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 ::load
Psych.load("--- a") # => 'a' Psych.load("---\n - a\n - b") # => ['a', 'b']
Psych.load_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
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.
# 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 ::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"
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The version of libyaml Psych is using
Deprecation guard
The version of Psych you are using
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 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 a list of objects as separate documents to a document stream.
Example:
Psych.dump_stream("foo\n ", {}) # => "--- ! \"foo\\n \"\n--- {}\n"
# File 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
Returns the version of libyaml being used
static VALUE libyaml_version(VALUE module) { int major, minor, patch; VALUE list[3]; yaml_get_version(&major, &minor, &patch); list[0] = INT2NUM((long)major); list[1] = INT2NUM((long)minor); list[2] = INT2NUM((long)patch); return rb_ary_new4((long)3, list); }
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 psych/lib/psych.rb, line 271 def self.load yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: false, symbolize_names: 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 if result symbolize_names!(result) if symbolize_names result 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
.
# File psych/lib/psych.rb, line 576 def self.load_file filename, fallback: false File.open(filename, 'r:bom|utf-8') { |f| self.load f, filename: filename, fallback: fallback } 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 psych/lib/psych.rb, line 554 def self.load_stream yaml, legacy_filename = NOT_GIVEN, filename: nil, fallback: [] 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 end else parse_stream(yaml, filename: filename).children.map(&:to_ruby) end return fallback if result.is_a?(Array) && result.empty? result 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 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 file at filename
. Returns the Psych::Nodes::Document.
Raises a Psych::SyntaxError when a YAML syntax error is detected.
# File 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 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 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
Returns a default parser
# File psych/lib/psych.rb, line 415 def self.parser Psych::Parser.new(TreeBuilder.new) end
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"}
# File psych/lib/psych.rb, line 328 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 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 else Visitors::NoAliasRuby.new scanner, class_loader end result = visitor.accept result symbolize_names!(result) if symbolize_names result end
Dump Ruby object
to a JSON
string.
# File psych/lib/psych.rb, line 533 def self.to_json object visitor = Psych::Visitors::JSONTree.create visitor << object visitor.tree.yaml end