Help:Template

A template is a Wikipedia page created to be included in other pages. Templates usually contain repetitive material that might need to show up on any number of articles or pages. They are commonly used for boilerplate messages, standard warnings or notices, infoboxes, navigational boxes, and similar purposes.

The most common method of inclusion is called transclusion]], where the wikitext of the target page contains a reference to the template, using the  syntax. Another method is substitution, where the content of the template is copied into the wikitext of the target page, just once, when it is saved.

General description
Most templates are pages in the Template namespace, which means that they have titles in the form "Template:XXXX". It is possible, however, to transclude and substitute from any namespace, and so some template pages are placed in other namespaces, such as the User namespace. Template pages have associated talk pages.

Templates can contain any desired wikitext, including calls to other templates. They have some limited programming capacities: customizable values (called parameters), calculation and branchings (using parser functions), and access to wiki-specific variables (magic words), such as dates, times, and page names. They may also contain tags which define which parts of the wikitext are to be included when the template is transcluded or substituted. This means that the appearance of the template page itself need not be the same as that of the transcluded content (for example, it can contain documentation, categories, etc. for the template).

How to do it: To transclude a template into an article or page, type  in the wikitext at the place where the template is to appear. The first letter may be indifferently lower- or upper-case.

The prefix  before the template name is the default one and need not be included. However, for templates stored in other namespaces, the prefix, such as, must be specified. To transclude a page in mainspace, precede its title with a colon, as.

Note: Attempting to transclude a template that does not exist produces a red link, just like linking to any other nonexistent page. Following the link allows one to create that particular template. It is not possible to transclude pages between projects – to use a template on another language project, a copy of the template must be created in that project.

Parameters
The basic transclusion syntax given above can be extended by the addition of parameters, which are used to control the template's output. The syntax for this is where each "parameter" may either contain just a value (these are called unnamed parameters) or be of the form  (named parameters). The first, second, etc. unnamed parameters are equivalent to parameters named "1", "2", etc.

Whitespace characters (spaces, tabs, returns) are stripped from the beginnings and ends of named parameter names and values, but not from the middle: thus  has the same effect as. This does not apply to unnamed parameters, where the whitespace characters are preserved.

What parameters (if any) can or should be passed to a template, and how they are to be named, depends on the coding of that template. Named parameters can be defined in any order. Superfluous or misnamed parameters will be ignored; undefined parameters will be assigned default values. If a parameter is defined more than once, the last value takes effect.

The value of a parameter can be the empty string (pipe or equals sign followed immediately by the next pipe or the closing braces). This is different from omitting the parameter altogether, which leaves it undefined, although templates are often coded so as to behave the same in both cases.

Substitution
If a template is to be substituted in the wikitext rather than transcluded, add the modifier  after the initial pair of braces, as in. Parameters can be added normally.

If the page to be substituted is in  mainspace, the word "subst" must be followed by two colons, e.g:

Other details
Template names are exactly like other page names: case-sensitive except for the first letter, with spaces indistinguishable from underscores. If the symbol # (normally used to link to a section of a page) appears in a transclusion, then it and any characters that follow it are ignored. If a redirect is transcluded, the redirect's target will be transcluded instead.

Notice that the same double-brace syntax is used for many MediaWiki variables and parser functions (see Help:Magic words). For example, the code  may look like a template call, but it is actually a variable whose value is the namespace prefix of the current page.

Basic template usage examples
If you wish to experiment with any of these, please use the template sandbox.

An example of a very simple template can be found at Template:Lambda, which places the lambda symbol, λ. Click the "Edit" tab on that template page to see the template code (its wikitext). The "active" part of that code, called the expansion of the template, is. (The remainder of the wikitext is enclosed between ... tags, which means that it is displayed on the template page itself, but will not be included along with the template.)

To transclude the Template:Lambda onto another page (e.g. in the Sandbox), type  (or   – the case of the first letter is not significant) somewhere into the wikitext of the target page, and save it. The page will be displayed as if the template call were replaced by the expansion of the template, i.e. as if the wikitext contained  at that point. The displayed page will therefore contain the text "λ".

For example, type " " and you will see "The 11th letter of the Greek alphabet is the lambda (𝜆)". You can use templates without knowing the details of their code – you only need to remember what result they produce (this is usually described on the template page).

Another way to use this template is to substitute it. If you type " " and save the page, you will again see "The 11th letter of the Greek alphabet is the lambda (λ)". However this time, if you look again at the saved wikitext, you will see that the template calls really were replaced by the expansion of the template when you saved the page. The link between the output text and the template is now broken, and the output will not be affected by changes which might be made to the template at some future time (as it would be in the case of transclusion).

