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.
# 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!
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.
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.
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
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
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
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.
# 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)
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
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.
# Returns Psych::Nodes::Stream
Psych.parse_stream("---\n - a\n - b")
# Returns Psych::Nodes::Document
Psych.parse("---\n - a\n - b")
# 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
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.
# 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"
# 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
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.
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>
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
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.
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"
# File ext/psych/lib/psych.rb, line 506
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.
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)
# File ext/psych/lib/psych.rb, line 523
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 ext/psych/lib/psych.rb, line 274
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.to_ruby(symbolize_names: symbolize_names, freeze: freeze)
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
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.
# File ext/psych/lib/psych.rb, line 580
def self.load_file filename, **kwargs
File.open(filename, 'r:bom|utf-8') { |f|
self.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.
# File ext/psych/lib/psych.rb, line 554
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 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 ext/psych/lib/psych.rb, line 384
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 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 ext/psych/lib/psych.rb, line 406
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 ext/psych/lib/psych.rb, line 448
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 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 ext/psych/lib/psych.rb, line 415 def self.parser Psych::Parser.new(TreeBuilder.new) end
Returns a default parser
# File ext/psych/lib/psych.rb, line 329
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 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"}
# File ext/psych/lib/psych.rb, line 591
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 ext/psych/lib/psych.rb, line 533 def self.to_json object visitor = Psych::Visitors::JSONTree.create visitor << object visitor.tree.yaml end
Dump Ruby object to a JSON string.
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Licensed under the Ruby License.
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Licensed under their own licenses.