gistfile1.py

import datetime
import yaml

class Obj1(object):
    def __init__(self, dt):
        self.dt = dt

dt = datetime.datetime(2011, 9, 1, 13, 20, 30, 405060)
o1 = Obj1(dt)

yaml.dump(o1)
> "!!python/object:__main__.Obj1 {dt: !!timestamp '2011-09-01 13:20:30.405060'}\n"

yaml.dump(o1, Dumper=yaml.CDumper)
> "!!python/object:__main__.Obj1 {dt: ! '2011-09-01 13:20:30.405060'}\n"