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Switching an R package from XML to xml2

Switching from XML to xml2


The R package XML for parsing and manipulation of XML documents in R is not actively maintained anymore, but used by many:

The R package xml2 is an actively maintained, more recent alternative.

This file documents useful resources and steps for moving from XML to xml2.


Finding usage

The itdepends package helps with finding all usages of XML, see

devtools::install_github("jimhester/itdepends") library("itdepends") itdepends::dep_locate("XML")

Mapping functions


XML xml2 Comment
XML::getNodeSet(doc = <document object>, path = "<XPath expression>") or XML::xpathApply(...) xml2::xml_find_all(..) and xml2::xml_find_one(..) with x = <node>, xpath = "<XPath 1.0 expression>" Find matching nodes value of a node's attribute
XML::htmlTreeParse(<path>, asText = <treat file as text>) xml2::read_html(<path, URL, connection, or literal xml>) parse HTML document
XML::isXMLString("<string>") No direct equivalent, can try to parse... Heuristically determine if string is XML
XML::toString.XMLNode(<node>) as.character(<document or node>) object to character
XML::xmlAttrs(node = <node object>) xml2::xml_attrs(x = <document, node, or node set>) Get the attributes of a node, both return a named character vector.
XML::xmlApply(X = <node>) and XML::xmlSApply(..) functions xml2::xml_attrs(..) and xml2::xml_contents(..) are vectorized Apply function to each child of a node
XML::xmlChildren(x = <node object>)[["<name of the sub-node>"]] xml2::xml_child(x = <node>, search = <number, or name of the sub-node>) (only elements) and xml2::xml_contents(..) for all nodes Get sub-nodes of a node
XML::xmlElementsByTagName(el = <node object>, name = "<name to match>") xml2::xml_find_all(x = <document, node, node set>, xpath = "<name to match>") Retrieve children matching tag name (children/sub-elements)
XML::xmlGetAttr(node = <node object>, name = "<attribute name>", default = "<default>") xml2::xml_attr(x = <document, node, or node set>, attr = "<attribute name>") Get value of a node's attribute
XML::xmlName(node = <node object>) xml2::xml_name(x = <document, node, or node set>) Get name of a node
XML::xmlParse(..) xml2::read_xml(..) Unexposed method in XML ?
XML::xmlParseDoc(file = <file name> or "<xml content>", asText = !file.exists(file)) xml2::read_xml(x = <string, connection, URL, or raw vector>) parse XML document
XML::xmlParseString(content = "<string>") xml2::read_xml(x = <string, connection, URL, or raw vector>) convenience function XML to node/tree
XML::xmlRoot(x = <node object>) xml2::xml_root(x = <document, node, or node set> Get top-level node
XML::xmlSize(obj = <node or document object>) xml2::xml_length() Note that xml_length(..) does not need to go to the root first, i.e. XML::xmlSize(XML::xmlRoot(old)) == xml2::xml_length(new)
XML::xmlToList(node = <xml node or document>) xml2::as_list(x = <document, node, or node set>) convert to R-like list; difference: as_list does not drop the root element
XML::xmlTreeParse(file = <file name> or "<xml content>", asText = !file.exists(file)) parse XML document
if(!is.null(<node object>[["<child name>"]])) { (inherits(xml_child(<node object>, "<child name>"), "xml_missing") Checking for child node existence
XML::xmlValue(<node object>) xml2::xml_text(x = <document, node, or node set>) Get/Set contents of a leaf node

Common snippets

XML xml2 Comment
if (!is.null(XML::xmlChildren(x = obj)[[<node name>]])) if (!inherits(xml2::xml_find_first(x = obj, xpath = <node name>), "xml_missing") Check if element exists.
if(!is.null(XML::xmlAttrs(node = obj)[["href"]])) if(! = obj, attr = "href"))) Checking for potentiall non-existing attribute


XML xml2 Comment
XML::addAttributes(node = <node object>, ..., .attrs = <character vector with attribute names>, append = <replace or add>) xml2::xml_set_attrs(x = <document, node, node set>, value = <named character vector>) to set multiple attributes and overwrite existing ones, or xml2::xml_set_attr(x = <node>, attr = <name>, value = <value>) to append a single attribute Add attributes to a node; in xml2 no re-assigning the object is needed, i.e. no doc <- XML::addAttributes(node = doc, ...)
XML::addChildren(node = <node object>, kids = list()) xml2::xml_add_child(.x = <document or nodeset>, .value = <document, node or nodeset>) Add child nodes to a node
XML::saveXML(doc = <xml document object>, file = "<file name>") xml2::write_xml(x = <document or node>, file = "<path or connection">) Write XML document to string or file
XML::xmlNamespaceDefinitions(x = <node>) xml2::xml_ns(x = <document, node, or node set>) Get namespace definitions from a node
XML::xmlNode(name = "<node name>") xml2::xml_new_document %>% xml2::xml_add_child("<node name>") or (preferred in docs) xml2::xml_new_root("<node name>") Create a new node
XML::xmlValue() xml2::xml_text(x = <document, node, or node set>) Get/Set contents of a leaf node


XML xml2 Comment
XMLAbstractDocument xml_document ..
XMLAbstractNode, XMLCommentNode, XMLTextNode, ... xml_node ..
? xml_missing ..


