Help:Template demo calling alternative for redirect

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The contents of one page can be placed onto another page by using a template.<ref name="intro">This process is called transclusionhttp://en.wikipedia.org/wiki/Wikipedia:Transclusion. If a page is specially intended for this purpose it is called a template. In terms of programming languages, the wikitext of a page can be considered a string expression. In this view a template call is the call of a string function. The template page is the function body. The resulting string may are either be fixed or depend on parameters and/or variables. Other than in many programming languages the declaration of the function does not start with a formal parameter list, it consists of the function body only, which is a string expression for the result. See also compound document.</ref>

This page and Help:Advanced templates is a reference manual on templates in MediaWiki. See also User:Happy-melon/Templates. For an introduction, see A quick guide to templates and mw:Help:Templates. For syntax often used within templates, see Help:Parser function, Help:Extension:ParserFunctions (on MediaWiki.org) and other "Advanced functioning" help pages listed below.


Contents

General

A template is one of the double-brace structures that can be in a page, see also Help:Expansion.

To insert a template on a page, use this format: {{name of template}}. For example: {{tc}}. The contents of the page Template:Tc is placed on a second page. If the page does not exist a red link to the page appears.

Template namespace

The template namespace is the namespace of pages with prefix "Template:". For example: Template:Tc. The namespace in this example is "Template". The page name is "Template:Tc".

One way of making a template is typing the tag first, and then following the red link.

One can call a template that calls another template. If a template calls itself directly or indirectly ("recursion"), the second-level inclusion is replaced by a link (possibly a self-link), with the HTML error comment: 

Template loop detected:

See also Repetition within a page.

A list of pages embedded in a page (as recorded in the templatelinks table), all with links, is given on the edit page. Notes:

  • on a section edit page first the whole list is given, i.e. also the templates used in other sections; after pressing preview only those in the section, updated for the edits made, if any; preview on first edit gives the full list;
  • similarly, when editing an old version of a page, the list is first given for the current version, while after pressing preview the templates that used to be included are shown, updated for the edits on the old version made in the current session, if any;
  • if the name of an embedded page is an expression, the current template name is shown. E.g.:

(Supported in version 1.4 and higher.)

In a template name the character "#" and any characters after that are ignored: since {{tc}} gives in, {{tc#abc}} gives in.

Talk pages

Even if a template is intended for use on talk pages, the template page should not be put in a talk namespace, because there would be no page for discussing it. If it is for general use it is often put as usual in the template namespace, see e.g. w:Wikipedia:Talk page templateshttp://en.wikipedia.org/wiki/Wikipedia:Talk page templates and w:Wikipedia:Template messages/User talk namespacehttp://en.wikipedia.org/wiki/Wikipedia:Template messages/User talk namespace. An auxiliary template for personal use for a user talk page can best be put in a subpage of the user page, not one of the user talk page, for example "User:Abc/Talk intro" rather than "User talk:Abc/Intro"; this allows discussion of the template at "User talk:Abc/Talk intro".

Case sensitivity

For the name of a page used as a template, whether in the template namespace or not, the regular rules apply, see Help:Page name. Thus, for a template in the template namespace, the part of the name after "Template:" is case sensitive except (on most wikis) for the first letter, and a blank space is the same as an underscore "_".

However, parameter names are case-sensitive, including the first letter, and spaces are different from underscores.

Use of templates across wikis

A template only works on a page in the same wiki. To use it in another wiki, one has to copy it to there. Bugzilla:1126 requests to lift that restriction, and allow interwiki use of templates.

The adjustments needed in the copy, to work on the other wiki, can be reduced by using {{SERVER}}, localurl, and generic namespace names, and writing links in a way that works on multiple wikis, e.g. m:Help:Table. For copying multiple templates, export and import can be helpful. It is also convenient if the template names can be the same on the other wiki. Therefore, when choosing template names, check which names are in use on other wikis to which people might want to copy the templates. Note that if a template is referred to by something like abc{{NAMESPACE}}, a wiki with different namespace names requires a different template name, or a redirect.

Transclusion across wikis, hence changing contents from a central place, is only possible:

  • for content in the form of an image on Commons, to any page,
  • for editable content on an image page on Commons, including content transcluded from other pages on Commons, to the corresponding local image pages, if there is no local image with the same name

Sometimes multiple wikis require pages which are partly the same and partly different. For example, on some wikis "m:" is used as interwiki link to Meta, and on some wikis "MetaWikipedia:". In this case a common wikitext can be used, calling wiki-specific templates. In the example the wikitext of the page could contain {{meta}}, and we would have on each wiki a Template:Meta containing the required prefix.

Parameters

See also Using parameters in templates

A template can contain parameters by putting a parameter name in three right and left curly brackets {{{ }}}.<ref name="other">Also commonly called: brackets/curly parenthesis</ref><ref name="opt"> Optionally with a pipe and a default.</ref> The values are specified in the template call.<ref>A more technical explanation: The parameter names in assignments matching a formal parameter are selected. Only the wikitexts for the parameter values of these are expanded, as well as the defaults of the undefined ones, if specified. The template call is replaced by the template content, where the parameter tags with matching names are replaced by their values or defaults. The result of this step can be viewed by applying subst. While what was explained involved expansion of wikitext for the template name, all parameter names and selected parameters can involve recursion in expanding the template. To illustrate the above, a page with {{t1demo|{{ta}}|{{tb}}|{{tc}}={{td}}|1={{te}}|2={{tf}}}}, with Template:T1demo only using parameter {{{1}}}, and giving start-Template:Te-end, transcludes templates tc and te only.</ref><ref name="place">A parameter in a template is a placeholderhttp://en.wikipedia.org/index.php/Free_variables_and_bound_variables for the parameter value specified in the template call. A parameter is also called an argument, e.g. in "template argument size".</ref> Parameters not only allow an editor to substitute unique text on a page, without changing the original template, but they may even control which action the template performs.

A template call can use named parameters and/or unnamed parameters, using one of the following forms (the number of parameters, here two, can vary):

  • {{templatename|parname1=parvalue1|parname2=parvalue2}} with tags {{{parname1|default}}}, {{{parname2|default}}}, etc. in the template, or
  • {{templatename|parvalue1|parvalue2}} with tags {{{1|default}}}, {{{2|default}}}, etc. in the template, or
  • a mix of the two.

