method introspection in Ruby 1.9.2

_introspection.rb

include Test::Unit::Assertions 
  #=> Object
m = method(:assert_in_delta)
  #=> #<Method: Object(MiniTest::Assertions)#assert_in_delta>
m.methods - Object.methods
  #=> [:call, :[], :arity, :to_proc, :receiver, :owner, :unbind, :source_location, :parameters]
m.parameters
  #=> [[:req, :exp], [:req, :act], [:opt, :delta], [:opt, :msg]]
m.arity
  #=> -3
m.source_location
  #=> ["/Users/andrew/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/minitest/unit.rb", 122]

example.rb

class Car
  def initialize(fuel_type, passenger_capacity, door = 3)
    puts "TODO"
  end
end

Car.instance_method(:initialize).parameters
  #=> [[:req, :fuel_type], [:req, :passenger_capacity], [:opt, :door]]

Method introspection works on all methods in Ruby. Also methods defined in Kernel (without source_location).

This is a great way to dynamically inspect the names of the arguments passed to any method.