The following steps were applied in switching from XML to xml2 for the package sos4R. This is not a "clean" process, but hopefully provides useful input for other's doing the switch. Ideally the lessons learned on what can be "regex-ed" and what needs manual interaction go into the above tables at a later stage.

  1. Make sure all functions use named parameters and package prefix with the following regular expressions
  • addAttributes\((?!node) replaced with XML::addAttributes(node =
  • addChildren\(node replaced with XML::addChildren(node
  • getNodeSet\((?!doc) replaced with XML::getNodeSet(doc =
  • isXMLString\((?!str) replaced with XML::isXMLString(str =
  • saveXML\((?!doc) replaced with XML::saveXML(doc =
  • xmlAttrs\((?!node) replaced with XML::xmlAttrs(node =
  • xmlChildren\((?!x) replaced with XML::xmlChildren(x =
  • xmlElementsByTagName replaced with XML::xmlElementsByTagName
  • xmlGetAttr\((?!node) replaced with XML::xmlGetAttr(node =
  • xmlName\((?!node) replaced with XML::xmlName(node =
  • xmlNode\((?!name) and xmlNode\(name = replaced with XML::xmlNode(name =
  • xmlParse\( replaced with XML::xmlParse(file =
  • xmlParseDoc\((?!file) replaced with XML::xmlParseDoc(file =
  • xmlParseString\( replaced with XML::xmlParseString(content =
  • xmlRoot\((?!x) replaced with XML::xmlRoot(x =
  • xmlSize\( replaced with XML::xmlSize(obj =
  • xmlToList\( replaced with XML::xmlToList(node =
  • xmlTreeParse\( replaced with XML::xmlTreeParse(file =
  • xmlValue\((?!x) replaced with XML::xmlValue(x =
  1. Imports: XML instead of Depends:
  2. Run tests - skip the ones unrelated to XML handling
  3. Commit:
  4. Do the switch (parsing functions first, all searches in files *.R, files in /sandbox/ ignored for manual corrections; order driven by running a basic parsing test and see where it fails next)
  • XML::xmlParseDoc
    • Replace XML::xmlParseDoc(file = with xml2::read_xml(x = (26 occurrences)
    • Fix parameters
      • drop , asText = TRUE by replacing it with `` (blank, 11 occurrences)
      • turn options into vector with strings
      • replace c(XML::NOERROR, XML::RECOVER) with SosDefaultParsingOptions()
      • use xmlParseOptions everywhere
  • XML::xmlParseString
    • Replaced manually by simplifying the implementation of encodeXML for signature "character"
  • XML::xmlParse
    • Replace single occurrence manually and refactored method parseFile
  • XML::xmlRoot
    • Replace XML::xmlRoot with xml2::xml_root (25 occurrences)
  • XML::xmlName
    • Replace XML::xmlName(node = with xml2::xml_name(x = (30 occurrences)
    • Manually added , ns = SosAllNamespaces() later to have names with prefix
  • XML::xmlAttrs
    • Replace XML::xmlAttrs(node = with xml2::xml_attrs(x = (3 occurrences)
    • Fix further occurrences manually by searching for xmlAttrs (must have slipped by before)
    • xml2::xml_attrs(x = obj)[["href"]] does not work because if attribute href does not exist there will be a "subscript out of bounds" error. Need to use
  • Search for xml2::xml_attrs\(x = (.*)\[\[ and fix manually to xml2::xml_attrs(x = obj, attr = "<attribute name>") and update subsequent is.null(..) checks to use
  • XML::xmlGetAttr
    • Replace XML::xmlGetAttr\(node = (.*), name = with xml2::xml_attr(x = $1, attr = (55 occurrences)
    • Manually fix the ones with spread across multiple lines and with missing name = , can also fix indentation then or remove newline
    • Manually fix where xmlGetAttr was used withn lapply(..) or sapply(..)
  • XML::xmlValue
    • Replace XML::xmlValue\(x = with xml2::xml_text(x = (45 occurrences)
  • XML::xmlChildren
    • Replace XML::xmlChildren\(x = with xml2::xml_children(x = (22 occurrences)
    • The common pattern XML::xmlChildren(x = obj)[[gmlTimeInstantName]] does not work because xml2::xml_children(..) does not return a named list. Need to run xml2::xml_find_all(x = obj, xpath = gmlTimeInstant) or xml2::xml_find_first(..) then. Search for xml2::xml_children\(x = (.*)\[\[ to fix those manually (10 results)
      • ..find_first returns missing node:, "f")) or inherits(xml2::xml_find_first(x, "f"), "xml_missing")
      • ..find_all returns (potentially empty) nodeset: length(xml2::xml_find_all(x, "f"))
