# Psych - ri - phpman = **Psych** (from gem psych-5.4.0) ------------------------------------------------------------------------ = **Overview** Psych is a YAML parser and emitter. Psych leverages libyaml [Home page: ] or [git repo: ] 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ASyntaxError/markdown) => 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) # => # # 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown) and [Psych::TreeBuilder](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ATreeBuilder/markdown). The AST can be examined and manipulated freely. Please see ## [Psych::parsestream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3Aparsestream/markdown), [Psych::Nodes](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes/markdown), and [Psych::Nodes::Node](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ANode/markdown) for more information on dealing with YAML syntax trees. ==== **Reading from a string** # Returns [Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown) Psych.parse_stream("---\n - a\n - b") # Returns [Psych::Nodes::Document](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ADocument/markdown) Psych.parse("---\n - a\n - b") ==== **Reading from a file** # Returns [Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown) Psych.parse_stream(File.read('database.yml')) # Returns [Psych::Nodes::Document](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ADocument/markdown) Psych.parse_file('database.yml') ==== **Exception handling** begin # The second argument changes only the exception contents Psych.parse("--- `", "file.txt") rescue [Psych::SyntaxError](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ASyntaxError/markdown) => 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes/markdown), [Psych::Nodes::Node](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ANode/markdown), and [Psych::TreeBuilder](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ATreeBuilder/markdown) for more information on building a YAML AST. ==== **Writing to a string** # We need [Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown) (not [Psych::Nodes::Document](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ADocument/markdown)) 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown) (not [Psych::Nodes::Document](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ADocument/markdown)) 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown) for more information on using the event based parser. ==== **Reading to [Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown) structure** parser = [Psych::Parser](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown).new(TreeBuilder.new) # => #<[Psych::Parser](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown)> parser = Psych.parser # it's an alias for the above parser.parse("---\n - a\n - b") # => #<[Psych::Parser](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown)> parser.handler # => #<[Psych::TreeBuilder](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ATreeBuilder/markdown)> parser.handler.root # => #<[Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown)> ==== **Receiving an events stream** recorder = [Psych::Handlers::Recorder](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AHandlers%3A%3ARecorder/markdown).new parser = [Psych::Parser](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown).new(recorder) parser.parse("---\n - a\n - b") recorder.events # => [list of [event, args] lists] # event is one of: [Psych::Handler::EVENTS](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AHandler%3A%3AEVENTS/markdown) # 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AEmitter/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AEmitter/markdown) for more information. ==== **Writing to a Ruby structure** Psych.parser.parse("--- a") # => #<[Psych::Parser](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AParser/markdown)> parser.handler.first # => #<[Psych::Nodes::Stream](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3AStream/markdown)> parser.handler.first.to_ruby # => ["a"] parser.handler.root.first # => #<[Psych::Nodes::Document](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3ANodes%3A%3ADocument/markdown)> parser.handler.root.first.to_ruby # => "a" # You can instantiate an Emitter manually [Psych::Visitors::ToRuby](https://www.chedong.com/phpMan.php/perldoc/Psych%3A%3AVisitors%3A%3AToRuby/markdown).new.accept(parser.handler.root.first) # => "a" ------------------------------------------------------------------------ = **Constants:** DEFAULT_SNAKEYAML_VERSION: [not documented] LIBYAML_VERSION: ### The version of libyaml Psych is using VERSION: ### The version of Psych you are using = **Class methods:** dump dump_stream libyaml_version load load_file load_stream parse parse_file parse_stream parser safe_dump safe_load safe_load_file safe_load_stream to_json unsafe_load unsafe_load_file