Examples with parameters
An example of a template that takes parameters is the template for. Try typing  in the sandbox— it will produce the following text:

The template for takes two unnamed parameters, but the same template can also be used with different numbers of parameters to give slightly different results, as explained in the documentation on the template page. For example,  produces

Note the usage of an empty parameter – in this instance, the consecutive pipes mean that the first parameter that was "passed" to the template is an empty string, which in this template will result in the text "other uses".

Other templates, particularly more complex ones, take named parameters (or a mixture of named and unnamed ones). A simple example is Template:Payoff matrix, used to generate a 2-by-2 grid. Type:

to produce the grid you can see on the right.

See the template page for more possibilities. Notice that the template is used here without defining all its possible parameters – undefined parameters are given default values.

The spaces around the equal signs and before and after the parameters are used only for clarity – they are not needed, and are ignored when the template is evaluated (although this is not the case with unnamed parameters). However parameter names are fully case sensitive – for example, it is not possible to replace "DR" by "dr" or "Dr" in the above example. Parameters with names that are not used by the template are simply ignored.

Usage hints and workarounds
The following points may be worth noting when using templates:
 * An unnamed parameter cannot contain an ordinary equals sign, as this would be interpreted as setting off a named parameter. (This does not apply if the equals sign comes within another template call or other item which the parser handles separately.) To pass an equals sign in an unnamed parameter (for example in a URL with key/value pairs), replace the equals sign with the special template, which returns an equals sign that will not be specially interpreted. Another method is to replace the unnamed parameter (and any subsequent unnamed parameters) with named parameters – the first unnamed parameter is equivalent to a named parameter with the name "1", and so on. So to call template done with the parameter "a=b", type either  or.
 * Similarly, it is not possible to use an ordinary pipe character | in template parameters, as it will be interpreted as a separator. (Again, this does not apply if it comes within another separately parsed item, such as a piped wikilink.) This time the problem can be solved by using the magic word in place of the pipe, or (if the pipe is not intended to be parsed specially at a higher level) using the HTML entity.
 * Remember that whitespace characters (spaces, tabs, carriage returns and line feeds) are not automatically stripped from the start and end of unnamed parameters (as they are from named parameters). Including such characters (or any other non-visible characters in any parameters) may in some cases affect the template's behaviour in unexpected ways. (Template designers can use Strip whitespace to remove unwanted whitespace in unnamed parameters.)
 * In documentation and discussions it is often convenient to be able to produce the template call syntax, with a link to the template in question, but without actually calling the template. This can be done easily using the "tl" template (the template link template). For example, " " produces "Example". There is an extended version, tlx, which also supports parameters.
 * When a template is changed (when the template or one of its subtemplates is edited), the change will be reflected on all pages on which the template is transcluded. However the change may not become visible on all pages immediately – a previously cached version of a page, based on the previous version of the template, may continue to be displayed for some time. Use the purge function to force a page to be displayed using the latest versions of templates. (This includes the template page itself, if it contains usage examples.)
 * When viewing old versions of pages, remember that templates will be transcluded as they are now, not necessarily as they were when the old page version was active.
 * To list all pages onto which a template is transcluded, use What links here on the template page. (This will not include pages where the template has been substituted.)
 * To get a list of templates transcluded on a page, click "Edit", and find the list below the edit window. This list also includes the sub-templates used by the templates that are directly transcluded. To get such a list for a page section, an old version of the page, or your newly edited version prior to saving, click "Show preview" on the appropriate edit page. (For an old version, the subtemplate tree will be constructed according to the templates' current state.)
 * There are limits to the number and complexity of the templates that an article may have. See the "Expand limits" section for help in resolving this.
 * If you'd like the template to leave a time stamp or signature you can write . But this will only work if you substitute the template. If you transclude it, you'll just get.
 * To improve readability, usually programmers like to split the code with newlines and indent it. Unfortunately MediaWiki software does not allow this functionality and in many instances these purpose-built newlines are treated by the software as content. One possible workaround is to add  before each newline character and   after it.

Creating and editing templates
Templates are created and edited in much the same way as any other page: choose an appropriate name, navigate to that page, then click the Edit tab or create a new page as needed. As mentioned above, templates are normally placed in the Template namespace, though templates intended for your own personal use or for experimentation can be created in your own user space. Anything that can be included on a normal page or article can be included on a template, including other templates (called subtemplates). Templates often make use of programming features – parameters, parser functions and other magic words – which allow the transcluded content to vary depending on context. There are also special tags to control which information is transcluded and which is not.

Before creating a template, do a quick search for existing templates to see if there's already a template that does what you want, or a similar template whose code can be copied and modified (or left in place and expanded). Look for generic templates on which the new template can be based (for example, navbox templates can be easily created by calling the generic Template:Navbox).

