module Psych - Documentation for Ruby 4.0
Description: module Psych: Overview Psych is a YAML parser and emitter. pyyaml.
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# module Psych

# Overview

`Psych` is a `YAML` parser and emitter. `Psych` leverages libyaml [Home page: pyyaml.org/wiki/LibYAML] or [git repo: github.com/yaml/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 `Psych.load`

#### Reading from a string

    Psych.safe_load("--- a")             # => 'a'
    Psych.safe_load("---\n - a\n - b")   # => ['a', 'b']
    # From a trusted string:
    Psych.load("--- !ruby/range\nbegin: 0\nend: 42\nexcl: false\n") # => 0..42

#### Reading from a file

    Psych.safe_load_file("data.yml", permitted_classes: [Date])
    Psych.load_file("trusted_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 `Psych.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 `Psych::Parser` and `Psych::TreeBuilder`. The AST can be examined and manipulated freely. Please see `Psych::parse_stream`, `Psych::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 `Psych::Nodes`, `Psych::Nodes::Node`, and `Psych::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 `Psych::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 `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.

#### 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"

### Constants

DEFAULT_SNAKEYAML_VERSION
LIBYAML_VERSION

  The version of libyaml `Psych` is using

VERSION

  The version of `Psych` you are using

### Public Class Methods

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
Source

    # File ext/psych/lib/psych.rb, line 515
    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 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. For unlimited line width use `-1`.

  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`.

`:stringify_names`

  Dump symbol keys in `Hash` objects as string.

  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)

    # Dump hash with symbol keys as string
    Psych.dump({a: "b"}, stringify_names: true) # => "---\na: b\n"

dump_stream (*objects)
Source

    # File ext/psych/lib/psych.rb, line 613
    def self.dump_stream *objects
      visitor = Psych::Visitors::YAMLTree.create({})
      objects.each do |o|
        visitor << o
      end
      visitor.tree.yaml
    end

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

Example:

    Psych.dump_stream("foo\n  ", {}) # => "--- ! \"foo\\n  \"\n--- {}\n"

libyaml_version
Source

    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);
    }

Returns the version of libyaml being used

load (yaml, permitted_classes: [Symbol], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true)
Source

    # File ext/psych/lib/psych.rb, line 369
    def self.load yaml, permitted_classes: [Symbol], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true
      safe_load yaml, permitted_classes: permitted_classes,
                      permitted_symbols: permitted_symbols,
                      aliases: aliases,
                      filename: filename,
                      fallback: fallback,
                      symbolize_names: symbolize_names,
                      freeze: freeze,
                      strict_integer: strict_integer,
                      parse_symbols: parse_symbols
    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. If `yaml` is empty, it returns the specified `fallback` return value, which defaults to `nil`.

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`. This method is similar to `safe_load` except that `Symbol` objects are allowed by default.

load_file (filename, **kwargs)
Source

    # File ext/psych/lib/psych.rb, line 716
    def self.load_file filename, **kwargs
      File.open(filename, 'r:bom|utf-8') { |f|
        self.load f, filename: filename, **kwargs
      }
    end

Loads 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 `nil`. See load for options.

load_stream (yaml, filename: nil, fallback: [], **kwargs) { |to_ruby(**kwargs)| ... }
Source

    # File ext/psych/lib/psych.rb, line 644
    def self.load_stream yaml, filename: nil, fallback: [], **kwargs
      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

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']

parse (yaml, filename: nil)
Source

    # File ext/psych/lib/psych.rb, line 400
    def self.parse yaml, filename: nil
      parse_stream(yaml, filename: filename) do |node|
        return node
      end

      false
    end

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.

parse_file (filename, fallback: false)
Source

    # File ext/psych/lib/psych.rb, line 412
    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 a file at `filename`. Returns the `Psych::Nodes::Document`.

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

parse_stream (yaml, filename: nil, &block)
Source

    # File ext/psych/lib/psych.rb, line 454
    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

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.

parser ()
Source

    # File ext/psych/lib/psych.rb, line 421
    def self.parser
      Psych::Parser.new(TreeBuilder.new)
    end

Returns a default parser

safe_dump(o) → string of yaml
safe_dump(o, options) → string of yaml
safe_dump(o, io) → io object passed in
safe_dump(o, io, options) → io object passed in
Source

    # File ext/psych/lib/psych.rb, line 596
    def self.safe_dump o, io = nil, options = {}
      if Hash === io
        options = io
        io      = nil
      end

      visitor = Psych::Visitors::RestrictedYAMLTree.create options
      visitor << o
      visitor.tree.yaml io, options
    end

Safely 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. By default, only the following classes are allowed to be serialized:

- `TrueClass`

- `FalseClass`

- `NilClass`

- `Integer`

- `Float`

- `String`

- `Array`

- `Hash`

Arbitrary classes can be allowed by adding those classes to the `permitted_classes` keyword argument. They are additive. For example, to allow `Date` serialization:

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

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

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

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. For unlimited line width use `-1`.

