def test_change_forecasting_config_of_products(self):
    "Change forecasting configuration of products"
    forecast_group = self.Forecast_Group.search([('name', '=', 'C')])
    product_product = self.Product_Product.search([('name', '=', 'Large Cabinet')])
    product_product.forecast_group_id = forecast_group # enter the computing method 1st time
    self.assertEqual(
        [product_product.no_periods, product_product.period_type, product_product.first_date_of_period],
        [forecast_group.periods_to_forecast, forecast_group.period_type, forecast_group.first_date])

    product_product.auto_update = False # enter the computing method 2nd time
    

In the above code i am trying to edit the computed filed mannally in a test method

And this is the method using for computing

@api.depends('auto_update',
             'forecast_group_id',
             'forecast_group_id.period_type',
             'forecast_group_id.periods_to_forecast',
             'forecast_group_id.first_date')

def _compute_forecasting(self):
    # custom prefetch
    cache = {
        record['id']: {'first_date_of_period': record['first_date_of_period'],'period_type': record['period_type'], 'no_periods': record['no_periods']}
        for record in self.read(['first_date_of_period', 'period_type', 'no_periods'])
    }# removing this line will make the product variable below return to empty value
    for product in self:
        if product.auto_update:
            product.first_date_of_period = product.forecast_group_id.first_date
            product.period_type = product.forecast_group_id.period_type
            product.no_periods = product.forecast_group_id.periods_to_forecast

My computed fields:

period_type = fields.Selection(PeriodType.LIST_PERIODS, compute='_compute_forecasting', store=True)
no_periods = fields.Integer(string="Number of Periods", compute='_compute_forecasting', store=True)
first_date_of_period = fields.Datetime(compute='_compute_forecasting', store=True)

From what I know i should not be allowed to change the computed field manually without using inverse method, but when i tested this Odoo did not throw any error. Can anyone explain for me   ?

And there is another question, if i remove the "cache" variable, the second time the computing function triggered the new value of product that is changed in the first time the function is triggered is not return. Why does this happen ?