i tried this method but i get 0 as the value, i don't know why even though there are no errors.

Thank you for your help!

if you use compute field, return default value by your function
in your case _compute_t_overtime_w

for further assistant .give me your complete code of function _compute_t_overtime_w

Edit your code of _compute_t_overtime_w :

@api.multi
def _compute_t_overtime_w(self):
self.overtime_weekdays = sum((self.attendance_ids.mapped('overtime_50_numeric')))
if self.overtime_weakdays==0:
self.overtime_weakdays="your expected default value"

your code and question are confusing,
where is the field overtime_w in the given code??
get clarified to get solutions!!

'overtime_w' is on the same model as the 'overtime_w_edit' field.

Sorry i write it wrong:
@api.multi
def _compute_t_overtime_w(self):
self.overtime_w = sum((self.attendance_ids.mapped('overtime_50_numeric')))
This is the right code.