There is no hard rule about what name to choose for a template – make it short but reasonably descriptive. If similar templates exist, try to follow a consistent naming pattern. Templates can be renamed without breaking existing transclusions, provided a redirect to the new template name is left behind.

Be extremely careful when editing existing templates – changes made can affect a large number of pages, often in ways you might not expect. For this reason many high-use templates are protected against editing except by administrators.

Handling parameters
The values of the parameters which can be fed to a template are represented in the template code by items enclosed between triple braces:
 * the code  will be replaced by the value of the parameter named xxx
 * the codes,   etc. will be replaced by the first, second etc. unnamed parameter (or the value of a parameter named 1, 2, etc.); these are sometimes known as positional parameters

If a parameter is not assigned a value, then no replacement will take place – this means that if no value is passed for parameter xxx, the value of the expression  inside the template will literally be   . A more intuitive behavior can be achieved by specifying default parameter values. This is done with the pipe syntax :  specifies the default value dflt for the first unnamed parameter. Most often, this is used to specify null default values ( or  ).

Note that you cannot assign a second parameter as the default; it will simply return the 'name' of the parameter as text. For example suppose the template 'Template:Hello' is, and is called with. Parameter '1' does not exist, so it will call the default. The text returned is the "param" or what would be expected to be the parameter name, not the parameter's assignment "world". This would be best implemented by a sequence of if parser statements. Also note that if a template is called with the parameter specified as blank (e.g. ), the default for the parameter will not appear. If that is undesired one can check if the parameter exists through the parser code instead. This will produce the text "default" even if the parameter is specified as empty.

Because of the multiple uses of double-brace and triple-brace syntax, expressions can sometimes be ambiguous. It may be helpful or necessary to include spaces to resolve such ambiguity, for example by writing  or , rather than typing five consecutive braces. However, watch out for unwanted whitespace appearing in template expansions.

Parameters do not get expanded when they are wrapped in  tags. They however aren't expanded either if placed within the actual XML-style opening tag. Thus, the following will not work within a template:
 * Special case: Parameters within XML-style opening tag

because the parameter is not expanded. Instead, you can however use the  parser function, which for example is used in  to generate the  element (see also Help:Magic words) Therefore, the following example will work:

If a parameter's value is or ends with a URL, check whether it is displayed in Wikipedia with the link overextending by one or more characters after the URL so that clicking the link causes an error or failure. Ensure that, after processing by the software, a soft space (not hard or nonbreaking) follows the URL, regardless of whether you or a user supplied the URL, or it was generated by automated processing. Possibly, the source code could contain or generate a space that is discarded in the processing or there might not be any space there. Correct the source code, perhaps by forcing a soft space to appear after the URL. The Spaces template may be useful.
 * Caution: Overextending URLs

System variables and conditional logic
Template code often makes use of the variables and parser functions described at Help:Magic words, in order to make the template's behavior depend on the environment (such as the current time or namespace) or on the parameter values that are passed to it. They can also be used for arithmetical calculations. Note that full string manipulation is not available (some templates providing such functionality have been created, but they are inefficient and imperfect) and neither are certain standard programming features such as loops and variable assignment.

Some of the most often used variables and functions are listed below. For more, see Help:Magic words, and the fuller documentation at the MediaWiki pages mw:Help:Magic words and mw:Help:Extension:ParserFunctions.

The ParserFunctions extension provides more programming-oriented parser functions.

The PAGENAME and NAMESPACE variables are particularly useful, and frequently used, to change template behavior based on context. For example, if the template transcludes a category link (e.g. cleanup templates, which transclude a link categorizing the page as a page which needs cleanup), it will often check the NAMESPACE variable to make sure that talk pages, user pages or anywhere else the tag might incidentally be placed do not themselves get categorized as pages needing cleanup.

Nesting templates
Templates may contain other templates – this is usually called "nesting". As the template is processed, the wikitext produced by any nested templates is transcluded into the nesting template, so that the final product is essentially processed from the most deeply nested template out. While fairly straightforward in application, it involves some noteworthy quirks and tricks.

To pass a parameter value to a nested template, place a parameter tag as the value of one of the nested template's parameters.
 * Examples:
 * Template:A contains "the quick brown jumps over..." This takes the value passed to the third positional parameter of Template:A and passes it as the first positional parameter of Template:B, then returns the wikitext produced by B as part of the phrase.
 * Template:A contains "the quick brown jumps over..." As previously, except the third positional parameter of Template:A is passed to the named parameter "waldo" of Template:B.

Template parameters themselves can be chosen conditionally.
 * Examples:
 * Template:A contains "the quick brown jumps over..." Template:A passes the word "fox" as a named parameter of Template:B whose name is A's third positional parameter.

