Editing Help:Template (section)

=== Parameters ===
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The basic transclusion syntax for a template can be further controlled using [[parameters]], which allow you to customize a template’s output. The syntax is <code><nowiki>{{Template name|</nowiki>''parameter''<nowiki>|</nowiki>''parameter''<nowiki>|...}}</nowiki></code>, where <code>{{var|Template name}}</code> is the template’s name, and each <code>{{var|parameter}}</code> can be either a simple value (known as an ''{{dfn|unnamed parameter}}'') or in the form <code>{{var|name}}={{var|value}}</code> (known as a ''{{dfn|named parameter}}''). The first, second, third, etc., unnamed parameters are named <code>1</code>, <code>2</code>, <code>3</code>, etc., so using <code><nowiki>{{Template name|1=value1|2=value2}}</nowiki></code> is equivalent to <code><nowiki>{{Template name|value1|value2}}</nowiki></code>.

Each template has specific parameters it can accept, as defined within its code. You can list named parameters in any order in a template call. Extra or misnamed parameters have no effect, while missing parameters cause a default value to be used. If a parameter is repeated, the last defined parameter value is used.

The value of a parameter can be an [[empty string]], such as when the pipe (<code>|</code>) or equals sign is followed immediately by the next pipe or the closing braces. For example, <code><nowiki>{{Template name|1=|2=test}}</nowiki></code> produces the same output as <code><nowiki>{{Template name||2=test}}</nowiki></code>. This is different from not specifying the parameter at all, which results in a default value, although templates are often coded so as to behave the same in both cases.

If a template call specifies a parameter which is not defined in the template, it has no effect. Editors sometime specify a parameter they know is not defined in the template,  For example, editors sometimes include a parameter like {{tag|reason|open}} to add a brief explanation within the source as a [[Wikipedia:Manual of Style/Hidden text|hidden comment]]. (But some templates, such as [[Template:Requested move|Requested move]], are programmed to show the reason parameter if provided). Certain templates, especially complex ones like [[Wikipedia:Infobox|infoboxes]], may use the [[Module:Check for unknown parameters|check for unknown parameters module]] to alert editors about any [[:Category:Unknown parameters|unrecognized parameters]] to help avoid unintentional errors.

If a parameter is specified as blank (e.g., {{para|1}}), it is treated as an empty string, which differs from leaving it out altogether (in which case the default value applies). However, templates are often coded to act the same in both cases.

==== Whitespace handling ====

Leading and trailing [[Wikipedia:Whitespace|whitespace]] (including line breaks) around {{em|named}} parameters and values is [[Whitespace collapsing|collapsed]] automatically, but spaces in the middle of the value are kept. For instance, <code><nowiki>{{ ... | myparam = this is a test }}</nowiki></code> has the same output as <code><nowiki>{{ ... |myparam=this is a test}}</nowiki></code>. However, with {{em|unnamed}} parameters, all whitespace is retained as preserved in the output.

The collapsing of line breaks around parameters can be used to [[Wikipedia:Template namespace#Readability of the code|improve the readability of a template call]] with many parameters by placing each parameter specification in its own line.

==== Variable length parameter lists ====
In MediaWiki, templates cannot automatically handle an unknown or unlimited number of parameters. Each possible parameter usually has to be predefined in the template code. For example, a template might be set up to use three specific parameters, such as <code>1</code>, <code>2</code>, and <code>3</code>. If someone includes additional parameters beyond those, they will not affect expansion of the template.

However, there are some ways to work around this:

*Setting a limit: You can write the template to handle a fixed number of parameters by manually specifying each one (e.g., up to 10 or 20 parameters).

*Using templates or [[Wikipedia:Lua|modules]]: For cases needing a flexible number of inputs, templates can use Lua modules or helper templates (like {{mfl|separated entries|main}} or {{tl|separated entries}}). These helper tools provide more advanced handling, such as counting or iterating through parameters, enabling the use of multiple inputs without setting an exact number.

However, modules can bypass this limitation. For simpler cases, the separated entries module expands all sequential parameters and lets you set custom delimiters. For more advanced usage, the {{mfl|params}} module enables counting, listing, mapping, filtering, and handling a variable number of parameters without prior knowledge of the exact number.

This workaround allows it to <em>appear</em> as if a template handles a variable number of parameters because the module processes each one iteratively up to a certain point, but true infinite flexibility is not built into the core MediaWiki system.{{Clarify|date=April 2025}} Several examples are collected under [[:Category:Variadic templates]].