instance-duck-punch.rb

class MyClass
  def my_method
    puts "Hello"
  end
end

def with_method(object, method_name, replacement_method, &block)
	old_method = object.method(method_name)
	object.class.class_eval { define_method method_name, replacement_method}
	
	block.call
	
	object.class.class_eval { define_method method_name, old_method }
end

my_object = MyClass.new
 
my_object.my_method

replacement_method = Proc.new { puts "Goodbye" }

with_method(my_object, 'my_method', replacement_method) { my_object.my_method }

my_object.my_method