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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/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
recorder = Psych::Handlers::Recorder.new
parser = Psych::Parser.new(recorder)

parser.parse("---\n - a\n - b")
recorder.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)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 623

attr_accessor :domain_types

.dump_tags (rw)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 622

attr_accessor :dump_tags

.load_tags (rw)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 621

attr_accessor :load_tags

Class Method Details

.add_builtin_type(type_tag, &block)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 588

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)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 582

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_tag(tag, klass)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 600

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

Currently supported options are:

:indentation

Number of space characters used to indent. Acceptable value should be in 0..9 range, otherwise option is ignored.

Default: 2.

:line_width

Max character to wrap line at.

Default: 0 (meaning “wrap at 81”).

:canonical

Write “canonical” YAML form (very verbose, yet strictly formal).

Default: false.

:header

Write %YAML [version] at the beginning of document.

Default: false.

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 504

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 521

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

[ GitHub ]

  
# File 'ext/psych/psych.c', line 7

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

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

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

    return rb_ary_new4((long)3, list);
}

.load(yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: false, symbolize_names: false, freeze: 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. If yaml is empty, it returns the specified fallback return value, which defaults to false.

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("--- `", filename: "file.txt")
rescue Psych::SyntaxError => ex
  ex.file    # => 'file.txt'
  ex.message # => "(file.txt): found character that cannot start any token"
end

When the optional symbolize_names keyword argument is set to a true value, returns symbols for keys in Hash objects (default: strings).

Psych.load("---\n foo: bar")                         # => {"foo"=>"bar"}
Psych.load("---\n foo: bar", symbolize_names: true)  # => {:foo=>"bar"}

Raises a TypeError when yaml parameter is NilClass

NOTE: This method *should not* be used to parse untrusted documents, such as YAML documents that are supplied via user input. Instead, please use the safe_load method.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 271

def self.load yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: false, symbolize_names: false, freeze: false
  if legacy_filename != NOT_GIVEN
    warn_with_uplevel 'Passing filename with the 2nd argument of Psych.load is deprecated. Use keyword argument like Psych.load(yaml, filename: ...) instead.', uplevel: 1 if $VERBOSE
    filename = legacy_filename
  end

  result = parse(yaml, filename: filename)
  return fallback unless result
  result = result.to_ruby(symbolize_names: symbolize_names, freeze: freeze) if result
  result
end

.load_file(filename, **kwargs)

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 fallback return value, which defaults to false.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 574

def self.load_file filename, **kwargs
  File.open(filename, 'r:bom|utf-8') { |f|
    self.load f, filename: filename, **kwargs
  }
end

.load_stream(yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: [], **kwargs)

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 552

def self.load_stream yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: [], **kwargs
  if legacy_filename != NOT_GIVEN
    warn_with_uplevel 'Passing filename with the 2nd argument of Psych.load_stream is deprecated. Use keyword argument like Psych.load_stream(yaml, filename: ...) instead.', uplevel: 1 if $VERBOSE
    filename = legacy_filename
  end

  result = if block_given?
             parse_stream(yaml, filename: filename) do |node|
               yield node.to_ruby(**kwargs)
             end
           else
             parse_stream(yaml, filename: filename).children.map { |node| node.to_ruby(**kwargs) }
           end

  return fallback if result.is_a?(Array) && result.empty?
  result
end

.parse(yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: NOT_GIVEN)

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("--- `", filename: "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 382

def self.parse yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: NOT_GIVEN
  if legacy_filename != NOT_GIVEN
    warn_with_uplevel 'Passing filename with the 2nd argument of Psych.parse is deprecated. Use keyword argument like Psych.parse(yaml, filename: ...) instead.', uplevel: 1 if $VERBOSE
    filename = legacy_filename
  end

  parse_stream(yaml, filename: filename) do |node|
    return node
  end

  if fallback != NOT_GIVEN
    warn_with_uplevel 'Passing the `fallback` keyword argument of Psych.parse is deprecated.', uplevel: 1 if $VERBOSE
    fallback
  else
    false
  end
end

