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NeXML is a file format for phylogenetic data. It is inspired by the modular architecture of the commonly-used NEXUS file format (hence the name) in that a NeXML instance document can contain:

  • sets of Operational Taxonomic Units (OTUs), i.e. the tips in phylogenetic trees, and that which comparative observations are made on. Often these are species ("taxa").
  • sets of phylogenetic trees (or reticulate trees, i.e. networks)
  • sets of comparative data, i.e. molecular sequences, morphological categorical data, continuous data, and other types.

The elements in a NeXML document can be annotated using RDFa (, which means that every object that can be parsed out of a NeXML document must be an object that, in turn, can be annotated with predicates (and their namespaces) and other objects (with, perhaps, their own namespaces). The advantage over previous file formats is that we can retain all metadata for all objects within one file, regardless where the metadata come from.

NeXML can be transformed to RDF using an XSL stylesheet. As such, NeXML forms an intermediate format between traditional flat file formats (with predictable structure but no semantics) and RDF (with loose structure, but lots of semantics) that is both easy to work with, yet ready for the Semantic Web.

To learn more, visit


Currently all the parsing is done at the start( i.e. no streaming ). This is likely to change later. Parse an NeXML file:

  doc = "trees.xml" )
  nexml = doc.parse
  nexml.class #Bio::NeXML::Nexml


Bio::NeXML::Writer class provides a wrapper over libxml-ruby to create any NeXML document. This class defines a set of serialize_* instance methods which can be called on the appropriate object to get its NeXML representation. The method returns a XML::Node object. To get the raw NeXML representation to_s method should be called on the return value.

NeXML defines three top level containers: otus, trees, characters which bear parent-child relation with other NeXML elements. In effect, a valid NeXML document has only three type of immediate children. Naturally, a typical working paradigm would be to create Bio::NeXML::Otus, Bio::NeXML::Trees, and Bio::NeXML::Characters objects and write them to the NeXML file.

  # Parse a test file. This will give us Bio::NeXML::Otus,
  # Bio::NeXML::Trees, and Bio::NeXML::Characters object.
  doc1 = 'test.xml'
  nexml = doc1.parse

  # Create a Writer object,
  writer =

  # add otus, trees and characters to it,
  writer << nexml.otus
  writer << nexml.trees
  writer << nexml.characters

  # save it. 'sample.xml'

Bio::NeXML::Writer internally calls some serialize_* method at the lowest level. If need be, these serialize_* methods can be called to obtain raw NeXML representation of any NeXML element.

  # Create an otus object with a child otu element
  taxa1 = 'taxa1', 'A taxa block'
  o1 = 'o1', 'A taxon'
  taxa1 << o1

  # Obtain the raw NeXML representation of the otus object created
  writer =
  writer.serialize_otus( taxa1 ).to_s
  # => "<otus label=\"A taxa block\" id=\"taxa1\">\n  <otu label=\"A taxon\" id=\"o1\"/>\n</otus>"

Unit tests for serializer are filled with such use cases.


  #get a hash of otus objects indexed with 'id'

  #get an array of otus objects

  #get an otus by id
  taxa1 = nexml.get_otus_by_id "taxa1"

  #iterate over each otus object
  nexml.each_otus do |taxa|
    puts taxa.label

  nexml.trees_set #return a hash of trees object indexed with 'id'
  nexml.trees #return an array of trees objects.

  #iterate over each trees object
  nexml.each_trees do |trees|
    puts trees.label

  #find a trees by id
  trees1 = nexml.get_trees_by_id 'trees1'

  # characters
  nexml.characters_set #return a hash of characters object indexed with 'id'
  nexml.characters #return an array of characters object

  #iterate over each characters object
  nexml.each_characters do |ch|
    puts ch.label

  #find a characters object by id
  characters = nexml.get_characters_by_id 'chars1'


Taxa blocks and taxons are stored internally as a Ruby hash for faster 'id' based lookup. Consider [ this] NeXML snippet

  #get the id of otus # "taxa1"

  #get the label of otus
  taxa1.label # "Primary taxa block"

  #get a hash of child otu objects indexed with id

  #get an array of child otu objects

  #get an otu object by id
  #get_otu_by_id is an alias of []
  t1 = taxa1[ 't1' ]

  #add an otu object to otus
  t1.add_otu( otu_object )
  #to add more than one otu object at a time use << or otus= method
  t1 << [otu_object1, otu_object2]
  t1.otus = otu_object1, otu_object2

  #or iterate over each otu object
  #each_otu is an alias for each
  taxa1.each do |taxon|
    puts taxon.label

  #check if an otu with given id belongs to an otus or not
  #include? and has? are alias for has_otu?
  taxa1.has_otu? 't2' # => true
  taxa1.has? 't8' # => false

  #an otus object in enumerable &:id # => array of otu ids {|t| t.class == "Lemurs" } #maybe in future


