# Introspection
# View an object's methods
# Inspecting an Object
You can find the public methods an object can respond to using either the methods or public_methods methods, which return an array of symbols:
class Foo
def bar; 42; end
end
f = Foo.new
def f.yay; 17; end
p f.methods.sort
#=> [:!, :!=, :!~, :<=>, :==, :===, :=~, :__id__, :__send__, :bar, :class, :clone,
#=> :define_singleton_method, :display, :dup, :enum_for, :eql?, :equal?, :extend,
#=> :freeze, :frozen?, :hash, :inspect, :instance_eval, :instance_exec,
#=> :instance_of?, :instance_variable_defined?, :instance_variable_get,
#=> :instance_variable_set, :instance_variables, :is_a?, :itself, :kind_of?,
#=> :method, :methods, :nil?, :object_id, :private_methods, :protected_methods,
#=> :public_method, :public_methods, :public_send, :remove_instance_variable,
#=> :respond_to?, :send, :singleton_class, :singleton_method, :singleton_methods,
#=> :taint, :tainted?, :tap, :to_enum, :to_s, :trust, :untaint, :untrust,
#=> :untrusted?, :yay]
For a more targeted list, you can remove methods common to all objects, e.g.
p (f.methods - Object.methods).sort
#=> [:bar,:yay]
Alternatively, you can pass false to methods or public_methods:
p f.methods(false) # public and protected singleton methods of `f`
#=> [:yay]
p f.public_methods(false)
#=> [:yay, :bar]
You can find the private and protected methods of an object using private_methods and protected_methods:
p f.private_methods.sort
#=> [:Array, :Complex, :DelegateClass, :Float, :Hash, :Integer, :Rational, :String,
#=> :__callee__, :__dir__, :__method__, :`, :abort, :at_exit, :autoload, :autoload?,
#=> :binding, :block_given?, :caller, :caller_locations, :catch,
#=> :default_src_encoding, :eval, :exec, :exit, :exit!, :fail, :fork, :format, :gem,
#=> :gem_original_require, :gets, :global_variables, :initialize, :initialize_clone,
#=> :initialize_copy, :initialize_dup, :irb_binding, :iterator?, :lambda, :load,
#=> :local_variables, :loop, :method_missing, :open, :p, :print, :printf, :proc,
#=> :putc, :puts, :raise, :rand, :readline, :readlines, :require, :require_relative,
#=> :respond_to_missing?, :select, :set_trace_func, :singleton_method_added,
#=> :singleton_method_removed, :singleton_method_undefined, :sleep, :spawn,
#=> :sprintf, :srand, :syscall, :system, :test, :throw, :trace_var, :trap,
#=> :untrace_var, :warn]
p f.protected_methods
#=> []
As with methods and public_methods, you can pass false to private_methods and protected_methods to trim away inherited methods.
# Inspecting a Class or Module
In addition to methods, public_methods, protected_methods, and private_methods, classes and modules expose instance_methods, public_instance_methods, protected_instance_methods, and private_instance_methods to determine the methods exposed for objects that inherit from the class or module. As above, you can pass false to these methods to exclude inherited methods:
p Foo.instance_methods.sort
#=> [:!, :!=, :!~, :<=>, :==, :===, :=~, :__id__, :__send__, :bar, :class,
#=> :clone, :define_singleton_method, :display, :dup, :enum_for, :eql?,
#=> :equal?, :extend, :freeze, :frozen?, :hash, :inspect, :instance_eval,
#=> :instance_exec, :instance_of?, :instance_variable_defined?,
#=> :instance_variable_get, :instance_variable_set, :instance_variables,
#=> :is_a?, :itself, :kind_of?, :method, :methods, :nil?, :object_id,
#=> :private_methods, :protected_methods, :public_method, :public_methods,
#=> :public_send, :remove_instance_variable, :respond_to?, :send,
#=> :singleton_class, :singleton_method, :singleton_methods, :taint,
#=> :tainted?, :tap, :to_enum, :to_s, :trust, :untaint, :untrust, :untrusted?]