  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`.

`:stringify_names`

  Dump symbol keys in `Hash` objects as string.

  Default: `false`.

Example:

    # Dump an array, get back a YAML string
    Psych.safe_dump(['a', 'b'])  # => "---\n- a\n- b\n"

    # Dump an array to an IO object
    Psych.safe_dump(['a', 'b'], StringIO.new)  # => #<StringIO:0x000001009d0890>

    # Dump an array with indentation set
    Psych.safe_dump(['a', ['b']], indentation: 3) # => "---\n- a\n-  - b\n"

    # Dump an array to an IO with indentation set
    Psych.safe_dump(['a', ['b']], StringIO.new, indentation: 3)

    # Dump hash with symbol keys as string
    Psych.dump({a: "b"}, stringify_names: true) # => "---\na: b\n"

safe_load (yaml, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true)
Source

    # File ext/psych/lib/psych.rb, line 323
    def self.safe_load yaml, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true
      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, strict_integer: strict_integer, parse_symbols: parse_symbols
      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

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

- `TrueClass`

- `FalseClass`

- `NilClass`

- `Integer`

- `Float`

- `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::AliasesNotEnabled` 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"}

safe_load_file (filename, **kwargs)
Source

    # File ext/psych/lib/psych.rb, line 705
    def self.safe_load_file filename, **kwargs
      File.open(filename, 'r:bom|utf-8') { |f|
        self.safe_load f, filename: filename, **kwargs
      }
    end

Safely loads 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 `nil`. See `safe_load` for options.

safe_load_stream (yaml, filename: nil, permitted_classes: [], aliases: false) { |doc| ... }
Source

    # File ext/psych/lib/psych.rb, line 671
    def self.safe_load_stream yaml, filename: nil, permitted_classes: [], aliases: false
      documents = parse_stream(yaml, filename: filename).children.map do |child|
        stream = Psych::Nodes::Stream.new
        stream.children << child
        safe_load(stream.to_yaml, permitted_classes: permitted_classes, aliases: aliases)
      end

      if block_given?
        documents.each { |doc| yield doc }
        nil
      else
        documents
      end
    end

Load multiple documents given in `yaml`. Returns the parsed documents as a list.

Example:

    Psych.safe_load_stream("--- foo\n...\n--- bar\n...") # => ['foo', 'bar']

    list = []
    Psych.safe_load_stream("--- foo\n...\n--- bar\n...") do |ruby|
      list << ruby
    end
    list # => ['foo', 'bar']

to_json (object)
Source

    # File ext/psych/lib/psych.rb, line 623
    def self.to_json object
      visitor = Psych::Visitors::JSONTree.create
      visitor << object
      visitor.tree.yaml
    end

Dump Ruby `object` to a `JSON` string.

unsafe_load (yaml, filename: nil, fallback: false, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true)
Source

    # File ext/psych/lib/psych.rb, line 272
    def self.unsafe_load yaml, filename: nil, fallback: false, symbolize_names: false, freeze: false, strict_integer: false, parse_symbols: true
      result = parse(yaml, filename: filename)
      return fallback unless result
      result.to_ruby(symbolize_names: symbolize_names, freeze: freeze, strict_integer: strict_integer, parse_symbols: parse_symbols)
    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. 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.unsafe_load("--- a")             # => 'a'
    Psych.unsafe_load("---\n - a\n - b")   # => ['a', 'b']

    begin
      Psych.unsafe_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.unsafe_load("---\n foo: bar")                         # => {"foo"=>"bar"}
    Psych.unsafe_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 load method or the `safe_load` method.

unsafe_load_file (filename, **kwargs)
Source

    # File ext/psych/lib/psych.rb, line 694
    def self.unsafe_load_file filename, **kwargs
      File.open(filename, 'r:bom|utf-8') { |f|
        self.unsafe_load f, filename: filename, **kwargs
      }
    end

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`.

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_file` method.