Module

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 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")

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')

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

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


No documentation available

The version of libyaml Psych is using

Deprecation guard

The version of Psych you are using

Class Methods


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)

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

Example:

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

Returns the version of libyaml being used

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.

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.

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

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

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.

Returns a default parser

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

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"}

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 false. See safe_load for options.

Dump Ruby object to a JSON string.