module Psych
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"
Constants
Public 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)
# File psych/lib/psych.rb, line 505 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 595 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(major); list[1] = INT2NUM(minor); list[2] = INT2NUM(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. This method is similar to `safe_load` except that `Symbol` objects are allowed by default.
# File 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
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 psych/lib/psych.rb, line 669 def self.load_file filename, **kwargs File.open(filename, 'r:bom|utf-8') { |f| self.load f, filename: filename, **kwargs } 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 626 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
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 398 def self.parse yaml, filename: nil parse_stream(yaml, filename: filename) do |node| return node end false 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 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 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 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
Returns a default parser
# File psych/lib/psych.rb, line 419 def self.parser Psych::Parser.new(TreeBuilder.new) 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:
-
TrueClass
-
FalseClass
-
NilClass
-
Integer
-
Float
-
String
-
Array
-
Hash
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..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.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)
# File psych/lib/psych.rb, line 578 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 load the yaml string in yaml
. By default, only the following classes are allowed to be deserialized:
-
TrueClass
-
FalseClass
-
NilClass
-
Integer
-
Float
-
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::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 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 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 psych/lib/psych.rb, line 658 def self.safe_load_file filename, **kwargs File.open(filename, 'r:bom|utf-8') { |f| self.safe_load f, filename: filename, **kwargs } end
Dump Ruby object
to a JSON
string.
# File psych/lib/psych.rb, line 605 def self.to_json object visitor = Psych::Visitors::JSONTree.create visitor << object visitor.tree.yaml 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 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 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 psych/lib/psych.rb, line 647 def self.unsafe_load_file filename, **kwargs File.open(filename, 'r:bom|utf-8') { |f| self.unsafe_load f, filename: filename, **kwargs } end