Overview

Psych is a YAML parser and emitter. Psych leverages libyaml [Home page: pyyaml.org/wiki/LibYAML] or [Git repo: github.com/zerotao/libyaml] for its 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.

I NEED TO PARSE OR EMIT YAML RIGHT NOW!

# Parse some YAML
Psych.load("--- foo") # => "foo"

# Emit some YAML
Psych.dump("foo")     # => "--- foo\n...\n"
{ :a => 'b'}.to_yaml  # => "---\n:a: b\n"

Got more time on your hands? Keep on reading!

YAML Parsing

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.

Low level parsing

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.

Mid level parsing

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.

High level parsing

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

YAML Emitting

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.

Low level emitting

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.

Mid level emitting

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.

High level emitting

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.

YAML Ain't Markup Language

This module provides a Ruby interface for data serialization in YAML format.

The underlying implementation is the libyaml wrapper Psych.

Usage

Working with YAML can be very simple, for example:

require 'yaml' # STEP ONE, REQUIRE YAML!
# Parse a YAML string
YAML.load("--- foo") #=> "foo"

# Emit some YAML
YAML.dump("foo")     # => "--- foo\n...\n"
{ :a => 'b'}.to_yaml  # => "---\n:a: b\n"

Security

Do not use YAML to load untrusted data. Doing so is unsafe and could allow malicious input to execute arbitrary code inside your application. Please see doc/security.rdoc for more information.

History

Syck was the original for YAML implementation in Ruby's standard library developed by why the lucky stiff.

You can still use Syck, if you prefer, for parsing and emitting YAML, but you must install the 'syck' gem now in order to use it.

In older Ruby versions, ie. <= 1.9, Syck is still provided, however it was completely removed with the release of Ruby 2.0.0.

More info

For more advanced details on the implementation see Psych, and also check out yaml.org for spec details and other helpful information.

Methods
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Constants
VERSION = '2.0.0'
 

The version is Psych you're using

LIBYAML_VERSION = Psych.libyaml_version.join '.'
 

The version of libyaml Psych is using

DEPRECATED = __FILE__
 
Class Public methods
add_private_type(type_tag, &block)
# File ext/psych/lib/psych/deprecated.rb, line 48
def self.add_private_type type_tag, &block
  warn "#{caller[0]}: add_private_type is deprecated, use add_domain_type" if $VERBOSE
  domain = 'x-private'
  key = [domain, type_tag].join ':'
  @domain_types[key] = [key, block]
end
add_ruby_type(type_tag, &block)
# File ext/psych/lib/psych/deprecated.rb, line 41
def self.add_ruby_type type_tag, &block
  warn "#{caller[0]}: add_ruby_type is deprecated, use add_domain_type" if $VERBOSE
  domain = 'ruby.yaml.org,2002'
  key = ['tag', domain, type_tag].join ':'
  @domain_types[key] = [key, block]
end
detect_implicit(thing)
# File ext/psych/lib/psych/deprecated.rb, line 34
def self.detect_implicit thing
  warn "#{caller[0]}: detect_implicit is deprecated" if $VERBOSE
  return '' unless String === thing
  return 'null' if '' == thing
  ScalarScanner.new.tokenize(thing).class.name.downcase
end
Psych.dump(o) → string of yaml
Psych.dump(o, options) → string of yaml
Psych.dump(o, io) → io object passed in
Psych.dump(o, io, options) → io object passed in

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 ext/psych/lib/psych.rb, line 237
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_stream(*objects)

Dump a list of objects as separate documents to a document stream.

Example:

Psych.dump_stream("foo\n  ", {}) # => "--- ! \"foo\\n  \"\n--- {}\n"
# File ext/psych/lib/psych.rb, line 254
def self.dump_stream *objects
  visitor = Psych::Visitors::YAMLTree.new {}
  objects.each do |o|
    visitor << o
  end
  visitor.tree.yaml
end
Psych.libyaml_version

Returns the version of libyaml being used

static VALUE libyaml_version(VALUE module)
{
    int major, minor, patch;
    VALUE list[3];

    yaml_get_version(&major, &minor, &patch);

    list[0] = INT2NUM((long)major);
    list[1] = INT2NUM((long)minor);
    list[2] = INT2NUM((long)patch);

    return rb_ary_new4((long)3, list);
}
load(yaml, filename = nil)

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 # => "(file.txt): found character that cannot start any token"
end
# File ext/psych/lib/psych.rb, line 128
def self.load yaml, filename = nil
  result = parse(yaml, filename)
  result ? result.to_ruby : result
end
load_documents(yaml, &block)

This method is deprecated, use ::load_stream instead.

# File ext/psych/lib/psych/deprecated.rb, line 25
def self.load_documents yaml, &block
  if $VERBOSE
    warn "#{caller[0]}: load_documents is deprecated, use load_stream"
  end
  list = load_stream yaml
  return list unless block_given?
  list.each(&block)
end
load_file(filename)

Load the document contained in filename. Returns the yaml contained in filename as a ruby object

# File ext/psych/lib/psych.rb, line 298
def self.load_file filename
  File.open(filename, 'r:bom|utf-8') { |f| self.load f, filename }
end
load_stream(yaml, filename = nil)

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 ext/psych/lib/psych.rb, line 285
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
object_maker(klass, hash)
# File ext/psych/lib/psych/deprecated.rb, line 71
def self.object_maker klass, hash
  warn "#{caller[0]}: object_maker is deprecated" if $VERBOSE
  klass.allocate.tap do |obj|
    hash.each { |k,v| obj.instance_variable_set(:"@#{k}", v) }
  end
end
parse(yaml, filename = nil)

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 # => "(file.txt): found character that cannot start any token"
end

See Psych::Nodes for more information about YAML AST.

# File ext/psych/lib/psych.rb, line 152
def self.parse yaml, filename = nil
  parse_stream(yaml, filename) do |node|
    return node
  end
  false
end
parse_file(filename)

Parse a file at filename. Returns the YAML AST.

Raises a Psych::SyntaxError when a YAML syntax error is detected.

# File ext/psych/lib/psych.rb, line 163
def self.parse_file filename
  File.open filename, 'r:bom|utf-8' do |f|
    parse f, filename
  end
end
parse_stream(yaml, filename = nil, &block)

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 # => "(file.txt): found character that cannot start any token"
end

See Psych::Nodes for more information about YAML AST.

# File ext/psych/lib/psych.rb, line 202
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
parser()

Returns a default parser

# File ext/psych/lib/psych.rb, line 171
def self.parser
  Psych::Parser.new(TreeBuilder.new)
end
read_type_class(type, reference)
# File ext/psych/lib/psych/deprecated.rb, line 61
def self.read_type_class type, reference
  warn "#{caller[0]}: read_type_class is deprecated" if $VERBOSE
  _, _, type, name = type.split ':', 4

  reference = name.split('::').inject(reference) do |k,n|
    k.const_get(n.to_sym)
  end if name
  [type, reference]
end
tagurize(thing)
# File ext/psych/lib/psych/deprecated.rb, line 55
def self.tagurize thing
  warn "#{caller[0]}: add_private_type is deprecated, use add_domain_type" if $VERBOSE
  return thing unless String === thing
  "tag:yaml.org,2002:#{thing}"
end
to_json(o)

Dump Ruby object o to a JSON string.

# File ext/psych/lib/psych.rb, line 264
def self.to_json o
  visitor = Psych::Visitors::JSONTree.new
  visitor << o
  visitor.tree.yaml
end