try to compute the days differance 

error:

raise exception.with_traceback(None) from new_cause
AttributeError: 'test.testing' object has no attribute '_compute_p_days'

py:

from datetime import date, timedelta
from odoo.exceptions import ValidationError
from odoo import api, fields, models


class test_testing(models.Model):
    _name = 'test.testing'
    _description = 'test code'

    start_date = fields.Date(string = 'Start Date',
                             required = True)
    end_date = fields.Date(string = 'End Date',
                           required = True)
    p_days = fields.Float(string = 'Plan Days',
                          compute = '_compute_p_days',
                          default = lambda self: self._compute_p_days)
    g_days = fields.Float(string = 'Days Gone',
                          compute = '_compute_g_days',
                          default = lambda self: self._compute_g_days)


@api.depends('start_date')
def _compute_g_days(self):
    self.g_days = 0
    for record in self:
        if record.start_date > date.today():
            raise ValidationError('Start date should not greater than end date.')
        else:
            record.g_days = (date.today() - record.start_date).days


@api.depends('start_date', 'end_date')
def _compute_p_days(self):
    self.p_days = 0
    for record in self:
        if record.start_date > record.end_date:
            raise ValidationError('Start date should not greater than end date.')
        else:
            record.p_days = (record.end_date - record.start_date).days

XML:

xml version="1.0" encoding="utf-8"?>


    id="test_testing_view_form" model="ir.ui.view">
        name="name">test.testing.form
        name="model">test.testing
        name="arch" type="xml">
            string="Testing Code">
                
                    string="Information">
                        name="start_date" type="date"/>
                        name="end_date" type="date"/>
                        name="p_days"/>
                        name="g_days"/>
                    
                
            
        
    

    id="test_testing_tree" model="ir.ui.view">
        name="name">test.testing.tree
        name="model">test.testing
        name="arch" type="xml">
            string="Testing">
                name="start_date" type="date"/>
                name="end_date" type="date"/>
                name="p_days"/>
                name="g_days"/>
            
        
    

    id="test_testing_action" model="ir.actions.act_window">
        name="name">testing Code
        name="type">ir.actions.act_window
        name="res_model">test.testing
        name="view_mode">tree,form
        name="context">{}
        name="domain">[]
        name="help" type="html">
            
class="o_view_nocontent_smiling_face">
                Create your first project!
            

        
    

    
    id="menu_test"
              name="test"
              parent="test_root"
              action="test_testing_action"
              sequence="20"
    />