Added first example to plugin unit test (#8636)

* Start documentation chapter for plugin tests

* Added env variables to the doc

* Fix style errors

* Further style bugs

* Reformat environment variables

* Reformat environment variables

* Add comments from wolflu05

* Add text to the intro

* Added first example the plugin unit test

* Addred line it function

* Typo

* Typo

* Typo

docs/docs/extend/plugins/test.md

@@ -0,0 +1,81 @@
+---
+Title: Unit Tests
+---
+
+## Unit Tests
+For complicated plugins it makes sense to add unit tests the code to ensure
+that plugins work correctly and are compatible with future versions too.
+You can run these tests as part of your ci against the current stable and
+latest tag to get notified when something breaks before it gets released as
+part of stable. InvenTree offers a framework for testing. Please refer
+to [Unit Tests](../../develop/contributing.md) for more information.
+
+### Prerequisites
+For plugin testing the following environment variables must be set to True:
+
+| Name | Function | Value |
+| --- | --- | --- |
+| INVENTREE_PLUGINS_ENABLED | Enables the use of 3rd party plugins | True |
+| INVENTREE_PLUGIN_TESTING | Enables enables all plugins no matter of their active state in the db or built-in flag | True |
+| INVENTREE_PLUGIN_TESTING_SETUP | Enables the url mixin | True |
+
+### Test program
+
+A file called test_plugin_name.py should be added to the plugin directory. It can have the
+following structure:
+
+```
+# Basic unit tests for the plugin
+from InvenTree.unit_test import InvenTreeTestCase
+
+class TestMyPlugin(InvenTreeTestCase):
+    def test_my_function(self):
+        do some work here...
+```
+
+The test can be executed using invoke:
+
+```
+invoke dev.test -r module.file.class
+```
+
+Plugins are usually installed outside of the InventTree directory, e.g. in .local/lib/...
+I that case module must be omitted.
+
+```
+invoke dev.test -r plugin_directory.test_plugin_name.TestMyPlugin
+```
+
+### do some work here... A simple Example
+A simple example is shown here. Assume the plugin has a function that converts a price string
+that comes from a supplier API to a float value. The price might have the form "1.456,34 €".
+It can be different based on country and local settings.
+The function in the plugin will convert it to a float 1456.34. It is in the class MySupplier
+and has the following structure:
+
+```
+class MySupplier():
+
+    def reformat_price(self, string_price):
+
+        ...
+        return float_price
+```
+
+This function needs to be tested. The test can look like this:
+
+```
+from .myplugin import MySupplier
+
+def test_reformat_price(self):
+
+    self.assertEqual(MySupplier.reformat_price(self, '1.456,34 €'), 1456.34)
+    self.assertEqual(MySupplier.reformat_price(self, '1,45645 €'), 1.45645)
+    self.assertEqual(MySupplier.reformat_price(self, '1,56 $'), 1.56)
+    self.assertEqual(MySupplier.reformat_price(self, ''), 0)
+    self.assertEqual(MySupplier.reformat_price(self, 'Mumpitz'), 0)
+```
+
+The function assertEqual flags an error in case the two arguments are not equal. In equal case
+no error is flagged and the test passes. The test function tests five different
+input variations. More might be added based on the requirements.

docs/mkdocs.yml

@@ -199,6 +199,7 @@ nav:
       - Developing a Plugin: extend/how_to_plugin.md
       - Model Metadata: extend/plugins/metadata.md
       - Tags: extend/plugins/tags.md
+      - Unit Test: extend/plugins/test.md
     - Plugin Mixins:
       - Action Mixin: extend/plugins/action.md
       - API Mixin: extend/plugins/api.md