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"
The version of libyaml Psych is using
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 512
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..9range, otherwise option is ignored.Default:
2. :line_width-
Max character to wrap line at.
Default:
0(meaning “wrap at 81”). :canonical-
Write “canonical”
YAMLform (very verbose, yet strictly formal).Default:
false. :header-
Write
%YAML [version]at the beginning of document.Default:
false. :stringify_names-
Dump symbol keys in
Hashobjects 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"
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 609
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"
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
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 368
def self.load yaml, permitted_classes: [Symbol], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false
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
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.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.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 683
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 false. See load for options.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 640
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']
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 398
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.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 410
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.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 452
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.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 419
def self.parser
Psych::Parser.new(TreeBuilder.new)
end
Returns a default parser
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 592
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:
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..9range, otherwise option is ignored.Default:
2. :line_width-
Max character to wrap line at.
Default:
0(meaning “wrap at 81”). :canonical-
Write “canonical”
YAMLform (very verbose, yet strictly formal).Default:
false. :header-
Write
%YAML [version]at the beginning of document.Default:
false. :stringify_names-
Dump symbol keys in
Hashobjects 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"
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 322
def self.safe_load yaml, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false
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
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:
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"}
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 672
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 false. See safe_load for options.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 619
def self.to_json object
visitor = Psych::Visitors::JSONTree.create
visitor << object
visitor.tree.yaml
end
Dump Ruby object to a JSON string.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 271
def self.unsafe_load yaml, filename: nil, fallback: false, symbolize_names: false, freeze: false, strict_integer: false
result = parse(yaml, filename: filename)
return fallback unless result
result.to_ruby(symbolize_names: symbolize_names, freeze: freeze, strict_integer: strict_integer)
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.
# File tmp/rubies/ruby-3.4.0-preview1/ext/psych/lib/psych.rb, line 661
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.