Hi

Try the following code,

from datetime import datetime

from_date = fields.Date(string="Register date")
final_date = fields.Date(string="Last date")
total_days = fields.Integer(string="TOTAL DAYS")

@api.onchange('from_date', 'final_date', 'total_days')
def calculate_date(self):
if self.from_date and self.final_date:
d1 = datetime.strptime(str(self.from_date), '%Y-%m-%d')
d2 = datetime.strptime(str(self.final_date), '%Y-%m-%d')
d3 = d2 - d1
self.total_days = str(d3.days)

Hope it helps