The defaults are optional, together with the pipe character "|" in front, see below.

For example:

Example 1
The Template:Weather page has the text:
The weather in {{{1}}} is always {{{2}}}.
An editor can then add the template {{Weather}} on several other wikipages.
On the State of Maine page:
{{Weather|Maine|cold}}
Displays:
The weather in Maine is always cold.
On the State of Florida page:
{{Weather|Florida|hot}}
Displays:
The weather in Florida is always hot.
 Example 2
On wikipedia, the {{Further}} template allows users to quickly add further information links.
The template is written as follows:
Further information: {{{1}}}<ref>This was the original further template up until 20:56, 19 June 2008, when much more complex formating was added, which is beyond the scope of this article.</ref>
The template can be used (called) in the following format:
{{Further|[[Article 1]], [[Article 2]], and [[Article Something#3|Article 3]]}}
For example, if an editor types in
{{Further|[[Cats]], [[Dogs]], and [[Fish]]}}
the following appears on the page:
Further information: Cats, Dogs, and Fish

For some technical details see the following footnote.<ref name="technobabble">Template parameters are called-by-valuehttp://en.wikipedia.org/index.php/Parameter_(computer_science)#Calling_conventions, and therefore input parameters only (see also the section "Template expansion").

Thus a template call has, after the template name, zero or more parameter definitions, separated by raw pipes (raw means here: in the wikitext, not just in the expanded wikitext, and not inside double or triple braces or double brackets). Furthermore, each parameter definition containing a raw equals sign is split around the first such equals sign into a parameter name and parameter value. In this stage the wikitexts for the parameter names are expanded, but not yet those for the parameter values. For any remaining parameter definitions the implicit numbers of the parameters are 1, 2, 3, etc. (so if there are also named parameters the implicitly numbered parameters are numbered according to the position among themselves, not the position in the mixed list as a whole). If a parameter would be assigned a value more than once (more than once as named parameter, or once as implicitly numbered one and once or more as named one), working from left to right through the mixed list only the last assignment is retained. The result is an associative array of parameter names and corresponding wikitexts for the parameter values. </ref>

Parameter definition with explicit or implicit parameter name

If a parameter name as used in a parameter tag such as {{{3}}} or {{{3|stu}}} is a not too large positive integer, in the template call it can both be referenced explicitly with, as in 3=pqr, or implicitly by putting pqr in the third position among the unnamed parameter definitions.

Comparison between named and implicitly numbered parameters:

  • To specify the value of a particular implicitly numbered parameter all previous ones have to be defined too (often the empty string is used as value if the assignment is only for this reason).
  • With implicit numbering the call is shorter (except if many extra parameters have to be defined because of what was just mentioned).
  • Named parameters can be specified in any order.
  • An implicitly numbered parameter can be assigned a value containing "=" only indirectly (see below).
  • Spaces and newlines are stripped from the start and end of parameter values of named parameters, but not of implicitly numbered parameters. Therefore the best name for a separator parameter (specifying how items of output are separated) is "1". This has e.g. been done in Template:For. See also Help:Newlines and spaces; also, compare conditional whitespace.
  • If an unnamed parameter is unused it does not contribute to the preprocessor node count, while for each unused named parameter the count is 1. This is relevant in data templates containing data in the form of values of parameters of a variable inner template, see Help:Array.

The default (optional, together with the pipe character in front) can be different for every occurrence, and applies if no value is specified when calling the template, not to be confused with the case that the empty value is specified.

To distinguish between a parameter being defined and non-empty on one hand, or undefined or empty on the other hand, use #if: with a blank default:

{{ #if: {{{param|}}} | param is defined and non-empty | param is undefined or empty}}.

To distinguish between defined (and possibly empty) and undefined, use:

{{ #ifeq: {{{param|+}}} | {{{param|-}}} | param is defined | param is undefined }}.

Example: named parameter substitution

Let's create a template called Name-example (that is, the template will be Template:Name-example), with a parameter for the first name which we will call firstName, and a parameter for the last name which we will call lastName.


Type the following in the new Template:Name-example: 

:I am a template example, my first name is '''{{{firstName}}}''' and my last name is '''{{{lastName}}}'''.
:
:You can reference my page at [[{{{lastName}}}, {{{firstName}}}]].

On a second page, type this: {{Name-example}}

The result is: Template:Name-example

Because the template has no parameters.

But if we type this on the second page: {{Name-example | firstName=John | lastName=Smith}}

The result will look like this:
Template:Name-example

To analyse the working, one can apply Special:ExpandTemplates to see the parse tree and the expanded wikitext as intermediate results: 

:I am a template example, my first name is '''John''' and my last name is '''Smith'''.
:
:You can reference my page at [[Smith, John]].

When this wikitext is applied directly the resulting rendering is the same.

Example: numbered parameter substitution

An example of how the contents of a parameter function.

Let us first define a template with the name t1demo, which will

  • expect a single argument
  • give back the text of that argument, with no spaces before or after it, between "start-" and "-end":
start-{{{1}}}-end

the definition of course being on page Template:t1demo.

  1. Calling the template named t1demo with a parameter value of "a", i.e.,
    {{t1demo|a}}
    gives
    "start-a-end"
  2. Calling the template named t1demo with a parameter value of " " (a blank space), i.e.,
    {{t1demo| }}
    gives
    "start- -end" .
  3. Calling the template named t1demo with a parameter value of "" (an empty string), i.e.,
    {{t1demo|}}
    gives
    "start--end".
  4. Calling the template named t1demo and not passing any parameters, i.e.,
    {{t1demo}}
    gives
    "start-{{{1}}}-end".

If something like {{{1}}} or in fact any {{{name}}} is visible on an ordinary page it typically indicates that a mandatory parameter of a template used on this page isn't defined.

Parameter names

Parameter names (as opposed to template names) are case-sensitive, even with respect to the first letter, and spaces are distinguished from underscores. The empty stringhttp://en.wikipedia.org/index.php/String_(computer_science)#Formal_theory is also a valid parameter name, see Template:T empty string as parameter name.

Parameter 02 is distinguished from parameter 2: using Template:t pl0 containing "[[:Template:T pl0]]", {{t pl0|5|6|7}} gives Template:T pl0

Note that parameters in the system messages (MediaWiki namespace) are written differently: as $1, $2, etc.

