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xml2 1.0.0

Hadley Wickham Headshot
Written by Hadley Wickham
2016-07-05

Please note that the information presented in this post reflects the package as it stood when initially released, and may now be outdated. For the most up-to-date information, kindly refer to https://xml2.r-lib.org/.

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:

```{r}
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:

```{r}
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:

```{r}
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/>
Hadley Wickham Headshot

Hadley Wickham

Chief Scientist, Posit
Hadley is Chief Scientist at Posit PBC, winner of the 2019 COPSS award, and a member of the R Foundation. He builds tools (both computational and cognitive) to make data science easier, faster, and more fun. His work includes packages for data science (like the tidyverse, which includes ggplot2, dplyr, and tidyr)and principled software development (e.g. roxygen2, testthat, and pkgdown). He is also a writer, educator, and speaker promoting the use of R for data science.

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