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Module: Psych

Relationships & Source Files
Namespace Children
Modules:
Classes:
Exceptions:
Defined in: ext/psych/lib/psych.rb,
ext/psych/psych.c,
ext/psych/psych_emitter.c,
ext/psych/psych_parser.c,
ext/psych/psych_to_ruby.c,
ext/psych/psych_yaml_tree.c,
ext/psych/lib/psych/class_loader.rb,
ext/psych/lib/psych/coder.rb,
ext/psych/lib/psych/deprecated.rb,
ext/psych/lib/psych/exception.rb,
ext/psych/lib/psych/handler.rb,
ext/psych/lib/psych/nodes.rb,
ext/psych/lib/psych/omap.rb,
ext/psych/lib/psych/parser.rb,
ext/psych/lib/psych/scalar_scanner.rb,
ext/psych/lib/psych/set.rb,
ext/psych/lib/psych/stream.rb,
ext/psych/lib/psych/streaming.rb,
ext/psych/lib/psych/syntax_error.rb,
ext/psych/lib/psych/tree_builder.rb,
ext/psych/lib/psych/versions.rb,
ext/psych/lib/psych/handlers/document_stream.rb,
ext/psych/lib/psych/handlers/recorder.rb,
ext/psych/lib/psych/json/ruby_events.rb,
ext/psych/lib/psych/json/stream.rb,
ext/psych/lib/psych/json/tree_builder.rb,
ext/psych/lib/psych/json/yaml_events.rb,
ext/psych/lib/psych/nodes/alias.rb,
ext/psych/lib/psych/nodes/document.rb,
ext/psych/lib/psych/nodes/mapping.rb,
ext/psych/lib/psych/nodes/node.rb,
ext/psych/lib/psych/nodes/scalar.rb,
ext/psych/lib/psych/nodes/sequence.rb,
ext/psych/lib/psych/nodes/stream.rb,
ext/psych/lib/psych/visitors/depth_first.rb,
ext/psych/lib/psych/visitors/emitter.rb,
ext/psych/lib/psych/visitors/json_tree.rb,
ext/psych/lib/psych/visitors/to_ruby.rb,
ext/psych/lib/psych/visitors/visitor.rb,
ext/psych/lib/psych/visitors/yaml_tree.rb

Overview

Psych is a YAML parser and emitter. Psych leverages libyaml [Home page: pyyaml.org/wiki/LibYAML] or [HG repo: bitbucket.org/xi/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.

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.

High-level API

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

Reading from a string
Psych.load("--- a")             # => 'a'
Psych.load("---\n - a\n - b")   # => ['a', 'b']
Reading from a file
Psych.load_file("database.yml")
Exception handling
begin
  # The second argument changes only the exception contents
  Psych.parse("--- `", "file.txt")
rescue Psych::SyntaxError => ex
  ex.file    # => 'file.txt'
  ex.message # => "(file.txt): found character that cannot start any token"
end
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.

Writing to a string
# 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)
Writing to a file

Currently there is no direct API for dumping Ruby structure to file:

File.open('database.yml', 'w') do |file|
  file.write(Psych.dump(['a', 'b']))
end

Mid-level API

Parsing

Psych provides access to an AST produced from parsing a YAML document. This tree is built using the Parser and TreeBuilder. The AST can be examined and manipulated freely. Please see .parse_stream, Nodes, and ::Psych::Nodes::Node for more information on dealing with YAML syntax trees.

Reading from a string
# Returns Psych::Nodes::Stream
Psych.parse_stream("---\n - a\n - b")

# Returns Psych::Nodes::Document
Psych.parse("---\n - a\n - b")
Reading from a file
# Returns Psych::Nodes::Stream
Psych.parse_stream(File.read('database.yml'))

# Returns Psych::Nodes::Document
Psych.parse_file('database.yml')
Exception handling
begin
  # The second argument changes only the exception contents
  Psych.parse("--- `", "file.txt")
rescue Psych::SyntaxError => ex
  ex.file    # => 'file.txt'
  ex.message # => "(file.txt): found character that cannot start any token"
end
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 Nodes, ::Psych::Nodes::Node, and TreeBuilder for more information on building a YAML AST.

Writing to a string
# We need Psych::Nodes::Stream (not Psych::Nodes::Document)
stream = Psych.parse_stream("---\n - a\n - b")

stream.to_yaml # => "---\n- a\n- b\n"
Writing to a file
# We need Psych::Nodes::Stream (not Psych::Nodes::Document)
stream = Psych.parse_stream(File.read('database.yml'))

File.open('database.yml', 'w') do |file|
  file.write(stream.to_yaml)
end

Low-level API

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 Parser for more information on using the event based parser.

Reading to ::Psych::Nodes::Stream structure
parser = Psych::Parser.new(TreeBuilder.new) # => #<Psych::Parser>
parser = Psych.parser                       # it's an alias for the above

parser.parse("---\n - a\n - b")             # => #<Psych::Parser>
parser.handler                              # => #<Psych::TreeBuilder>
parser.handler.root                         # => #<Psych::Nodes::Stream>
Receiving an events stream
parser = Psych::Parser.new(Psych::Handlers::Recorder.new)

parser.parse("---\n - a\n - b")
parser.events # => [list of [event, args] lists]
              # event is one of: Psych::Handler::EVENTS
              # args are the arguments passed to the event
Emitting

The lowest level emitter is an event based system. Events are sent to a 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 Emitter for more information.