  #get an otu's id # => "t1"

  #get an otu's label
  t1.label # => "Homo sapiens"


Trees and tree and network are stored internally as a Ruby hash for faster 'id' based lookup.

  trees1.class #Bio::NeXML::Trees

  #get the taxa block to which the trees is linked to
  trees1.otus #returns an otus object


  trees1.tree_set #return a hash or tree objects indexed with 'id'
  tress1.trees #return an arrayof trees object

  #iterate over each tree object
  trees1.each_tree do |t|
    puts t.label

  #get a tree object with its 'tree1'
  tree1 = trees1[ 'tree1' ]
  #or, with a conventional method call
  tree1 = trees1.get_tree_by_id 'tree1'
  #or, from a nexml object
  tree1 = nexml.get_tree_by_id 'tree1'

  tree1.class #Bio::NeXML::IntTree or Bio::NeXML::FloatTree

  #check if a tree belongs to a trees or not
  #pass it a tree id
  tree1.has_tree? 'tree1' #return true or false

  #get the number of treess


  trees1.network_set #return a hash or network objects indexed with 'id'
  tress1.networks #return an arrayof network objects

  #iterate over each network object
  trees1.each_network do |n|
    puts n.label

  #get a network object with its id
  network1 = trees1[ 'network1' ]
  #or, with a conventional method call
  network1 = trees1.get_network_by_id 'network1'
  #or, from a nexml object
  network1 = nexml.get_tree_by_id 'network1'

  network1.class #Bio::NeXML::IntTree or Bio::NeXML::FloatTree

  #check if a network belongs to a trees or not
  #pass it a network id
  trees1.has_network? 'network1' #return true or false

  #get the number of networks

Tree and Network

  #iterate over both trees and networks
  trees1.each do |g|
    puts g.class

  #find if a tree or a network belongs to a trees or not
  #include? is an alias for has?
  trees1.has? 'tree1' #return true or false

  #total number of trees and networks

All the available methods from [ Bio::Tree] class can be called on a tree object.

  node1 = tree.get_node_by_name "n3" #note name is same as id
  tree1.parents node1

A trees object is an enumerable: &:id


  puts characters.class

  #get the taxa block to which the characters is linked to
  characters.otus #returns an otus object

  #get the child format element
  format = characters.format

  puts format.class

  #get the child matrix element
  matrix = characters.matrix

  puts matrix.class


  format.states_set #return a hash of states objects indexed with 'id'
  format.states #return an array of states object

  #iterate over each states object
  format.each_states do |states|
    puts states.label

  #get a states object by id
  states = format.get_states_by_id 'states1'

  #check if the states object with 'id' belongs to format or not
  format.has_states? 'states1'

  format.char_set #return a hash of char objects indexed with 'id'
  format.chars #return an array of char objects

  #iterate over each char object
  format.each_char do |char|
    puts char.label

  #get a char object by id
  char = format.get_char_by_id 'char1'

  #check if the char object with 'id' belongs to format or not
  format.has_char? 'char1'

  #get a states or a char object by id
  state = format[ 'states1' ]
  char = format[ 'char1' ]

  #check if a states or a char object with 'id' belongs to format or not
  format.has? 'states1'
  format.has? 'char1'

  #all objects, including char and states can be iterated over with each
  format.each do |obj|
    puts obj.class

  #format is enumerable &:id


  states.state_set #return a hash of state objects indexed with 'id'
  states.states #return an array of state objects

  #iterate over each state object
  states.each_state do |state|
  #or, use its alias each

  #get a state object by id
  state = states.get_state_by_id 'state1'
  #or, use hash notation
  state = states[ 'state1' ]

  #check if a state belongs to states or not
  states.has_state? 'state1'
  #or, use its alias has? and include?
  #get the symbol associated with the state

  #find if the state is ambiguous

  #find the kind of ambiguity

  #find if it is an uncertain state set

  #find if it is a polymorphic state set

  #get the members of a state set as an array

  #or iterate over each member
  state.each do |member|
    puts member.class #same as self

  #a state is Enumerable over its members{ |member| == "rna5" }


  #get the id

  #get the label

  #get the states object the char is linked to

  #get the codon position for DnaChar and RnaChar objects



Contributing to bio-nexml

  • Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet
  • Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it
  • Fork the project
  • Start a feature/bugfix branch
  • Commit and push until you are happy with your contribution
  • Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
  • Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.


The research leading to these results has received funding from the [European Community's] Seventh Framework Programme ([FP7/2007-2013] under grant agreement n¡ [237046].

Citing bio-nexml

If you use this software, please cite:

NeXML: rich, extensible, and verifiable representation of comparative data and metadata


Biogem: an effective tool based approach for scaling up open source software development in bioinformatics

Copyright (c) 2011 Rutger Vos and Anurag Priyam. See LICENSE.txt for further details.


BioRuby plugin to read and write NeXML data







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