Declaring a default value

Main article: Help:Parameter default.

Let us define a template with the name t which contains a single numbered parameter 1 with a default of pqr. The only difference between the effects of t and t1demo appears when they are called with no parameter (and no "|"):

start-{{{1|pqr}}}-end

the definition of course being on page Template:t.

  1. Calling the template named t with a parameter value of "a", i.e.,
    {{t|a}}
    gives
    "start-a-end".
  2. Calling the template named t with a parameter value of " ", i.e.,
    {{t| }}
    gives
    "start- -end".
  3. Calling the template named t with a parameter value of "", i.e.,
    {{t|}}
    gives
    "start--end".
  4. Calling the template named t with named parameter 1=, i.e.,
    {{t|1=no surprise}}
    gives
    "start-no surprise-end".
  5. Calling the template named t with 1= after an unnamed parameter, i.e.,
    {{t|no|1=surprise}}
    gives
    "start-surprise-end".
  6. Calling the template named t with 1= before an unnamed parameter, i.e.,
    {{t|1=no|surprise}}
    gives
    "start-surprise-end".
  7. Calling the template named t and no parameter at all, i.e.,
    {{t}}
    gives
    "start-pqr-end".
  8. Calling the template named t and no named or unnamed parameter 1, i.e.,
    {{t|2=two}}
    gives
    "start-pqr-end".

Empty vs. undefined

{{t2demo|| a }} makes the first parameter equal to the empty string rather than leaving it undefined: it gives Template:T2demo, just like {{t2demo|1=|2= a }}: Template:T2demo. By specifying parameter name "2" explicitly, the first unnamed parameter can be left undefined:

{{t2demo|2= a }} gives Template:T2demo.

If the second parameter should not be trimmed it has to be unnamed, which means that we can assign the first parameter the empty string, but we cannot leave it undefined.

Making emptiness and undefinedness equivalent

It is often convenient if assigning the empty string to a parameter has the same effect as not assigning any value to it, as the difference is somewhat abstract, and as mentioned, in some cases there is not even the choice. Also, for example "p=" may be put for convenience, to be filled in later, and/or meant to make explicit that the template has a parameter p, but as this time we do not give it a value.

Making emptiness and undefinedness equivalent can for example be achieved by exclusively using {{{p|}}}, not {{{p}}} and not {{{p|q}}} for a non-empty q, or for example using {{#if:{{{p|}}}|..{{{p}}}..|..}} (i.e., using {{{p}}} only in branches of branching functions where it is defined and non-empty).

Restrictions on parameter values

An unnamed parameter can be assigned a value containing an equals sign, but only indirectly.<ref>bugzilla:14235</ref> Methods (here with example using Template:Ti) include:

  • Write it as the default of an undefined parameter, as in {{{| a=b }}}: start- a=b -end.
  • Use Template:= containing "=", e.g. " a{{=}}b ": start- a=b -end.
  • If it is just for display, the equals sign can be replaced by the HTML entity &#x3d;: start- a=b -end.

The same example with "1= a=b " gives start-a=b-end.

Reasons for using an unnamed parameter include:

  • Spaces and newlines are stripped from the start and end of parameter values of named parameters (as demonstrated).
  • Not disturbing the numbering of following unnamed parameters (alternatively, the parameter definition can be prefixed with an extra "|", providing a dummy definition of an unnamed parameter, which is overwritten by the parameter definition concerned).

Any unmatched pairs of two consecutive braces or brackets must be placed in nowiki tags. Braces can be used for nested template parameters, nested templates or parser functions while brackets can be used for links. Unmatched pairs not placed in nowiki tags either prevent template expansion or are taken as closing braces for the template call.

For example, using Template:t1demo containing "start-{{{1}}}-end<noinclude> [[Category:Demo template]]</noinclude>":

  • {{t1demo|abc]]def[[ghi}} gives {{t1demo|abc]]def[[ghi}} (the template is not expanded; see also [1] where this example causes several sections not to be displayed).
  • {{t1demo|abc}}def}} gives start-abc-enddef}} (the first closing braces are taken as those for the template call).

Without restriction:

  • {{t1demo|abc]def[ghi}} gives start-abc]def[ghi-end.
  • [http://1 {{t1demo|abc]def}} gives start-abcdef-end.

A parameter value can be quite long, see Help:Long parameter demo.

A parameter value containing a pipe character

To define a parameter value containing a pipe character (|) which is not part of a template call, parser function call, piped link or image tag, use Template:! containing "|". Using Template:Tcw, compare {{t1demo|a{{!}}b}} giving start-a|b-end, {{t1demo|a&#124;b}} giving start-a|b-end and {{t1demo|a|b}} giving start-a-end. The difference between the first and the second is that in the first case the expanded wikitext is start-a|b-end, while in the second case it is start-a&#124;b-end. This is rendered the same in final output but makes a difference when used in table syntax: this requires the "real" pipe character.

Using Template:Tcw, 

{{2x|{{{!}} 
{{!}} A 
{{!}} B
{{!}}- 
{{!}} C
{{!}} D
{{!}}}
}}

gives

A B
C D
A B
C D


while 

{{2x|{&#124; 
&#124; A 
&#124; B
&#124;- 
&#124; C
&#124; D
&#124;}

}}

gives

{| | A | B |- | C | D |} {| | A | B |- | C | D |}


and 

{{2x|{| 
| A 
| B
|- 
| C
| D
|}
}}

gives

{{

Similar constructs can allow a parameter value to contain a double closing brace. Even a parameter name can contain "|" and "}}" in this way, but that seems of little use. However, a pagename cannot contain "|" or a brace.

Mix of named and unnamed parameters

In the case of a mix of named and unnamed parameters in a template tag, the unnamed parameters are numbered 1,2,3,.., so they are not numbered according to the position in the mixed list as a whole. For example, {{t sup|foo=1|2|bar=3|4|5|6|7}} using Template:Tcw gives Template:T sup.

However, {{t sup|3=1|2|1=3|4|5|6|7}} gives Template:T sup. The first number shown is '3', because the setting of parameter 1 to the value '2' (by the first unnamed parameter) is overwritten by the later direct assignment of the value '3' to parameter 1. Similarly, the direct assignment of the value '1' to parameter 3 is later overwritten by the third unnamed parameter. See also mw:Module talk:ArgumentPairs demo.

