>>> def print_a(inst):
  ... 	print inst.a
  ... 	
  >>> class Foo(object):
  ... 	def __init__(self, a):
  ... 		self.a = a
  ... 		
  >>> Foo.print_my_a = print_a
  >>> f = Foo(5)
  >>> f.print_my_a()
  5

Not sure why my previous post was downvoted, unless maybe your definition of monkey-patching is different than mine.

Well, I didn't downvote it, but you can't do this with _everything_ in Python, like you can in Ruby.

    $ irb
    irb(main):001:0> class Numeric
    irb(main):002:1>   def wtf
    irb(main):003:2>     puts "hey"
    irb(main):004:2>   end
    irb(main):005:1> end
    => nil
    irb(main):006:0> 5.wtf
    hey
    => nil