A template can call itself, but will stop after one iteration to prevent an infinite loop.

When a nested template contains unmatched braces – as in lb&#125; – the unmatched braces are treated as text during processing, and do not affect the parsing of braces in the nesting template. If the nested template is substituted, however, the substitution is processed first, and this will change how braces are parsed in the nesting template. This has little practical use, but can occasionally introduce unexpected errors.

Noinclude, includeonly, and onlyinclude
By default, when a template is transcluded (or substituted), the entire wikitext (code) of the template page gets included in that of the target page. However it is possible to modify that behaviour, using tags that specify which parts of the template code are to be included. This makes it possible to avoid transcluding information intended for display only on the template page itself, such as the template's documentation.

These tags can be nested inside each other, though (for a given page) this really only applies to the tag; nesting  and  tags is fairly pointless. Be careful not to split the tags, however. Constructions like abcdefghi will not work as expected. Use the "first opened, last closed" rule that is standard for XML.

Problems and workarounds

 * If the first included character of a template or parser function is one of four wiki markup characters       then it is processed as though it were at the beginning of a line (even when the template tag is not). This allows the creation of various kinds of lists in templates where the template may not always be in the correct place for a list. To avoid this, either use  before the markup, see Help:Nowiki, or use the HTML entities         respectively. In some cases the HTML entities will work when the  does not.
 * For issues involving the substitution of templates (for example, how to control whether subtemplates are substituted as well when the parent template is substituted), see Help:Substitution.
 * For debugging templates the following techniques are sometimes helpful:
 * Using "subst:" – substituting a template (rather than transcluding it) can show more clearly what is happening when the template is transcluded; see Help:Substitution.
 * Using "msgnw:" – this keyword (short for "message, nowiki") transcludes the wikitext of the template page, more or less, rather than the processed contents. It is not perfect: lists are rendered, comments are removed, and single newlines are replaced with spaces (which is particularly confounding when transcluding wikitext tables).
 * Using Special:ExpandTemplates to see the full recursive expansion of one or more templates.
 * To protect server resources and avoid infinite loops, the parser imposes certain limits on the depth of nesting of transclusions and on the page size with expanded templates. This may cause pages to break when using very complex templates, particularly if there are multiple such templates on the same page. A page's overall load on the server can be checked by examining the generated HTML for a page and looking for the "NewPP limit report" comments.
 * Do not use  wikimarkup to create section headers within a template which is intended for use in article space – this will create an   link that when transcluded will confusingly open the template for editing.
 * You may avoid  links to the template by including

Documentation and categories
Categorizing your template and documenting its proper usage will make it easier for other editors to find and use.

Documentation for users, together with the template's categories, is normally placed after the template code, inside  tags. It is normally necessary to put the opening tag immediately after the end of the code, with no intervening spaces or newlines, to avoid transcluding unwanted whitespace.

In the case of complex templates, the documentation (together with categories) is often kept on a separate subpage of the template page (named "Template:XXX/doc"). This also applies to many protected templates (to allow the information to be edited by non-administrators). This is achieved by placing the Documentation template after the main template code (within  tags). If the "/doc" subpage does not exist, a link will then appear enabling it to be created.

Some templates contain category definitions in their transcluded code, i.e. they are intended to place the target pages in particular categories. This is often done with maintenance categories (placing articles into ordinary content categories in this way is discouraged). When doing this, it may be necessary to use  tags to keep the template itself out of the category. While developing, testing, sandboxing, or demonstrating a template intended to apply a category, either temporarily replace each category with a test category.

Aliases
Aliases can be created with redirects. For example, Template:Tsh redirects to Template:Template shortcut. Then you can write instead of.

It's good to prepare template aliases which only differ in whitespaces and capitalization.

Template limits
"Post-expand include size" limit. When templates are rendered or expanded to HTML for viewing in your browser, they use memory. This is called the "post-expand include size" and has a limit of 2,048,000 bytes. This size is included as an invisible comment in the HTML output – use your browser's view source feature to show the raw HTML and search for newpp. The report will look like:

The example shows that template expansion is using 63k out of 2M of available memory.

Display problem. If too many templates are included on a page, the post-expand include size may exceed the limit. When this happens, templates after the limit will no longer expand and will instead display as a wikilink (for example, ). Common causes are the inclusion of too many citation templates and/or flag templates. To resolve this problem substitute templates, remove templates, or split the page.

Lua programming language
Since February 2013 Lua programming language is available for use via the Scribunto MediaWiki extension. Lua code can be embedded into wiki templates by employing the " " functionality of the Scribunto MediaWiki extension. The Lua source code is stored in pages called modules, and these individual modules are then invoked on template pages. For example, Module:Bananas can be invoked using the code  to print the text "Hello, world!".