When parameters do not expand

Template:MW 1.6 Parameters do not get expanded when they are inside XML-style tags, unless using the parser function #tag. Thus, the following will not work within a template, because the parameter is not expanded: 

 <myextension xparam={{{tparam}}}> ... </myextension>

Monitoring parameter usage

To monitor the usage of a parameter of a template (in the case that many pages use the template), this template can call an auxiliary template of which the name depends on whether the parameter is defined. The possible auxiliary templates need not exist, if they are e.g. used as parameter value of Template:Ti, to avoid displaying anything (with the new preprocessor Template:Ti cannot be used for this anymore). Applying "What links here" to the possible auxiliary templates shows which pages use the template with the parameter, and which pages use the template without the parameter. This parameter usage monitor facility should preferably be built into the template when creating the template or introducing a new parameter. If it is done afterwards "What links here" may not show a page until it has been refreshed by an ordinary or dummy edit or purge.

Applications of monitoring parameter usage include:

  • check where the parameter has not yet been specified, in order to add it
  • check where a parameter is used that is renamed, in order to change the template calls
  • check whether a parameter is used before abolishing it
  • when a template has been changed with respect to processing a particular parameter, check the pages which use the parameter (or some of them) to see if things work as desired.

In a similar way an auxiliary template can be called for a "template / parameter / parameter value" combination or "template / parameter / parameter value range" combination. "What links here" then shows which pages use the template with this parameter value, or with the parameter in the given range.

See Template:T optional parameter demo.

Similarly a tracking category can be used, see also Help:Category#Comparison with "What links here".

Template tag lay-out

To have a desired template tag lay-out, especially in the case of many parameters, additional dummy parameters can be used with a newline, spaces, and/or comments. In the case of unnamed parameters, this means that some parameter numbers are not used in the template content, so that their values, present in the template tag, do not affect the rendering of the template. For example, using Template:Tcw, 

{{t3d |a|b|c| 1
      |d|e|f| 2
      |g|h|i| 3
}}

gives:

Template:T3d

If parameters are named, dummy parameters can easily be inserted at any time. If they are unnamed, then, to avoid renumbering, one can insert named parameters: any text including an equals sign will do, if the text on the left is not one of the parameter numbers: this text is interpreted as the name of an unused parameter, hence ignored.

Example:

Using template:Chess position ( talk edit history links ) 

{{chess position|=

 8 |rd|nd|bd|qd|kd|bd|nd|rd|=
 7 |  |pd|pd|pd|pd|pd|pd|pd|=
 6 |pd|  |  |  |  |  |  |  |=
 5 |  |  |  |  |  |  |  |  |=
 4 |  |pl|  |  |  |  |pl|  |=
 3 |  |  |  |  |  |  |  |bl|=
 2 |pl|  |pl|pl|pl|pl|  |pl|=
 1 |rl|nl|bl|ql|kl|  |nl|rl|=
    a  b  c  d  e  f  g  h

|30}}

which gives

8 rd nd bd qd kd bd nd rd
7 File:Chess d40.png pd pd pd pd pd pd pd
6 pd File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png
5 File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png
4 File:Chess l40.png pl File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png pl File:Chess d40.png
3 File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png File:Chess l40.png File:Chess d40.png bl
2 pl File:Chess d40.png pl pl pl pl File:Chess l40.png pl
1 rl nl bl ql kl File:Chess l40.png nl rl
a b c d e f g h

In this example, the 1 to 8 row numbers and a to h column numbers (as well as their surrounding spaces and newlines) in the parameter list are hidden within a dummy parameter (with an empty name before the equal sign).

The template generates a HTML table where each cell contains an image whose name is dynamically generated from the 64 first automatically numbered parameters (there are intermediate parameters within the numbered list, but as these dummy parameters are named explicitly, they don't increment the parameter number used to generate default parameter names. So the parameter with autogenerated name "1" effectively contains the "rd" value).

The template is then called with a parameter list that contains:

  • 65 parameters automatically named from "1" to "65" (64 parameters for the cells content, and the 65th for the image width),
  • and a dummy named parameter (whose name is an empty string, and whose effective value results from the last assignment with the chessboard column names a to h and the surrounding spaces and newlines), which is not used in the internal template content.

Note also that all parameters are trimmed, so the parameter with autogenerated name "9" (at position a7 on the chessboard above) is assigned the empty string value.

Relative name for transcluded page

A page A can transclude a subpage A/B using {{/B}} or, with more control over the name of the page to be transcluded, this name can be constructed from {{PAGENAME}}.

This allows copies of the same wikitext in different pages to transclude different pages. Thus for example, the same wikitext, adapted from [2], containing a list of calls {{/lang|..}}, can be used in various ways on different pages, for example Template:Pim and Template:Pim, by varying the content of the subpage. A transcluded page need not use all the specified parameters. Two pages can share the same transcluded page using a redirect.

Template page

Without noinclude and includeonly parts the following are identical:

  • the rendering of the wikitext of the template (this is on the template page, and would be the same in other pages with that wikitext, except for page-dependent variables)
  • the rendering of the template on a page that calls it without parameters

Thus the defaults are shown, e.g. Template:T 1 containing "[[:Template:T 1]]" shows the same as {{t 1}}: "Template:T 1".

Template page if no defaults have been specified

In simple cases this corresponds to treating the parameter tags as ordinary text, for example:

"{{t2demo}}" (using Template:t2demo containing [[:Template:T2demo]]) gives Template:T2demo

Similarly, if part of the parameters is undefined, only those show up as {{{parameter number or name}}}.

However, often the rendered page produces a seemingly arbitrarily mutilated version of its content:

  • #expr and #ifexpr applied to an expression containing a parameter tag without default produces an error message. For example, a template that doubles parameter {{{1}}} with the wikitext {{#expr:2*{{{1}}}}} gives Expression error: Unrecognized punctuation character "{"..

For parameters without default a workaround is less needed because the result in the case a parameter is undefined is of limited use anyway. If one wants it anyway one can put nowiki tags around the parameter; for the first example this gives "{{{1}}}p".

Less surprising, but nevertheless limiting the usefulness of the result are:

  • #if applied to a parameter tag without default produces the then-part, the else-part is not shown.
  • #ifeq typically produces the else-part only.
  • Variable tags are not shown, but evaluated.
  • A switch typically shows the default only.

