WP:Template test cases

Templates are a very powerful feature of MediaWiki, but mistakes can be easily made, even by experienced users. Complex templates should therefore be accompanied by sandboxes and test cases to prevent bugs.
== For what kind of templates ==
It is recommended to apply the test case pattern to templates using ParserFunctions that can generate very different looking output, such as the ones that take many parameters, or the ones that have many branches of the #switch operator.
For navigational boxes, of which the outputs are almost identical on each transcluding page, the benefit may not outweight the time and effort.
Any template that is transcluded in thousands of pages should of course be tested before an edit is made.
== Prerequisite ==
The template must be properly documented following Wikipedia:Template documentation.
Furthermore, the template should not have too many interleaving <includeonly> and <noinclude> blocks. Ideally there is only one <noinclude> block, placed at the bottom of the template, and containing the template.
== How to create /sandbox and /testcases subpages ==
1: Suppose your template is named Template:X. Create a subpage with the name Template:X/sandbox and another with Template:X/testcases. When these subpages are present, the green /doc box produced by automatically provide links to them. Note that "/sandbox" and "/testcases" are spelled in all lower-case.
2: Copy only the code (everything that is not surrounded by <noinclude>) from your Template:X to Template:X/sandbox. And add the following at the end of the last line, in the same line:
:<noinclude>{{template sandbox notice}}</noinclude>
In other words, the result should be:
:[-- Template code --]<noinclude>{{template sandbox notice}}</noinclude>
Note! The template code must start directly before and physically touch the <noinclude> tag. Any blank space or newlines between the template code and the <noinclude> tag can cause the template to render unwanted spaces or newlines when used.
3: Copy a few transcluding examples from articles ({{X | .... }}) to your Template:X/testcases. In the test case subpage, duplicate each example, and for one copy of each example, replace X with X/sandbox. This is done so that you may compare the old and new outputs side by side in the future. Lastly, add the following line at the beginning of the test case page. It does not matter how many blank lines you put around it:
:{{template test cases notice}}
4: Thus Template:X/testcases will look like this:
 {{template test cases notice}}
 {{X | foo}}
 {{X/sandbox | foo}}
 {{X | bar}}
 {{X/sandbox | bar}}
4b: To make differences easier to identify, you can use Template:Testcase to lay out your test cases. Then, test cases which are changed in the sandbox are highlighted so you can double check that the change was intentional.
== How to use them subsequently ==
* Sync sandbox from the real template (i.e. step 2 above)
* Have fun editing the sandbox
* Verify the test cases
* If everything looks good, copy the code from the sandbox to the real template
If you chose not to compare the new and old outputs side by side described at the last bullet point of the previous section, it is recommended to use a tabbed browser and open the test case subpage before changing the sandbox, and another after changing the sandbox. This can ensure that the differences are compared accurately.
== Example ==
* 
* 
== See also ==
* Wikipedia:High-risk templates
* – To put at the bottom of /sandbox pages.
* – To put on top of /testcases pages.
ja:Wikipedia:テンプレートのサンドボックスとテストケース
pt:Predefinição:Teste