[IMP] purchase: Extract code from _compute_amount to allow inheritance

Purpose
=======

Currently the discount mechanism is not implemented in the purchase module.
This limitation is could be avoided if the _compute_amount arguments could
be overwritten.

While waiting the feature to be implemented in master, we simply introduce
a hook method to allow an easy inheritance of the argument values.

Closes #11235

PS: Also move compute methods under the fields definition to follow
    the coding guidelines

addons/purchase/models/purchase.py

@@ -402,36 +402,6 @@ class PurchaseOrderLine(models.Model):
     _description = 'Purchase Order Line'
     _order = 'order_id, sequence, id'
 
-    @api.depends('product_qty', 'price_unit', 'taxes_id')
-    def _compute_amount(self):
-        for line in self:
-            taxes = line.taxes_id.compute_all(line.price_unit, line.order_id.currency_id, line.product_qty, product=line.product_id, partner=line.order_id.partner_id)
-            line.update({
-                'price_tax': sum(t.get('amount', 0.0) for t in taxes.get('taxes', [])),
-                'price_total': taxes['total_included'],
-                'price_subtotal': taxes['total_excluded'],
-            })
-
-    @api.multi
-    def _compute_tax_id(self):
-        for line in self:
-            fpos = line.order_id.fiscal_position_id or line.order_id.partner_id.property_account_position_id
-            # If company_id is set, always filter taxes by the company
-            taxes = line.product_id.supplier_taxes_id.filtered(lambda r: not line.company_id or r.company_id == line.company_id)
-            line.taxes_id = fpos.map_tax(taxes, line.product_id, line.order_id.partner_id) if fpos else taxes
-
-    @api.depends('invoice_lines.invoice_id.state', 'invoice_lines.quantity')
-    def _compute_qty_invoiced(self):
-        for line in self:
-            qty = 0.0
-            for inv_line in line.invoice_lines:
-                if inv_line.invoice_id.state not in ['cancel']:
-                    if inv_line.invoice_id.type == 'in_invoice':
-                        qty += inv_line.uom_id._compute_quantity(inv_line.quantity, line.product_uom)
-                    elif inv_line.invoice_id.type == 'in_refund':
-                        qty -= inv_line.uom_id._compute_quantity(inv_line.quantity, line.product_uom)
-            line.qty_invoiced = qty
-
     name = fields.Text(string='Description', required=True)
     sequence = fields.Integer(string='Sequence', default=10)
     product_qty = fields.Float(string='Quantity', digits=dp.get_precision('Product Unit of Measure'), required=True)
@@ -465,6 +435,57 @@ def _compute_qty_invoiced(self):
     currency_id = fields.Many2one(related='order_id.currency_id', store=True, string='Currency', readonly=True)
     date_order = fields.Datetime(related='order_id.date_order', string='Order Date', readonly=True)
 
+    @api.depends('product_qty', 'price_unit', 'taxes_id')
+    def _compute_amount(self):
+        for line in self:
+            vals = line._prepare_compute_all_values()
+            taxes = line.taxes_id.compute_all(
+                vals['price_unit'],
+                vals['currency_id'],
+                vals['product_qty'],
+                vals['product'],
+                vals['partner'])
+            line.update({
+                'price_tax': sum(t.get('amount', 0.0) for t in taxes.get('taxes', [])),
+                'price_total': taxes['total_included'],
+                'price_subtotal': taxes['total_excluded'],
+            })
+
+    def _prepare_compute_all_values(self):
+        # Hook method to returns the different argument values for the
+        # compute_all method, due to the fact that discounts mechanism
+        # is not implemented yet on the purchase orders.
+        # This method should disappear as soon as this feature is
+        # also introduced like in the sales module.
+        self.ensure_one()
+        return {
+            'price_unit': self.price_unit,
+            'currency_id': self.order_id.currency_id,
+            'product_qty': self.product_qty,
+            'product': self.product_id,
+            'partner': self.order_id.partner_id,
+        }
+
+    @api.multi
+    def _compute_tax_id(self):
+        for line in self:
+            fpos = line.order_id.fiscal_position_id or line.order_id.partner_id.property_account_position_id
+            # If company_id is set, always filter taxes by the company
+            taxes = line.product_id.supplier_taxes_id.filtered(lambda r: not line.company_id or r.company_id == line.company_id)
+            line.taxes_id = fpos.map_tax(taxes, line.product_id, line.order_id.partner_id) if fpos else taxes
+
+    @api.depends('invoice_lines.invoice_id.state', 'invoice_lines.quantity')
+    def _compute_qty_invoiced(self):
+        for line in self:
+            qty = 0.0
+            for inv_line in line.invoice_lines:
+                if inv_line.invoice_id.state not in ['cancel']:
+                    if inv_line.invoice_id.type == 'in_invoice':
+                        qty += inv_line.uom_id._compute_quantity(inv_line.quantity, line.product_uom)
+                    elif inv_line.invoice_id.type == 'in_refund':
+                        qty -= inv_line.uom_id._compute_quantity(inv_line.quantity, line.product_uom)
+            line.qty_invoiced = qty
+
     @api.model
     def create(self, values):
         line = super(PurchaseOrderLine, self).create(values)