In such cases includeonly can be used to avoid uninformative and messy rendering on the template page, but either way a disadvantage during template development is that preview does not (meaningfully) reflect changes in the parts of the page outside the noinclude tags (i.e. the included parts). Therefore it can be useful to provide representative default values, even if that is only done temporarily during development of the template.

With noinclude an informative template page can be produced, containing more demos and test cases than just the single default set. Typically, the noinclude-part of a template page contains examples including or substituting the template. Changes in the working of the template (i.e. changes outside the noinclude-part) are not yet effective in these examples in preview and, in the case of substitution, in "show changes". However, they are reflected in the rendered page after saving.

Some formatting aspects may show up which are not readily seen from the edit box.

A parameter value depending on parameters

If a parameter is left undefined, the triple braced parameter name in the result does not function as a parameter if the page is called as a template from another page; if it should, do not omit the parameter value but specify it in terms of a parameter of the calling page; compare:

The most straightforward way of expressing a parameter value in terms of a parameter is using the same name and making it equal, e.g. {{Lan is|lr=af|le={{{le}}}}} in Template:Lan is mr.

To preserve possible undefinedness of parameter p one can put:

p{{#ifeq:{{{p|+}}}|{{{p|-}}}||dummy}}={{{p}}}

Note however that when applied to a numbered parameter it becomes named, hence spaces and newlines at the start and end of its value are not preserved.

See also Template:T ptd1,

msgnw

The prefix msgnw (short for "message, nowiki") is used to display within a page the wikitext of another page, with comments removed.

For the example Template:Msgnw demo:

Template:Xpdopocc

Note that with ExpandTemplates (see the link) "Remove comments" should be on to get the real result, otherwise it shows also the comment <!--Msgnw demo-->.

In the expanded wikitext, as seen with ExpandTemplates, the characters <>'[]{|& ><iki>

[[:Template:Msgnw demo]]

</nowiki></

Usage

A template is useful for any text for which one wants a copy in two or more pages, and there is no need for each copy to be edited independently to adapt it to the page it is in. Since parameters can be used, versions may to that extent even be different, and parameter values can be edited independently for each. Templates are not only convenient, they can also prevent the server doing duplicate work in the expansion of a page by reusing a parameter value that is expanded only once (like assigning a result to a variable or a dynamically valued constant in a computer program, and making multiple use of it); also they can "force" a useful uniformity. See also w:Subroutine#advantages of subprogramshttp://en.wikipedia.org/index.php/Subroutine#advantages_of_subprograms.

A template can also provide structure (e.g. a table structure, colors, and other formatting) while the content varies (fully or partly) through parameters and variables.

A template can also carry out computations, comparisons, etc. using ParserFunctions, where the result(s) vary again through parameters and variables. See e.g. m:Category:Mathematical templates.

Typical applications are:

Various combinations are possible. E.g., the last two could be combined, so that e.g. every language version of Wikipedia has the same Infobox Countries template (for each wiki a copy, because the template mechanism can not import across wikis), which has parameters for the data, and refers to templates with translations of standard terms.

See also Help:Advanced templates.

Composite pages

The wikitext of a page may (partly or fully) consist of tags for the inclusion of component pages. The "templates" are not necessarily in the template namespace, and may be more or less self-contained pages by themselves. A disadvantage of putting the components in the template namespace is that the component page may, by itself, be of a nature that it belongs in e.g. the main namespace, while the prefix would suggest otherwise; also the prefix would clutter the pagename.

Examples are:

This allows the user the choice between viewing the component pages separately or combinedly. One may want to view a page separately if one has a slow connection; also sometimes people have sentimental reasons for having a separate page about a topic, e.g. a village. Viewing the combined page is much more convenient if there are many small component pages; even if a subtopic page does not provide additional info, if it exists anyway, and is linked to, it is useful to display the content (making clear that the whole content is shown), it saves the trouble of checking the contents by following the link.

In the latter case the search function of the browser can be used for searching in the combined page. Similar to the section editing feature, it allows editing a component page; however, there is no possibility to have the combined wikitext in the edit box.

Edit history, recent changes, watching pages, and "what links here" work separately for the component pages and the composition page, not for the composite page. Related changes works for the component pages and for the composite page. Page protection can be set for selected component pages and for the composition page, not directly for the composite page.

The talk page of a composition page is used to talk about the composition and the page in general, and could in addition be a composite page of the talk pages of the component pages.

A composite page has an integrated TOC and section numbering; the numbering is not reset for each component page.

Editing a section of a component page can be done directly from the composite page, see editing sections of included templates. After saving, one ends up at the page for the component page to which the section belongs.

On wikis with the interlanguage link feature the composite page shows the combined interlanguage links of all component pages, hence possibly multiple links for one language or even for one page.

For easy access to the component pages, links to them on the composite page (for example by self-links on the component pages) are convenient. Alternatively, access is through section editing, or, if the component pages are in the template namespace, through the list of templates called from the composite page, at its edit page.

Conversely, for easy access from a component page to the composite page(s) that call(s) it (if there are not too many of them) it is convenient to link the component page to them (on a composite page one becomes a self-link).

See also w:Wikipedia talk:Template namespace#transcluding prose.

Pages with a common section

A section on the relationship of the subjects A and B, or a subtopic equally relevant for A as for B, can be put both in page A and in page B, by making it a separate page C, called as a template from A and B.

Remarks:

  • For readers reading both A and B, it is useful that the duplication be indicated by a message or a special lay-out; otherwise it is confusing and inconvenient. One can for example use Template:Common section.
  • After saving one ends up viewing the template page. For easy access to the calling pages it is convenient to link the called page to them (on a calling page one becomes a self-link). This may well be integrated in the message about the duplication. It is even more convenient than the two-step link as shown above.
  • The section level of the embedded section(s) has to be the same in each embedding page. The same applies to lay-out and style.
  • Pages like C can be put in
    • the same namespace as A and B; advantage:
      • the kind of content of C is the same as that of A and B
    • the template namespace; advantages:
      • C appears in the list of templates called from A and B, at their edit pages
      • C by itself may not satisfy the standards for pages in the namespace of A and B, such as providing context (however, the message about the duplication explains it)

Examples:

For various remarks which also apply here, see also the previous section.

