Don't set missing attribute on line in _on_change_lines_taxes

because it is sent back to the client as changed values, instead improve
taxable_lines to work with missing values

issue5103
review22691002

invoice.py

@@ -452,13 +452,6 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
         if self.lines:
             for line in self.lines:
                 self.untaxed_amount += getattr(line, 'amount', None) or 0
-                for attribute, default_value in (
-                        ('taxes', []),
-                        ('unit_price', Decimal(0)),
-                        ('quantity', 0.),
-                        ):
-                    if not getattr(line, attribute, None):
-                        setattr(line, attribute, default_value)
             computed_taxes = self._get_taxes()
 
         def is_zero(amount):
@@ -733,8 +726,22 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
 
     @property
     def taxable_lines(self):
-        return [(l.taxes, l.unit_price, l.quantity) for l in self.lines
-            if l.type == 'line']
+        taxable_lines = []
+        # In case we're called from an on_change we have to use some sensible
+        # defaults
+        for line in self.lines:
+            if getattr(line, 'type', None) != 'line':
+                continue
+            taxable_lines.append(tuple())
+            for attribute, default_value in [
+                    ('taxes', []),
+                    ('unit_price', Decimal(0)),
+                    ('quantity', 0.),
+                    ]:
+                value = getattr(line, attribute, None)
+                taxable_lines[-1] += (
+                    value if value is not None else default_value,)
+        return taxable_lines
 
     @property
     def tax_type(self):