implementation.rb

module PythonishDecorator
  def method_added(name)
    return unless @decorations
    
    decorations = @decorations.dup
    @decorations = nil
    
    alias_method "undecorated_#{name}", name
    
    define_method(name) do |*args|
      decorations.each {|klass| klass.call(self) }
      send("undecorated_#{name}", *args)
    end
    
    decorations.each {|klass| klass.init(klass, name) }
  end
  
  def decorate(klass)
    @decorations ||= []
    @decorations << klass
  end
end

usage.rb

class MyDecorator
  def self.init(klass, method)
    p [klass, method]
  end
  
  def self.call(obj)
    p obj
  end
end

class MyClass
  extend PythonishDecorator
  
  decorate MyDecorator
  def omg
    p "omg"
  end
end

MyClass.new.omg

# Output:
# 
# [MyDecorator, :omg]
# #<MyClass:0x10012a2e0>
# "omg"