  • Replaced occurrences of class XMLAbstractNode and XMLInternalDocument for slots in S4 classes with ANY and the default prototype to xml2::xml_missing(), will have to handle stuff manually around these classes
  • Add SosAllNamespaces() and add namespaces to all the xxxName constants in R/Constants.R
  • test_exceptionreports.R complete
  • test_sams.R added and parsing fixed
  • XML::getNodeSet manually switched to xml2::xml_find_all(..) and xml2::xml_find_one(..), because XPath-based getting of sub-nodes with xml2 also requires proper namespaces and some handling can be simplified because of vectorised xml2::xml_text(..).
  • XML::xmlSize
    • Updated single occurrence manually
  • XML::saveXML
    • Replaced XML::saveXML(doc = with xml2::write_xml(x = (6 occurrences), no parameters in saveXML besides doc and file were used
  • Update NAMESPACE to import xml2 and not XML
  • Parsing tests of test_sensors.R work
  • XML::isXMLString
    • Replace with own function using simple regex test: grepl("^<(.*)>$", "...")
  • get rid of .filterXmlChildren and .filterXmlOnlyNoneTexts manually using xml2::xml_child(..), xml2::xml_find_first(..) or xml2::xml_find_all(..)
    • also remove all ".noneText" objects (and by that fix all occurrences of xmlTagName)
    • > fix using (regex, 16 occurrences)
  • must fix all obj[[ because subsetting with [[ does not work with XML (107 occurrences at this point!)
    • trying to automate by replacing obj\[\[(.*?)\]\] with xml2::xml_child(x = obj, search = $1, ns = SosAllNamespaces())
    • revert the changes in summary functions where obj[[..]] was used (file PrintShowStructureSummary-methods.R)
    • does not work for multiple subsets, e.g. obj[["elementCount"]][["Count"]][["value"]] > search for SosAllNamespaces())[[ and fix manually to use XPath (4 occurrences)
    • re-check occurrences of .children[[
    • is.null\(\. with some XML object, should be which picks up on "xml_missing" objects
    • New tests added for...
      • parseOwsRange
      • parseSosFilter_Capabilities
      • parseOwsServiceIdentification
      • parseTime
      • parseSosObservationOffering (also for 2.0.0)
    • fix tests in test_sensors.R
  • [Continue with encoding functions]
  • XML::addAttributes
    • switched manually because sometimes .attrs is used, which is replaced with xml2::xml_set_attrs(), and sometimes not (single ...), which is replaced with xml2::xml_set_attr(), the _set_attr variants operate directly on the object (no need to re-assign), and often statements are multi-line (18 occurrences)
    • get rid of .sos100_NamespaceDefinitionsForAll
  • XML::xmlNode and XML::addChildren
    • manually switched to xml2::xml_new_root("<node name>") and xml2::xml_add_child("<node name>")
    • attrs parameter replaced with xml2::xml_set_attrs()
    • r-lib/xml2#239 is a problem
    • XML::addChildren with "append = TRUE" replace with a for loop and xml2::xml_add_child(..)

Limitations of regexes for the actual switch are due to multi-line statements and the result of functions not being the same. Especially the subsetting with [[ used extensively does not work the same way anymore.

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Your blog is very useful thank you for that.
However; I wanted to let you know that XML is back again after all that period with new version
Best regards,

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nuest commented Sep 3, 2020

Thanks @MohammedFCIS for the pointer. TBH, XML is not actively maintained anymore, and the fact that in the recent days, people had problems installing again is a good sign that switching to xml2 is worth the effort.

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BroVic commented Mar 19, 2023

@nuest A new version of XML was released today. So, it appears it is still being actively maintained. The real question would be what are the pros and cons between these 2 packages. Since you're experienced in both, your thoughts on this would be appreciated.

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XML is not being actively maintained, but the CRAN team (see try to mitigate problems arising from changes in development versions of R itself. Certainly no new project should rely on it; others should migrate away.

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