Psych is a YAML parser and emitter. Psych leverages libyaml for it's YAML parsing and emitting capabilities. In addition to wrapping libyaml, Psych also knows how to serialize and de-serialize most Ruby objects to and from the YAML format.
# Parse some YAML Psych.load("--- foo") # => "foo" # Emit some YAML Psych.dump("foo") # => "--- foo\n...\n" { :a => 'b'}.to_yaml # => "---\n:a: b\n"
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Psych provides a range of interfaces for parsing a YAML document ranging from low level to high level, depending on your parsing needs. At the lowest level, is an event based parser. Mid level is access to the raw YAML AST, and at the highest level is the ability to unmarshal YAML to ruby objects.
The lowest level parser should be used when the YAML input is already known, and the developer does not want to pay the price of building an AST or automatic detection and conversion to ruby objects. See Psych::Parser for more information on using the event based parser.
Psych provides access to an AST produced from parsing a YAML document. This tree is built using the Psych::Parser and Psych::TreeBuilder. The AST can be examined and manipulated freely. Please see ::parse_stream, Psych::Nodes, and Psych::Nodes::Node for more information on dealing with YAML syntax trees.
The high level YAML parser provided by Psych simply takes YAML as input and returns a Ruby data structure. For information on using the high level parser see ::load
Psych provides a range of interfaces ranging from low to high level for producing YAML documents. Very similar to the YAML parsing interfaces, Psych provides at the lowest level, an event based system, mid-level is building a YAML AST, and the highest level is converting a Ruby object straight to a YAML document.
The lowest level emitter is an event based system. Events are sent to a Psych::Emitter object. That object knows how to convert the events to a YAML document. This interface should be used when document format is known in advance or speed is a concern. See Psych::Emitter for more information.
At the mid level is building an AST. This AST is exactly the same as the AST used when parsing a YAML document. Users can build an AST by hand and the AST knows how to emit itself as a YAML document. See Psych::Nodes, Psych::Nodes::Node, and Psych::TreeBuilder for more information on building a YAML AST.
The high level emitter has the easiest interface. Psych simply takes a Ruby data structure and converts it to a YAML document. See ::dump for more information on dumping a Ruby data structure.
Dump Ruby object o to a YAML string.
Optional options may be passed in to control the output
format. If an IO object is passed in, the YAML will be dumped to that IO
object.
Example:
# Dump an array, get back a YAML string Psych.dump(['a', 'b']) # => "---\n- a\n- b\n" # Dump an array to an IO object Psych.dump(['a', 'b'], StringIO.new) # => #<StringIO:0x000001009d0890> # Dump an array with indentation set Psych.dump(['a', ['b']], :indentation => 3) # => "---\n- a\n- - b\n" # Dump an array to an IO with indentation set Psych.dump(['a', ['b']], StringIO.new, :indentation => 3)
# File rake/lib/psych.rb, line 235 def self.dump o, io = nil, options = {} if Hash === io options = io io = nil end visitor = Psych::Visitors::YAMLTree.new options visitor << o visitor.tree.yaml io, options end
Dump a list of objects as separate documents to a document stream.
Example:
Psych.dump_stream("foo\n ", {}) # => "--- ! \"foo\\n \"\n--- {}\n"
# File rake/lib/psych.rb, line 252 def self.dump_stream *objects visitor = Psych::Visitors::YAMLTree.new {} objects.each do |o| visitor << o end visitor.tree.yaml end
Load yaml in to a Ruby data structure. If multiple documents
are provided, the object contained in the first document will be returned.
filename will be used in the exception message if any
exception is raised while parsing.
Raises a Psych::SyntaxError when a YAML syntax error is detected.
Example:
Psych.load("--- a") # => 'a' Psych.load("---\n - a\n - b") # => ['a', 'b'] begin Psych.load("--- `", "file.txt") rescue Psych::SyntaxError => ex ex.file # => 'file.txt' ex.message # => "(foo.txt): found character that cannot start any token" end
# File rake/lib/psych.rb, line 126 def self.load yaml, filename = nil result = parse(yaml, filename) result ? result.to_ruby : result end
Load the document contained in filename. Returns the yaml
contained in filename as a ruby object
# File rake/lib/psych.rb, line 296 def self.load_file filename File.open(filename, 'r:bom|utf-8') { |f| self.load f, filename } end
Load multiple documents given in yaml. Returns the parsed
documents as a list. If a block is given, each document will be converted
to ruby and passed to the block during parsing
Example:
Psych.load_stream("--- foo\n...\n--- bar\n...") # => ['foo', 'bar'] list = [] Psych.load_stream("--- foo\n...\n--- bar\n...") do |ruby| list << ruby end list # => ['foo', 'bar']
# File rake/lib/psych.rb, line 283 def self.load_stream yaml, filename = nil if block_given? parse_stream(yaml, filename) do |node| yield node.to_ruby end else parse_stream(yaml, filename).children.map { |child| child.to_ruby } end end
Parse a YAML string in yaml. Returns
the first object of a YAML AST.
filename is used in the exception message if a
Psych::SyntaxError is raised.
Raises a Psych::SyntaxError when a YAML syntax error is detected.
Example:
Psych.parse("---\n - a\n - b") # => #<Psych::Nodes::Sequence:0x00> begin Psych.parse("--- `", "file.txt") rescue Psych::SyntaxError => ex ex.file # => 'file.txt' ex.message # => "(foo.txt): found character that cannot start any token" end
See Psych::Nodes for more information about YAML AST.
# File rake/lib/psych.rb, line 150 def self.parse yaml, filename = nil parse_stream(yaml, filename) do |node| return node end false end
Parse a YAML string in yaml. Returns
the full AST for the YAML document. This method can
handle multiple YAML documents contained in
yaml. filename is used in the exception message
if a Psych::SyntaxError is raised.
If a block is given, a Psych::Nodes::Document node will be yielded to the block as it’s being parsed.
Raises a Psych::SyntaxError when a YAML syntax error is detected.
Example:
Psych.parse_stream("---\n - a\n - b") # => #<Psych::Nodes::Stream:0x00> Psych.parse_stream("--- a\n--- b") do |node| node # => #<Psych::Nodes::Document:0x00> end begin Psych.parse_stream("--- `", "file.txt") rescue Psych::SyntaxError => ex ex.file # => 'file.txt' ex.message # => "(foo.txt): found character that cannot start any token" end
See Psych::Nodes for more information about YAML AST.
# File rake/lib/psych.rb, line 200 def self.parse_stream yaml, filename = nil, &block if block_given? parser = Psych::Parser.new(Handlers::DocumentStream.new(&block)) parser.parse yaml, filename else parser = self.parser parser.parse yaml, filename parser.handler.root end end
Returns a default parser
# File rake/lib/psych.rb, line 169 def self.parser Psych::Parser.new(TreeBuilder.new) end
Dump Ruby object o to a JSON string.
# File rake/lib/psych.rb, line 262 def self.to_json o visitor = Psych::Visitors::JSONTree.new visitor << o visitor.tree.yaml end