Module:Ns has subpages

This Lua module is used in system messages, and on approximately 2,220,000 pages, or roughly 5738% of all pages.
Changes to it can cause immediate changes to the Wikipedia user interface.
To avoid major disruption and server load, any changes should be tested in the module's /sandbox or /testcases subpages, or in your own module sandbox. The tested changes can be added to this page in a single edit. Please discuss changes on the talk page before implementing them.

This module is subject to page protection. It is a highly visible module in use by a very large number of pages, or is substituted very frequently. Because vandalism or mistakes would affect many pages, and even trivial editing might cause substantial load on the servers, it is protected from editing.

This module finds whether a given namespace can have subpages.

Usage

From wikitext

From wikitext this module must be used via the {{ns has subpages}} template. Please see the template page for documentation.

From Lua

Usually Lua modules should use mw.site.namespaces[namespace].hasSubpages rather than this module. But if you have a good reason, it can be accessed like this:

Load the module:

local mNsHasSubpages = require('Module:Ns has subpages')

The subpage information can be found with the ._main function:

mNsHasSubpages._main(ns, frame)

ns is the namespace name, number, or a page name. It defaults to the current namespace.
frame is a frame object with which we can call frame:callParserFunction if necessary. This is optional, and intended for internal use.

-- This module implements [[Template:Ns has subpages]].
-- While the template is fairly simple, this information is made available to
-- Lua directly, so using a module means that we don't have to update the
-- template as new namespaces are added.

local p = {}

function p._main(ns, frame)
	-- Get the current namespace if we were not passed one.
	if not ns then
		ns = mw.title.getCurrentTitle().namespace
	end

	-- Look up the namespace table from mw.site.namespaces. This should work
	-- for a majority of cases.
	local nsTable = mw.site.namespaces[ns]

	-- Try using string matching to get the namespace from page names.
	-- Do a quick and dirty bad title check to try and make sure we do the same
	-- thing as {{NAMESPACE}} in most cases.
	if not nsTable and type(ns) == 'string' and not ns:find('[<>|%[%]{}]') then
		local nsStripped = ns:gsub('^[_%s]*:', '')
		nsStripped = nsStripped:gsub(':.*$', '')
		nsTable = mw.site.namespaces[nsStripped]
	end

	-- If we still have no match then try the {{NAMESPACE}} parser function,
	-- which should catch the remainder of cases. Don't use a mw.title object,
	-- as this would increment the expensive function count for each new page
	-- tested.
	if not nsTable then
		frame = frame or mw.getCurrentFrame()
		local nsProcessed = frame:callParserFunction('NAMESPACE', ns)
		nsTable = nsProcessed and mw.site.namespaces[nsProcessed]
	end
	
	return nsTable and nsTable.hasSubpages
end

function p.main(frame)
	local ns = frame:getParent().args[1]
	if ns then
		ns = ns:match('^%s*(.-)%s*$') -- trim whitespace
		ns = tonumber(ns) or ns
	end
	local hasSubpages = p._main(ns, frame)
	return hasSubpages and 'yes' or ''
end

return p