.parse_caller(at) (private)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 611

def self.parse_caller(at)
  if /^(.?):(\d)(?::in `.*')?/ =~ at
    file = $1
    line = $2.to_i
    [file, line]
  end
end

.parse_file(filename, fallback: false)

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 404

def self.parse_file filename, fallback: false
  result = File.open filename, 'r:bom|utf-8' do |f|
    parse f, filename: filename
  end
  result || fallback
end

.parse_stream(yaml, legacy_filename = NOT_GIVEN, 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("--- `", filename: "file.txt")
rescue Psych::SyntaxError => ex
  ex.file    # => 'file.txt'
  ex.message # => "(file.txt): found character that cannot start any token"
end

Raises a TypeError when NilClass is passed.

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

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 446

def self.parse_stream yaml, legacy_filename = NOT_GIVEN, filename: nil, &block
  if legacy_filename != NOT_GIVEN
    warn_with_uplevel 'Passing filename with the 2nd argument of Psych.parse_stream is deprecated. Use keyword argument like Psych.parse_stream(yaml, filename: ...) instead.', uplevel: 1 if $VERBOSE
    filename = legacy_filename
  end

  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 413

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

.remove_type(type_tag)

This method is for internal use only.
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 594

def self.remove_type type_tag
  @domain_types.delete type_tag
end

.safe_load(yaml, legacy_permitted_classes = NOT_GIVEN, legacy_permitted_symbols = NOT_GIVEN, legacy_aliases = NOT_GIVEN, legacy_filename = NOT_GIVEN, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false)

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 permitted_classes keyword argument. They are additive. For example, to allow Date deserialization:

Psych.safe_load(yaml, permitted_classes: [Date])

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

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

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

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

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

filename will be used in the exception message if any exception is raised while parsing.

When the optional symbolize_names keyword argument is set to a true value, returns symbols for keys in Hash objects (default: strings).

Psych.safe_load("---\n foo: bar")                         # => {"foo"=>"bar"}
Psych.safe_load("---\n foo: bar", symbolize_names: true)  # => {:foo=>"bar"}
[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 327

def self.safe_load yaml, legacy_permitted_classes = NOT_GIVEN, legacy_permitted_symbols = NOT_GIVEN, legacy_aliases = NOT_GIVEN, legacy_filename = NOT_GIVEN, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false
  if legacy_permitted_classes != NOT_GIVEN
    warn_with_uplevel 'Passing permitted_classes with the 2nd argument of Psych.safe_load is deprecated. Use keyword argument like Psych.safe_load(yaml, permitted_classes: ...) instead.', uplevel: 1 if $VERBOSE
    permitted_classes = legacy_permitted_classes
  end

  if legacy_permitted_symbols != NOT_GIVEN
    warn_with_uplevel 'Passing permitted_symbols with the 3rd argument of Psych.safe_load is deprecated. Use keyword argument like Psych.safe_load(yaml, permitted_symbols: ...) instead.', uplevel: 1 if $VERBOSE
    permitted_symbols = legacy_permitted_symbols
  end

  if legacy_aliases != NOT_GIVEN
    warn_with_uplevel 'Passing aliases with the 4th argument of Psych.safe_load is deprecated. Use keyword argument like Psych.safe_load(yaml, aliases: ...) instead.', uplevel: 1 if $VERBOSE
    aliases = legacy_aliases
  end

  if legacy_filename != NOT_GIVEN
    warn_with_uplevel 'Passing filename with the 5th argument of Psych.safe_load is deprecated. Use keyword argument like Psych.safe_load(yaml, filename: ...) instead.', uplevel: 1 if $VERBOSE
    filename = legacy_filename
  end

  result = parse(yaml, filename: filename)
  return fallback unless result

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

.to_json(object)

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

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 531

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

.warn_with_uplevel(message, uplevel: 1) (private)

This method is for internal use only.

Workaround for emulating warn '...', uplevel: 1 in Ruby 2.4 or lower.

[ GitHub ]

  
# File 'ext/psych/lib/psych.rb', line 606

def self.warn_with_uplevel(message, uplevel: 1)
  at = parse_caller(caller[uplevel]).join(':')
  warn "#{at}: #{message}"
end