Here's another magical version that uses a class decorator:

  class Autoinit(object):
      def __call__(self, cls):
          orig_init = cls.__init__
          arg_count = orig_init.func_code.co_argcount
          arg_vars = orig_init.func_code.co_varnames[1:1 + arg_count]
  
          class Wrapped(cls):
              def __init__(self, *args, **kwargs):
                  for pos, var in enumerate(arg_vars[:len(args)]):
                      setattr(self, var, args[pos])
                  for var in kwargs:
                      setattr(self, var, kwargs[var])
                  return orig_init(self, *args, **kwargs)
  
          return Wrapped

  
  if __name__ == '__main__':
      @Autoinit()
      class Test(object):
          def __init__(self, a, b):
              pass
  
      t = Test(1, b=2)
      print t.a, t.b

[Disclaimer: This is my first class decorator. Please correct me if I'm wrong.]