Wikipedia:Substitution
 | This page documents an English Wikipedia editing guideline. It is a generally accepted standard that editors should attempt to follow, though it is best treated with common sense, and occasional exceptions may apply. Any substantive edit to this page should reflect consensus. When in doubt, discuss first on the talk page. |
 | This page in a nutshell: Substitution is the one-time, permanent copying of template content on Wikipedia. It differs from transclusion, which continually updates as the linked-to content changes. |
Substitution is a different way of handling templates than normally done on Wikipedia. Usually, templates are transcluded, which means that if the template changes, every page that uses it will update to comply with the change-in-question. This is the procedure used for most templates. However, some templates are instead substituted. This means that if the template changes, the page that uses it will not, and will continue to show the old version of the template. Some templates should always be substituted, while most should always be transcluded.
To substitute, add subst: after the opening braces, as in {{subst:page name}}. For example: substitution {{subst:Medicine}} in contrast to transclusion {{Medicine}}. The contents of Template:Medicine will be expanded (for any templates) and stored at each point where "{{subst:Medicine}}" is placed in a page. Note that ref tags refuse to run "subst:" unless temporarily renamed as "<xref name=xx>" or similar (see the bug report at T4700).
This page also lists templates that should either always or never be substituted except in the Wikipedia namespace. Automated tools (a.k.a. bots) that do such replacements will never be used on the original template. If you are going to use a bot to substitute templates, please read through the talk page first, as many are under dispute or change status over time, and substitution is permanent.
About subst:
[mali mi di yibu sheena n-niŋ]Definition
[mali mi di yibu sheena n-niŋ]The subst: keyword (short for "substitution") is used inside template code, placed as a prefix before the name of the template but within the curly braces ({{}}). It changes the way the MediaWiki software expands the template.
Ordinarily, a template will be expanded "on the fly"; that is, the template code on a page calls a separate page every time it is rendered. Although most page views are served from the cache, pages need to be rendered for previews and rendered again when the page changes. When someone is editing a page with a normal template call, they see the template call.
Placing "subst:" inside the curly braces tells the software to permanently replace the template with the text of the template (i.e., the text that is on the template's article page when the template is added to the page). Therefore, {{template}} becomes {{subst:template}}, until you save the page. The next editor sees not the template call, but instead the text of the template when you saved; it does not change even if the original template is edited.
Usage
[mali mi di yibu sheena n-niŋ]You can substitute a template tag by adding "subst:" to it. For example, use {{subst:test5}} instead of {{test5}}.
When this is a major component of an edit, it is strongly suggested that the template be mentioned in the edit summary (for instance, put "{{subst:test}}", "subst:test" or "test1 applied" in your edit summary) so other editors can easily see what you have done.
Additionally, when creating or editing templates that are commonly substituted, adding a hidden comment to the template page helps article editors to see how templates are being used (e.g. <!-- Substituted from Template:Documentation -->). Use {{subst:html comment}} if you want to write HTML comments that contain wikitext expansions.
Benefits
[mali mi di yibu sheena n-niŋ]- If there is a template you would like to modify for a single occasion, but you do not want to modify the template for all the pages that use it, and you do not want to make a new template, you can substitute the template and then with a second edit, make the modifications. For example, a template might be used with all the states of the United States and with the District of Columbia. The template might be worded correctly for the states, but not for DC. To fix the wording, you can substitute the template and then fix the wording.
- Substituting frequently used, but unprotected templates, limits the damage that can be caused by people vandalizing the template. See high-risk templates.
- Transcluding certain templates hides wikitext from newcomers, whereas substituting makes the wikitext visible and gives them the opportunity to learn its use (of course, there is a "how to edit" link at the bottom of each edit page).
- Substitution allows for recursive evaluation through macro templates.
Neutral
[mali mi di yibu sheena n-niŋ]- Substituting en masse may ultimately speed up the site, but this is not a reason to prefer substitution over transclusion. Don't worry about performance of Wikipedia's servers.