I use below code for apply globally discount in Purchase Order. Please guide me if any error in this code

class PurchaseOrder(models.Model): 
_inherit = "purchase.order"

@api.depends('order_line.price_subtotal')
def _amount_all(self):
"""
Compute the total amounts of the SO.
"""
for order in self:
amount_untaxed = amount_tax = amount_discount = 0.0
for line in order.order_line:
amount_untaxed += line.price_subtotal
amount_tax += line.price_tax
amount_discount += (line.product_qty * line.price_unit * line.discount)/100
order.update({
'amount_untaxed': order.currency_id.round(amount_untaxed),
'amount_tax': order.currency_id.round(amount_tax),
'amount_discount': order.currency_id.round(amount_discount),
'amount_total': amount_untaxed + amount_tax,
})

discount_type = fields.Selection([('percent', 'Percentage'), ('amount', 'Amount')], string='Discount type',
readonly=True,states={'draft': [('readonly', False)], 'sent': [('readonly', False)]},
default='percent')
discount_rate = fields.Float('Discount Rate', digits_compute=dp.get_precision('Account'),
readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]})
amount_untaxed = fields.Monetary(string='Untaxed Amount', store=True, readonly=True, compute='_amount_all',
track_visibility='always')
amount_tax = fields.Monetary(string='Taxes', store=True, readonly=True, compute='_amount_all',
track_visibility='always')
amount_total = fields.Monetary(string='Total', store=True, readonly=True, compute='_amount_all',
track_visibility='always')
amount_discount = fields.Monetary(string='Discount', store=True, readonly=True, compute='_amount_all',
digits_compute=dp.get_precision('Account'), track_visibility='always')

@api.onchange('discount_type', 'discount_rate', 'order_line')
def supply_rate(self):
for order in self:
if order.discount_type == 'percent':
for line in order.order_line:
line.discount = order.discount_rate
else:
total = discount = 0.0
for line in order.order_line:
total += round((line.product_qty * line.price_unit))
if order.discount_rate != 0:
discount = (order.discount_rate / total) * 100
else:
discount = order.discount_rate
for line in order.order_line:
line.discount = discount

@api.multi
def _prepare_invoice(self,):
invoice_vals = super(PurchaseOrder, self)._prepare_invoice()
invoice_vals.update({
'discount_type': self.discount_type,
'discount_rate': self.discount_rate
})
return invoice_vals

@api.multi
def button_dummy(self):
self.supply_rate()
return True

class AccountTax(models.Model): 
_inherit = 'account.tax'

@api.multi
def compute_all(self, price_unit, currency=None, product_qty=1.0, product=None, partner=None):
print "hello"
if len(self) == 0:
company_id = self.env.user.company_id
else:
company_id = self[0].company_id
if not currency:
currency = company_id.currency_id
taxes = []
prec = currency.decimal_places
round_tax = False if company_id.tax_calculation_rounding_method == 'round_globally' else True
round_total = True
if 'round' in self.env.context:
round_tax = bool(self.env.context['round'])
round_total = bool(self.env.context['round'])

if not round_tax:
prec += 5
# total_excluded = total_included = base = round(price_unit * quantity, prec)
total_excluded = total_included = base = (price_unit * product_qty)

for tax in self.sorted(key=lambda r: r.sequence):
if tax.amount_type == 'group':
ret = tax.children_tax_ids.compute_all(price_unit, currency, product_qty, product, partner)
total_excluded = ret['total_excluded']
base = ret['base']
total_included = ret['total_included']
tax_amount = total_included - total_excluded
taxes += ret['taxes']
continue

tax_amount = tax._compute_amount(base, price_unit, product_qty, product, partner)
if not round_tax:
tax_amount = round(tax_amount, prec)
else:
tax_amount = currency.round(tax_amount)

if tax.price_include:
total_excluded -= tax_amount
base -= tax_amount
else:
total_included += tax_amount

tax_base = base

if tax.include_base_amount:
base += tax_amount

taxes.append({
'id': tax.id,
'name': tax.with_context(**{'lang': partner.lang} if partner else {}).name,
'amount': tax_amount,
'sequence': tax.sequence,
'account_id': tax.account_id.id,
'refund_account_id': tax.refund_account_id.id,
'analytic': tax.analytic,
'base': tax_base,
})
print "total_excluded:",total_excluded
print "total_included:",total_included
return {
'taxes': sorted(taxes, key=lambda k: k['sequence']),
'total_excluded': total_excluded,
'total_included': total_included,
'base': base,
} 

class PurchaseOrderLine(models.Model):
_inherit = "purchase.order.line"

discount = fields.Float(string='Discount (%)', default=0.0)