hash_spy.rb

class HashSpy

  def initialize(hash={})
    @hash = hash
  end

  def method_missing(method_name, *args, &block)
    puts "***** hash access"
    puts "  before: #{@hash.inspect}"
    r = @hash.send(method_name, *args, &block)
    puts "  after: #{@hash.inspect}"
    puts "  backtrace:\n    #{caller.join("\n    ")}"
    r
  end

end

IMPORTANT_STUFF = HashSpy.new(
  :password => "too many secrets"
)

def change_password(h)
  h[:password] = "FAIL"
end

def print_password
  puts IMPORTANT_STUFF[:password]
end

print_password
change_password(IMPORTANT_STUFF)
print_password