_line_item_email(osv.Model):
    _name = 'sale.order.line'
    _inherit = 'sale.order.line'
    _columns = {
        'email_chain': fields.many2one(
            'sale.order_email.collection',
            'Email Chain',
            required=False
        ),
        'starting_day': fields.date('Starting Day', required=False),
    }