Repetition within a page

For a succession of similar pieces of content, as in a list, a computer program would use a loophttp://en.wikipedia.org/index.php/Conditional_branching#Loops. What comes closest in MediaWiki is putting the loop body in a template, and calling the template repeatedly, usually with a varying parameter value. This is somewhat primitive for a loop:

Example:

  • Template:List of Languages - the "loop body" is in Template:Lang def; compared with putting the content of the latter directly in the list the advantage is that the variable part (the value of "code") occurs once each time in the code, although it is used twice. This is important because the fixed part of the list item code can be copied while the variable part is typically inserted manually.

The list of template calls can be in another template, with the name of the called template equal to or depending on a parameter, see e.g. Template:Do for every Dutch municipality.

For repetition of the same text the loop body can be a parameter, e.g. Template:Ti.

A double loop is achieved by a second template that repeatedly calls the first. Similarly for a triple loop, etc. See e.g. w:Template:Ld, w:Template:L2d, and w:Template:L3d.

An example where copies of the same template are used for different loop levels is Template:Loop 2 (formerly a redirect could be used for each level, but that gives the error message "Template loop detected" now). The number of levels that the template can handle is determined by the number of parameters specified in the two template calls it comprises (because going up a level requires an extra parameter) and the number of existing copies.

Similarly Template:Lanlp 1 is used for both the inner and the outer of a double loop. Adding an item (here a language) adds to each row and also adds an extra row.

Providing effective repetition without repetition of similar template calls in the wikitext

A technique for repetition of similar template calls without actually putting this list in the wikitext is demonstrated in template:for ( talk edit history links ), and a more primitive method described below.

Example Template:Ut1:

{{for loop|; |call=t2demo|pc1n=2|pc1v=constant|abc|def|ghi}} gives:

Template:For loop

Page counts

Templates can prevent the server doing duplicate work in the expansion of a page by reusing a parameter value that is expanded only once (like assigning a result to a variable in a computer program).

Example: "{{Population Dutch municipality by code|193}}" → "Template:Population Dutch municipality by code" [4] with preprocessor node count 871/1000000<ref name="1fp">Excluding the fixed amount of 1 for the page.</ref>, post-expand include size 12/2048000 bytes, and template argument size 3/2048000 bytes (for these page counts<ref>The terms page count refers here to a count regarding a page, not a count of a number of pages. The term template limit is somewhat misleading since parser functions and even <nowiki> tags and section headers also contribute to some of the counts.</ref> see w:wp:Template limits). Thus mainly the preprocessor node count is a concern (especially on a page with many of these high counts): a switch is used, therefore 2 is added for each case. This municipality is the last of the 430 in the list.

Rounding the number to three significant digits requires a parameter of the round operator that is computed from the number:

{{#expr:{{Population Dutch municipality by code|193}}round(2-floor(ln{{Population Dutch municipality by code|193}}/ln10))}} gives Expression error: Unrecognized punctuation character "["., with a preprocessor node count of 1743<ref name="1fp"/>/1000000, a post-expand include size of 30, and a template argument size of 6. Thus the preprocessor node count is doubled, plus 1 for #expr.

Better is to use Template:Sigdig, because now the high count for the switch is not doubled:

{{sigdig|{{Population Dutch municipality by code|193}}|3}} gives Template:Sigdig with a preprocessor node count of 879, a post-expand include size of 24, and a template argument size of 16.

See also Help:Expansion#Efficiency.

Ambiguity of a reference to "this page"

If a page is included in another page, a reference to "this page" on the included page is ambiguous. Use noinclude or includeonly tags depending on what is meant. Alternatively, mention the page name explicitly, without using {{PAGENAME}}.

Internal links

In the case of an internal link in a template with the target depending on a parameter, and in the case of a link with the target depending on a template, existence detection works as usual.

Examples:

Noinclude, includeonly, and onlyinclude

This feature is not available before version 1.6. These features are applied as nesting pairs, or 'Blocks', as it is said they enclose the material so that they can be said to begin and end a block of wikitext (code).

noinclude blocks

Anything between <noinclude> and </noinclude> will be processed and displayed only when the page is being viewed directly; it will not be included or substituted. Possible applications are:

  1. Categorising templates, see template documentation.
  2. Interlanguage links to similar templates in other languages.
  3. Pages in the MediaWiki namespace.
includeonly blocks

The converse is <includeonly>. Text between <includeonly> and </includeonly> will be processed and displayed only when the page is being included. Applications include:

  1. Adding all pages containing a given template to a category, but not the template itself.
  2. Avoiding messy rendering on the template page, e.g. {{#expr:{{{1}}}}} gives Expression error: Unrecognized punctuation character "{". [5].

Note that spaces and newlines between the general content and the tagged part belong to the general content. If they are not desired the include tag should directly follow the content on the same line: 

<noinclude>this is </noinclude>fine<includeonly>, closing tags are
</includeonly><noinclude>
less critical, but must be specified.</noinclude>
nesting

Nesting an includeonly block within a noinclude block (or vice-versa) is legal but pointless.

onlyinclude blocks

With <onlyinclude>wikitext</onlyinclude> on a page, the display of the wikitext so surrounded and the rest of the page (except includeonly parts) is rendered on the page itself normally (note: this means interwiki translation links will behave as normal external links, unless they are further bracketed by <includeonly>). The demo page below holds a full screenful of text and code, including several includeonly blocks as well as a single onlyinclude block as a demonstration. It is advisable to look at that page in edit mode.

However, in transclusion, the page gives only the parts within onlyinclude blocks. Using the test line: 

Using [[Help:Template/onlyinclude demo]], 
     <nowiki>{{Help:Template/onlyinclude demo}}</nowiki> 
          gives '''{{Help:Template/onlyinclude demo}}'''.

When repeated below we get: Using Help:Template/onlyinclude demo, {{Help:Template/onlyinclude demo}} gives Help:Template/onlyinclude demo. When that page is included, it does not even transclude other blocks which are defined as includeonly parts, unless such are also within the onlyinclude block. So on a page with onlyinclude tags, text within includeonly tags is never rendered, i.e., it is reduced to wikitext comment. See also Help:Template/onlyinclude demo 2 and its talk page.

Caveats on splitting and interworkings

Attempts to nest split pairs of these tags, or the similar <nowiki> and </nowiki> pair, won't work as expected.