Writing to a Ruby structure
Psych.parser.parse("--- a")       # => #<Psych::Parser>

parser.handler.first              # => #<Psych::Nodes::Stream>
parser.handler.first.to_ruby      # => ["a"]

parser.handler.root.first         # => #<Psych::Nodes::Document>
parser.handler.root.first.to_ruby # => "a"

# You can instantiate an Emitter manually
Psych::Visitors::ToRuby.new.accept(parser.handler.root.first)
# => "a"

Constant Summary

Class Attribute Summary

Class Method Summary

Class Attribute Details

.domain_types (rw)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 507

attr_accessor :domain_types

.dump_tags (rw)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 506

attr_accessor :dump_tags

.load_tags (rw)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 505

attr_accessor :load_tags

Class Method Details

.add_builtin_type(type_tag, &block)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 487

def self.add_builtin_type type_tag, &block
  domain = 'yaml.org,2002'
  key = ['tag', domain, type_tag].join ':'
  @domain_types[key] = [key, block]
end

.add_domain_type(domain, type_tag, &block)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 481

def self.add_domain_type domain, type_tag, &block
  key = ['tag', domain, type_tag].join ':'
  @domain_types[key] = [key, block]
  @domain_types["tag:#{type_tag}"] = [key, block]
end

.add_private_type(type_tag, &block)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 50

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)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 43

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

.add_tag(tag, klass)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 499

def self.add_tag tag, klass
  @load_tags[tag] = klass.name
  @dump_tags[klass] = tag
end

.detect_implicit(thing)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 35

def self.detect_implicit thing
  warn "#{caller[0]}: detect_implicit is deprecated" if $VERBOSE
  return '' unless String === thing
  return 'null' if '' == thing
  ss = ScalarScanner.new(ClassLoader.new)
  ss.tokenize(thing).class.name.downcase
end

.dump(o) ⇒ string of yaml .dump(o, options) ⇒ string of yaml .dump(o, io) ⇒ io object passed in .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)
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 411

def self.dump o, io = nil, options = {}
  if Hash === io
    options = io
    io      = nil
  end

  visitor = Psych::Visitors::YAMLTree.create 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"
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 428

def self.dump_stream *objects
  visitor = Psych::Visitors::YAMLTree.create({})
  objects.each do |o|
    visitor << o
  end
  visitor.tree.yaml
end

.libyaml_version

Returns the version of libyaml being used

.load(yaml, filename = nil, fallback = false)

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
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 253

def self.load yaml, filename = nil, fallback = false
  result = parse(yaml, filename, fallback)
  result ? result.to_ruby : result
end

.load_documents(yaml, &block)

This method is deprecated, use .load_stream instead.

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 26

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, fallback = false)

Load the document contained in filename. Returns the yaml contained in filename as a Ruby object, or if the file is empty, it returns the specified default return value, which defaults to an empty Hash

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 473

def self.load_file filename, fallback = false
  File.open(filename, 'r:bom|utf-8') { |f|
    self.load f, filename, FALLBACK.new(fallback)
  }
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']
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 459

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)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 73

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, fallback = false)

Parse a YAML string in yaml. Returns the ::Psych::Nodes::Document. 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::Document: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.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 326

def self.parse yaml, filename = nil, fallback = false
  parse_stream(yaml, filename) do |node|
    return node
  end
  fallback
end

.parse_file(filename)

Parse a file at filename. Returns the ::Psych::Nodes::Document.

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

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 337

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 ::Psych::Nodes::Stream. 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.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 376

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

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 345

def self.parser
  Psych::Parser.new(TreeBuilder.new)
end

.read_type_class(type, reference)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 63

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

.remove_type(type_tag)

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 493

def self.remove_type type_tag
  @domain_types.delete type_tag
end

.safe_load(yaml, whitelist_classes = [], whitelist_symbols = [], aliases = false, filename = nil)

Safely load the yaml string in yaml. By default, only the following classes are allowed to be deserialized:

  • TrueClass

  • FalseClass

  • NilClass

  • Numeric

  • String

  • Array

  • Hash

Recursive data structures are not allowed by default. Arbitrary classes can be allowed by adding those classes to the whitelist. They are additive. For example, to allow Date deserialization:

Psych.safe_load(yaml, [Date])

Now the Date class can be loaded in addition to the classes listed above.

Aliases can be explicitly allowed by changing the aliases parameter. For example:

x = []
x << x
yaml = Psych.dump x
Psych.safe_load yaml               # => raises an exception
Psych.safe_load yaml, [], [], true # => loads the aliases

A ::Psych::DisallowedClass exception will be raised if the yaml contains a class that isn't in the whitelist.

A ::Psych::BadAlias exception will be raised if the yaml contains aliases but the aliases parameter is set to false.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 292

def self.safe_load yaml, whitelist_classes = [], whitelist_symbols = [], aliases = false, filename = nil
  result = parse(yaml, filename)
  return unless result

  class_loader = ClassLoader::Restricted.new(whitelist_classes.map(&:to_s),
                                             whitelist_symbols.map(&:to_s))
  scanner      = ScalarScanner.new class_loader
  if aliases
    visitor = Visitors::ToRuby.new scanner, class_loader
  else
    visitor = Visitors::NoAliasRuby.new scanner, class_loader
  end
  visitor.accept result
end

.tagurize(thing)

[ GitHub ]

  
# File 'ext/psych/lib/psych/deprecated.rb', line 57

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(object)

Dump Ruby object to a ::Psych::JSON string.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 438

def self.to_json object
  visitor = Psych::Visitors::JSONTree.create
  visitor << object
  visitor.tree.yaml
end