p Foo.instance_methods(false)
#=> [:bar]
Finally, if you forget the names of most of these in the future, you can find all of these methods using methods:
p f.methods.grep(/methods/)
#=> [:private_methods, :methods, :protected_methods, :public_methods,
#=> :singleton_methods]
p Foo.methods.grep(/methods/)
#=> [:public_instance_methods, :instance_methods, :private_instance_methods,
#=> :protected_instance_methods, :private_methods, :methods,
#=> :protected_methods, :public_methods, :singleton_methods]
# View an object's Instance Variables
It is possible to query an object about its instance variables using instance_variables, instance_variable_defined?, and instance_variable_get, and modify them using instance_variable_set and remove_instance_variable:
class Foo
attr_reader :bar
def initialize
@bar = 42
end
end
f = Foo.new
f.instance_variables #=> [:@bar]
f.instance_variable_defined?(:@baz) #=> false
f.instance_variable_defined?(:@bar) #=> true
f.instance_variable_get(:@bar) #=> 42
f.instance_variable_set(:@bar, 17) #=> 17
f.bar #=> 17
f.remove_instance_variable(:@bar) #=> 17
f.bar #=> nil
f.instance_variables #=> []
The names of instance variables include the @ symbol. You will get an error if you omit it:
f.instance_variable_defined?(:jim)
#=> NameError: `jim' is not allowed as an instance variable name
# View Global and Local Variables
The Kernel exposes methods for getting the list of global_variables (opens new window) and local_variables (opens new window):
cats = 42
$demo = "in progress"
p global_variables.sort
#=> [:$!, :$", :$$, :$&, :$', :$*, :$+, :$,, :$-0, :$-F, :$-I, :$-K, :$-W, :$-a,
#=> :$-d, :$-i, :$-l, :$-p, :$-v, :$-w, :$., :$/, :$0, :$1, :$2, :$3, :$4, :$5,
#=> :$6, :$7, :$8, :$9, :$:, :$;, :$<, :$=, :$>, :$?, :$@, :$DEBUG, :$FILENAME,
#=> :$KCODE, :$LOADED_FEATURES, :$LOAD_PATH, :$PROGRAM_NAME, :$SAFE, :$VERBOSE,
#=> :$\, :$_, :$`, :$binding, :$demo, :$stderr, :$stdin, :$stdout, :$~]
p local_variables
#=> [:cats]
Unlike instance variables there are no methods specifically for getting, setting, or removing global or local variables. Looking for such functionality is usually a sign that your code should be rewritten to use a Hash to store the values. However, if you must modify global or local variables by name, you can use eval with a string:
var = "$demo"
eval(var) #=> "in progress"
eval("#{var} = 17")
p $demo #=> 17
By default, eval will evaluate your variables in the current scope. To evaluate local variables in a different scope, you must capture the binding where the local variables exist.
def local_variable_get(name, bound=nil)
foo = :inside
eval(name,bound)
end
def test_1
foo = :outside
p local_variable_get("foo")
end
def test_2
foo = :outside
p local_variable_get("foo",binding)
end
test_1 #=> :inside
test_2 #=> :outside
In the above, test_1 did not pass a binding to local_variable_get, and so the eval was executed within the context of that method, where a local variable named foo was set to :inside.
# View Class Variables
Classes and modules have the same methods for introspecting instance variables as any other object. Class and modules also have similar methods for querying the class variables (@@these_things):
p Module.methods.grep(/class_variable/)
#=> [:class_variables, :class_variable_get, :remove_class_variable,
#=> :class_variable_defined?, :class_variable_set]
class Foo
@@instances = 0
def initialize
@@instances += 1
end
end
class Bar < Foo; end
5.times{ Foo.new }
3.times{ Bar.new }
p Foo.class_variables #=> [:@@instances]
p Bar.class_variables #=> [:@@instances]
p Foo.class_variable_get(:@@instances) #=> 8
p Bar.class_variable_get(:@@instances) #=> 8
Similar to instance variables, the name of class variables must begin with @@, or you will get an error:
p Bar.class_variable_defined?( :instances )
#=> NameError: `instances' is not allowed as a class variable name