If say <nowiki> begins within the general page content, or in a "noinclude" part, or in an "includeonly" part, then it also has to be closed within the same part.

The code ~<includeonly>~</includeonly>~~ will be displayed as ~~~ when the template is not included, ~~~~ when the template is included, but it will only be expanded as the active user when the template is subst'd, which is to say when it has been joined within the same block once again.

Substitution

Main article: Help:Substitution

Putting "subst:" after the double opening braces causes an automatic conversion of wikitext when the referring page is saved: the subst tag is replaced by the wikitext of the template, with the parameter values substituted for the parameters.

For optional multi-level substitution of templates add a substitution parameter with the empty string as default, e.g. {{{substp|}}}, after the opening braces in all calls of templates, parser functions and variables in templates, and add parameter definitions of the form "substq={{{substr|}}}" in all template calls within templates. Note that the use of multiple substitution parameters allows selective substitution. Maximum flexibility is obtained by making them all different. Sometimes a little less flexibility is needed; e.g. there may be a group of parser functions in a template for which it is sufficient to be able to choose between substitution of all or none in the group.

To reduce, in the wikitext, undefined parameters with default to the defaults themselves, use the alternative way of specifying a parameter default.

Redirection

When a page called for inclusion is a redirect page, the redirect target is included instead. As usual, a double redirect or broken redirect does not work.

A page that consists of nothing else than the inclusion of another page as a template, shows similarities to a redirect to that other page, but also many differences, including:

  • the result has as header the name of the referring page
  • there is no redirect message
  • the buttons such as edit (for editing the whole page), watch, talk, history, "what links here", and "last modified", etc., refer to the referring page (to go to the target page, a section edit link can be used, from there the whole page can be accessed)
  • the referring page is in the same categories as the target page, except when includeonly and/or noinclude tags are used
  • "double redirects", with one or both being this kind of "pseudo-redirect", work.

Compare Help:Template namespace with Help:Template namespace - demo of alternative for redirect.

In a page that in turn includes such a referring page there is no difference.


Note that embedding works for all pages where redirecting works, and it also fails for all pages where a redirect won't work.

Self-transclusion

A page can transclude itself, but is protected against an infinite loop: the transclusion in the transcluded copy of the template is replaced by the bolded pagename. See Template:Pim and Template:Pim.

Transcluding on page A page B redirecting to page A does not allow more repetition anymore, see Template:Pim with Template:Pim.

If due to the use of braces hidden in a template (see Help:Recursive conversion of wikitext) the saved wikitext of a page contains a substitution call to itself, then on the next save the substitution is applied for one level; the transclusion in the transcluded copy of the template is replaced by a link to the pagename. Thus it becomes a self link, i.e., it becomes bolded text with immediately following characters also bolded. This time the link is followed in the wikitext by the hidden message "<!-- WARNING: template loop detected -->"; see [6]; this edit was automatic by edit/save without making a change in the wikitext oneself.

Parsing braces

If some braces are "hidden" in a template, as in template:lb ( talk edit history links ), then parsing of pairs of double and triple braces is based on visible braces only. In the case of partial substitution, first parsing is done for the purpose of substitution, then parsing is done again for rendering:

  • {{t1|{{lb}}tc}}}} gives Template:T1}} - the last-but-one pair of closing braces is taken as closing braces of template:t1 ( talk edit history links )
  • {{t1|{{subst:lb}}tc}}}} gives the wikitext {{t1|{{tc}}}} rendered as Template:T1 - after substitution the last pair of closing braces is taken as closing braces of t1
  • {{subst:t1|{{subst:lb}}tc}}}} gives the wikitext start{{tc-end}} rendered as startTemplate:Tc-end - the last-but-one pair of closing braces is taken as closing braces of t1 for both substitutions
  • {{subst:subst}}t1|a}}, substituting template:subst ( talk edit history links ), gives the wikitext {{subst:t1|a}} rendered the same; in the next edit the wikitext is converted to "startaend".
  • {{subst:tsubst 1|a}}, substituting template:tsubst 1 ( talk edit history links ): ditto.

With parser function #if instead of template:t1:

  • {{#if:x|{{lb}}t1|a}}|b}} gives Template:Lbt1|b}} - the last-but-one pair of closing braces is taken as closing braces of #if.
  • {{#if:x|{{subst:lb}}t1|a}}|b}} gives the wikitext {{#if:x|{{t1|a}}|b}} rendered as Template:T1 - after substitution the last pair of closing braces is taken as closing braces of #if.
  • {{subst:#if:x|{{subst:lb}}t1|a}}|b}} gives the wikitext {{t1|b}} rendered as Template:T1 - the last-but-one pair of closing braces is taken as closing braces of #if for both substitutions.
  • {{subst:#if:x|{{subst:subst}}t1|a}}|b}} gives the wikitext "{{subst:t1|b}}" rendered the same: "{{subst:t1|b}}" (the substitution phase is finished; the ordinary template expansion phase ignores substitution calls); in the next edit the wikitext is converted to "startbend".
  • {{subst:#if:x|{{subst:#if:x|{{subst:subst}}t1|a}}|b}}|c}} gives the wikitext "{{subst:t1|c}}" rendered the same: "{{subst:t1|c}}".
  • {{subst:#if:x|{{subst:tsubst 1|a}}|b}} gives the wikitext "startaend". Apparently there is a substitution phase of parameter expansion resulting in the parameter value "{{subst:t1|a}}", and another substitution phase for the substitution of #if, taking the parameter value not as plain text, but interpreting the "subst:" in it again.

See also Help:Recursive conversion of wikitext.

Predefined templates

Variables and parser functions can be considered predefined templates. They do not have a page that defines in terms of wikitext what they do, because their action is something extra that cannot be constructed from more basic wikitext functionality.

Some additional differences:

  • Parser functions use ":" instead of the first "|".
  • An edit page does not list predefined templates used on the page.
  • No "What links here" is available for predefined templates, to find the pages where they are used.
  • Apart from #switch, predefined templates don't allow you to define named parameters, and therefore equal signs generally have no special effect, comparison:
    {{ #if: not blank | 2=abc }} gives 2=abc
    {{ ifdef | not blank | 2=abc }} gives Template:Ifdef

Ordinary and predefined templates have in common that their parameter values can depend on ordinary and predefined templates. In the case of named parameters, the names used in the template call can also depend on ordinary and predefined templates. This applies also for the named parameters of #switch. Since everything before the first pipe can depend on ordinary and predefined templates, a difference is that in the case of a parser function this applies to the whole combination of name, colon, and first parameter: {{#exp{{1x|r:}}2*3}} gives 6, while {{t1{{!}}a}} gives {{t1|a}}. Another difference is that a blank space is allowed between the template name and the first pipe, but not between the parser function name and the colon: {{t1 |a}} gives Template:T1, while {{#expr:2*3}} gives 6.

