Template:Category handler/doc

This is the category handler meta-template.

It helps other templates to automate both categorization and category suppression.

Already when used with its default settings this template prevents auto-categorization in some namespaces and on some pages where we usually don't want categorization. Thus even the most basic usage like " " sees to it that templates don't auto-categorize in the wrong places.

This template makes it easy to choose in what namespaces a template should and should not categorize. And it makes it easy to use different categories in different namespaces. And this template uses a central blacklist where we can add pages and page types where templates should not auto-categorize.

When not to use this template
If a template only needs to categorize in one of the namespaces main (articles), file (images) or category, then using this template is overkill. Then instead use one of main other, file other, or category other. But if your template needs to categorize in any other namespace, then we recommend you use this template, since it provides proper category suppression and makes it easy to select how to categorize in the different namespaces.

Namespaces
This template detects and groups all the different namespaces used on Wikipedia into several types. These types are used as parameter names in this template.


 * main = Main/article space, as in normal Wikipedia articles.
 * talk = Any talk space, such as page names that start with "Talk:", "User talk:", "File talk:" and so on.
 * user, wikipedia, file, mediawiki, template, help, category, portal and book = The other namespaces except the talk pages.
 * other = Any namespaces that were not specified as a parameter to the template. See examples below.

Basic usage
This template takes two or more parameters. Here's an example with the full template code for an article message box:

The above example uses the default settings for. That means the box will categorize on pages in the following namespaces:
 * main, file, help, category, portal and book

But it will not categorize in the following namespaces:
 * talk, user, wikipedia, mediawiki and template

And it will not categorize on blacklisted pages. (See section blacklist below.)

The reason this template does not categorize in some of the namespaces is that in those namespaces most templates are just demonstrated or listed, not used. Thus most templates should not categorize in those namespaces.

Any template that is meant for one or more of the namespaces where this template categorizes can use the basic syntax as shown above.

Advanced usage
This template takes one or more parameters named after the different page types as listed in section namespaces above. By using those parameters you can specify exactly in which namespaces your template should categorize. Like this:

The above box will only categorize in main and talk space. But it will not categorize on /archive pages since they are blacklisted. (See section blacklist below.) And if you need to demonstrate (discuss) that box on a talkpage, then you can feed " " to prevent that template from categorizing. (See section nocat below.) Like this:

Sometimes we want to use the same category in several namespaces, then do like this:

In the above example we use a numbered parameter to feed one of the categories, and then we tell this template to use that numbered parameter for both the help and user space.

This template understands the numbered parameters 1 to 10.

The other parameter defines what should be used in the remaining namespaces that have not explicitly been fed data.

Note the empty but defined talk parameter. That stops this template from showing what has been fed to the other parameter, when in talk space.

This template also has a parameter called all. It works like this:

The above example will categorize in all namespaces, but not on blacklisted pages. If you want to demonstrate that box on a page, then use " " to prevent the template from categorizing.

We suggest avoiding the all parameter, since templates should preferably only categorize in the namespaces they need to.

The all parameter can also be combined with the rest of the parameters. Like this:

If the above box is placed on an article, then it will add the categories "Somecat1" and "Somecat2". But on all other types of pages it will instead add "Somecat1" and "Somecat3". As the example shows, the all parameter works independently of the rest of the parameters.

Subpages
This template understands the subpage parameter. Like this:

If " " then this template will not categorize on subpages. For the rare occasion you only want to categorize on subpages, then use " ". If subpage is empty or undefined then this template categorizes both on basepages and on subpages.

Blacklist
This template has a blacklist of the pages and page types where templates should not auto-categorize. Thus templates that use this meta-template will for instance not categorize on /archive pages and on the subpages of Template messages.

If you want a template to categorize on a blacklisted page, then feed " " to the template when you place it on the page, thus skipping the blacklist check. Note that this template only categorizes if it has data for the namespace. For instance, if the basic syntax is used (see basic usage above), then even if you set " " the template will not categorize on a talk page, since it has no data for talk pages. But it has data for help space, so on a blacklisted help page it will categorize.

The blacklist is in the sub-template category handler/blacklist. To see or update the blacklist, go there.

The "nocat" parameter
This template understands the nocat parameter:


 * If " " then this template does not categorize.
 * If nocat is empty or undefined then this template categorizes as usual.
 * If " " this template categorizes even when on blacklisted pages. (See section blacklist above.)

Templates that use should forward nocat, so they too understand nocat. The code " " shown in the examples on this page does that.

The "categories" parameter
For backwards compatibility this template also understands the categories parameter. It works the same as nocat. Like this:


 * If " " then this template does not categorize.
 * If categories is empty or undefined then this template categorizes as usual.
 * If " " this template categorizes even when on blacklisted pages.

When adding this template to a template that already uses the " " logic, then you can do the following to not break any existing usage:

The "category2" parameter
For backwards compatibility this template kind of supports the old "category =" parameter. But the parameter name "category" is already used in this template to feed category data for when in category space. So instead this template uses category2 for the usage similar to nocat. Like this:


 * If " " (empty but defined), or " ", or if category2 is fed any other data (except as described in the next two points), then this template does not categorize.
 * If category2 is undefined or if " ", then this template categorizes as usual.
 * If " " this template categorizes even when on blacklisted pages.

When adding this template to a template that already uses the " " logic, then you can do like this to not break any existing usage:

Note that the " " is necessary, it helps this template to detect if the category parameter is defined but empty, or undefined.

Categories and text
Besides from categories, you can feed anything else to this template, for instance some text. Like this:

When the template above is shown on anything else than a talk page, it will look like this (note the text below the box):

That text will not show on blacklisted pages, so don't use this method to show any important information. Feeding " " to the template hides the text, just as it suppresses any categories.

The "page" parameter
For testing and demonstration purposes this template can take a parameter named page. Like this:

In the above code we on purpose left out the brackets around the category names so we see the output on the page. No matter on what kind of page the code above is used it will return this:



The page parameter makes this template behave exactly as if on that page. Even the blacklist works. The pagename doesn't have to be an existing page.

If the page parameter is empty or undefined, the name of the current page determines the result.

You can make it so your template also understands the page parameter. That means you can test how your template will categorize on different pages, without having to actually edit those pages. Then do like this:

Parameters
List of all parameters:

Note that empty values to the "main" ... "other" parameters have special meaning (see examples above). The "all" parameter doesn't understand numbered parameters, since there should never be a need for that.

Technical details
The centralised category suppression blacklist is in the sub-template category handler/blacklist. To see or update the blacklist, go there.

For its internal parameter processing, this template uses the sub-template category handler/numbered.

For more technical details see the talk page.