Add a variable to a class instance at runtime

Add a variable to a class instance at runtime
You are encouraged to solve this task according to the task description, using any language you may know.

This demonstrates how to dynamically add variables to a class instance at runtime. This is useful when the methods/variables are based on a data file that isn't available until runtime. Hal Fulton gives an example of creating an OO CSV parser at An Exercise in Metaprogramming with Ruby

Python

class empty(object):
  pass

e = empty()
e.foo = 1  ## (alternatively: setattr(e, "foo", 1))
print e.foo

Ruby

class Empty
end

e = Empty.new
e.instance_variable_set("@foo", 1)
e.instance_eval("class << self; attr_accessor :foo; end")
puts e.foo