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. This is referred to as "monkeypatching" by Pythonistas and some others. It's possible in most dynamic OO languages such as Python, Ruby, and Smalltalk.

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