Template expansion

Expansion of wikitext can be relevant even if the wikitext resulting from these subevaluations cannot have effect on the overall wikitext, because the expansion can have side effects:

  • Subevaluations producing links add on the target page to the list of incoming links (What links here), based on the pagelinks table, even if there is no link on the resulting page.
  • Subevaluations involving template calls add on the template page in "What links here" and on the edit page of the page concerned to inclusion items, based on the templatelinks table.
  • Some extensions may perform some action during a subevaluation, e.g. with VariablesExtension a subevaluation may involve assignment of a value to a variable.

In the case of #ifexpr etc., either the then-part or the else-part is expanded, not both. Also the wikitext for the value of an unused template parameter is not expanded.

Template argument size

On Wikimedia wikis the maximum template argument size is 2_048_000 bytes per page.

Example:

{{5x|{{4x|abc}}}} has a template argument size of 72 bytes: the argument abcabcabcabc is counted 5 times, the argument abc 4 times.

See also w:Wikipedia:Template limits.

Templates in different versions of MediaWiki

See also:

Version 1.6, 2006-04-05

Version 1.4, end 2004

Including the same template more than five times in the same page (see bugzilla:95) works from version 1.4, now on all Wikimedia sites. Also new was that subst can be used with parameters.

See release notes and [7] [8].

Version 1.3.4, 2004-09-28

  • Fixed lots of template-related bugs, esp. for cases where template variables are used for links, images, etc. Also a template name can now depend on the value of a parameter.[9] [10]
  • Fixed broken edit links being shown for sections from included templates

See release notes.

Version 1.3.0, 2004-08-11

Introduction of template parameters. Introduction of the Template namespace. The prefix "msg:" became optional. See release notes and MediaWiki roadmap.

Version 1.1.0, 2003-12-08

Templates without parameters were introduced along with the introduction of user-editable interface messages. They shared the new MediaWiki namespace. A page MediaWiki:mytemplate was referred to with {{msg:mytemplate}}. See release notes and description and usage on the English Wikipedia.

History

When ParserFunctions were not available yet, multiple assignments of parameters with the same name (and thus allowing parameter overrides) were ingeniously used to perform branching, e.g. in the now deleted en:Template:If defined.

Revision history of pages containing templates

Pages in the stored page history consist of wikitext with possible references to templates and images. When viewing an old version of a page, these refer to the current versions of the templates and images, if these still exist. Thus the former composite page is not reconstructed.

Other pages not primarily meant for direct viewing

Pages not primarily meant for direct viewing include, apart from templates:

Labeled section transclusion

Template:Lst

It allows transclusion of a section of a page, where "section" is not necessarily a section in the sense of Help:Section, it can be any marked part of a page, even a non-contiguous part, and sections can overlap. It allows content that is reused to reside on some master page instead of in a separate template (with that content only, or a collection of that kind of content). For example, a population figure of a town can be in the wikitext of the town page, and be reused elsewhere in a table of towns, or conversely.

This provides integrated editing and an integrated edit history of the combination of specific and reused content on the master page. Note however that not having a separate edit history of a piece of reused content can also be a disadvantage.

Example of transclusion from Help:Advanced templates:

"{{#lst:Help:Advanced templates|vartemp}}" → "{{#lst:Help:Advanced templates|vartemp}}" [11]

Example of transclusion from the page itself:

p=<section begin=p/>12<section end=p/>, p<sup>2</sup>={{#expr:{{#lst:Help:Template|p}}^2}} gives p=<section begin=p/>12<section end=p/>, p2=Expression error: Unrecognized punctuation character "{"..

Section tags have to be explicit in the plain wikitext, not just in the expanded wikitext, and not in a parameter:

Using Template:2x, {{2x|1=<section begin=a/>a<section end=a/>}} gives <section begin=a/>a<section end=a/><section begin=a/>a<section end=a/>. However, "{{#lst:Help:Template|a}}" → "{{#lst:Help:Template|a}}" [12], so neither the expanded wikitext effectively contains the section tags, nor does the parameter value "<section begin=a/>a<section end=a/>" directly define a section.

However, {{subst:2x|1=<section begin=b/>b<section end=b/>}} produces the expected wikitext <section begin=b/>b<section end=b/><section begin=b/>b<section end=b/> rendered as <section begin=b/>b<section end=b/><section begin=b/>b<section end=b/>, so "{{#lst:Help:Template|b}}" → "{{#lst:Help:Template|b}}" [13].

Also, the section tags do not work if they are between <math> tags. However, they can between span tags, so the section content can be hidden. For example:

<span style="display:none"><section begin=q/>12<section end=q/></span>{{#tag:math|q={{#lst:Help:Template|q}}, q^2={{#expr:{{#lst:Help:Template|q}}^2}}}} gives <section begin=q/>12<section end=q/><math>q={{#lst:Help:Template|q}}, q^2=Expression error: Unrecognized punctuation character "{".</math>

For some more examples see Help:Array#Labeled section transclusion.

Alternatives

An alternative that does not require the extension puts the sections (pieces of content) to be reused inside <onlyinclude> tags, or the rest is put in <noinclude> tags. {{#ifeq:{{{1}}}|k|k'th section}} can be used to select a section. Another method, used in s:Bible (King James)/Philemon, makes each section the default value of a parameter; this allows selection of one or more sections by giving the parameters other than the corresponding ones the empty string as value. Thus the excluded sections have to be specified; to avoid making it cumbersome to select just one section the auxiliary template s:Template:Sec is used.

These alternatives have no straightforward way of allowing overlapping sections.

Transcluding a page from the start to a specified position in the expanded wikitext can be done with #padleft without any preparation of that page. If one wants to control the exact end of the transcluded part, without a sentence or paragraph cut off, every time the first part of the page is edited the truncation parameter has to be adjusted.

Notes

<references />

See also


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