[IMP] Taxes: changes specific to 18.0

- changes for Python taxes
- changes to the `Included in Price` setting

Author: antoine162

content/applications/finance/accounting/taxes.rst

@@ -81,7 +81,7 @@ price. The following options are available:
   tax-excluded sales price
 - :ref:`Percentage of Price Tax Included <taxes/computation/percentage-of-price-tax-included>`: a
   percentage of the tax-included total
-- :ref:`Python Code <taxes/computation/python-code>`: a custom user-defined formula
+- :ref:`Custom Formula <taxes/computation/python-code>`: a custom user-defined formula
 
 .. _taxes/computation/group-of-taxes:
 
@@ -217,34 +217,45 @@ which determines whether the sales price should be treated as tax-excluded or ta
 
 .. _taxes/computation/python-code:
 
-Python code
-***********
+Custom formula
+**************
 
 .. important::
    If a tax can be expressed as a multiple of the quantity of the product to which it applies, it
    can be defined as a :ref:`Fixed <taxes/computation/fixed>` tax. Doing so is strongly recommended
-   over defining a :guilabel:`Python Code` tax.
+   over defining a :guilabel:`Custom Formula` tax.
+
+.. note::
+   To use :guilabel:`Custom Formula` taxes, :ref:`install <general/install>` the :guilabel:`Define
+   Taxes as Python Code` (`account_python_tax`) module.
 
-A tax defined as :guilabel:`Python Code` consists of two snippets of Python code that are executed
-in a local environment that can access the unit price, quantity, product, and partner.
-:guilabel:`Python Code` defines the amount of the tax, and :guilabel:`Applicable Code` defines
-whether the tax is applied. Enter a formula for each field at the bottom of the
-:guilabel:`Definition` tab.
+For a :guilabel:`Custom Formula` tax, the tax amount is computed according to a Python expression
+defined in the :guilabel:`Formula` field. The Python expression may contain the following tokens:
 
-.. example::
-   A product has a sales price of $1000, and we apply a :guilabel:`Python Code` tax with the
-   following configuration:
+- any of the following variables:
 
-   - :guilabel:`Python Code`: `result = price_unit * 0.10` and
-   - :guilabel:`Applicable Code`: `result = True`.
+  - `price_unit`: the unit price of the product
+  - `base`: the taxable basis on which the tax is applied - may differ from the `price_unit` if
+    other taxes are applied first
+  - `quantity`: the quantity of the product
+  - `product`: the product record - product fields can also be accessed
+
+- integers and floating-point numbers
+
+- the following permitted tokens: `(`, `)`, `+`, `-`, `*`, `/`, `,`, `<`, `>`, `<=`, `>=`, `and`,
+  `or`, `None`, `min`, and `max`
+
+.. example::
+   A product has a sales price of $1000, and we apply a :guilabel:`Custom Formula` tax with a
+   :guilabel:`Formula` of `max(base, 500) * 0.10 + min(base - 500, 0) * 0.20`
 
    We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
    | sales price | without tax |          |          |
    +=============+=============+==========+==========+
-   | 1,000       | 1,000       | 100      | 1,100.00 |
+   | 1,000       | 1,000       | 150      | 1,150.00 |
    +-------------+-------------+----------+----------+
 
 .. _taxes/active:
@@ -365,10 +376,21 @@ invoice line.
 Included in price
 ~~~~~~~~~~~~~~~~~
 
-With this option activated, the tax will treat the sales price on which it is applied as a total
-including the tax amount. The tax computation will split the sales price into a base amount and a
-tax amount. This makes it suitable for B2C sales in most countries where prices are quoted
-tax-inclusive.
+.. tip::
+   To set a company-wide default for this setting, go to :menuselection:`Accounting -->
+   Configuration --> Settings`, find the :guilabel:`Taxes` section, and set the :guilabel:`Prices`
+   setting to :guilabel:`Tax Excluded` or :guilabel:`Tax Included`. This setting cannot be changed
+   once invoices have been created.
+
+When set to :guilabel:`Tax Included`, the tax will always treat the sales price on which it is
+applied as a total including the tax amount. The tax computation will split the sales price into a
+base amount and a tax amount. This makes it suitable for B2C sales in most countries where prices
+are quoted tax-inclusive.
+
+When set to :guilabel:`Tax Excluded`, the tax will always treat the sales price on which it is
+applied as a price excluding the tax amount.
+
+When set to :guilabel:`Default`, the company-wide default will be used.
 
 `Total = Sales Price = Computed Tax-Excluded price + Tax`
 
@@ -387,8 +409,6 @@ tax-inclusive.
    For a guide on configuring tax-excluded and tax-included prices for B2B and B2C customers,
    see :doc:`taxes/B2B_B2C`.
 
-   .. image:: taxes/toggle-button.png
-
 .. _taxes/base-subsequent:
 
 Affect base of subsequent taxes