'callback takes two parameters: rv and exc. One is always None.
if callback is actually a tuple, the first one is called with
the return value on normal exit, the second is called with
exc_info on an exception.'

def __init__(self, func, callback=None, args=(), kw={}):

    BaseTask.__init__(self)
    self.func = func
    self.callback = callback
    self.args = args
    self.kw = kw
