xml2 1.0.0

This article is originally published at https://www.rstudio.com/blog/

We are pleased to announced that xml2 1.0.0 is now available on CRAN. Xml2 is a wrapper around the comprehensive libxml2 C library, and makes it easy to work with XML and HTML files in R. Install the latest version with:

install.packages("xml2")

There are three major improvements in 1.0.0:

1. You can now modify and create XML documents.

2. xml_find_first() replaces xml_find_one(), and provides better semantics for missing nodes.

3. Improved namespace handling when working with XPath.

There are many other small improvements and bug fixes: please see the release notes for a complete list.

Modification and creation

xml2 now supports modification and creation of XML nodes. This includes new functions xml_new_document(), xml_new_child(), xml_new_sibling(), xml_set_namespace(), xml_remove(), xml_replace(), xml_root(), and replacement methods for xml_name(), xml_attr(), xml_attrs() and xml_text().

The basic process of creating an XML document by hand looks something like this:

root <- xml_new_document() %>% xml_add_child("root")

root %>%
  xml_add_child("a1", x = "1", y = "2") %>%
  xml_add_child("b") %>%
  xml_add_child("c") %>%
  invisible()

root %>%
  xml_add_child("a2") %>%
  xml_add_sibling("a3") %>%
  invisible()

cat(as.character(root))
#> <?xml version="1.0"?>
#> <root><a1 x="1" y="2"><b><c/></b></a1><a2/><a3/></root>

For a complete description of creation and mutation, please see vignette("modification", package = "xml2").

xml_find_first()

xml_find_one() has been deprecated in favor of xml_find_first(). xml_find_first() now always returns a single node: if there are multiple matches, it returns the first (without a warning), and if there are no matches, it returns a new xml_missing object.

This makes it much easier to work with ragged/inconsistent hierarchies:

x1 <- read_xml("<a>
  <b></b>
  <b><c>See</c></b>
  <b><c>Sea</c><c /></b>
</a>")

c <- x1 %>%
  xml_find_all(".//b") %>%
  xml_find_first(".//c")
c
#> {xml_nodeset (3)}
#> [1] <NA>
#> [2] <c>See</c>
#> [3] <c>Sea</c>

Missing nodes are replaced by missing values in functions that return vectors:

xml_name(c)
#> [1] NA  "c" "c"
xml_text(c)
#> [1] NA    "See" "Sea"

XPath and namespaces

XPath is challenging to use if your document contains any namespaces:

x <- read_xml('
 <root>
   <doc1 xmlns = "http://foo.com"><baz /></doc1>
   <doc2 xmlns = "http://bar.com"><baz /></doc2>
 </root>
')
x %>% xml_find_all(".//baz")
#> {xml_nodeset (0)}

To make life slightly easier, the default xml_ns() object is automatically passed to xml_find_*():

x %>% xml_ns()
#> d1 <-> http://foo.com
#> d2 <-> http://bar.com
x %>% xml_find_all(".//d1:baz")
#> {xml_nodeset (1)}
#> [1] <baz/>

If you just want to avoid the hassle of namespaces altogether, we have a new nuclear option: xml_ns_strip():

xml_ns_strip(x)
x %>% xml_find_all(".//baz")
#> {xml_nodeset (2)}
#> [1] <baz/>
#> [2] <baz/>

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