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-3.1.3/ext/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 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 tmp/rubies/ruby-3.1.3/ext/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
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-3.1.3/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-3.1.3/ext/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
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-3.1.3/ext/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
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-3.1.3/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-3.1.3/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-3.1.3/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-3.1.3/ext/psych/lib/psych.rb, line 419
def self.parser
Psych::Parser.new(TreeBuilder.new)
end
Returns a default parser
# File tmp/rubies/ruby-3.1.3/ext/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 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..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 tmp/rubies/ruby-3.1.3/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::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 tmp/rubies/ruby-3.1.3/ext/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
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-3.1.3/ext/psych/lib/psych.rb, line 605
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-3.1.3/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-3.1.3/ext/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
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.