'A variant of the apply() method which returns an
ApplyResult object.
If callback is specified then it should be a callable which
accepts a single argument. When the result becomes ready,
callback is applied to it (unless the call failed). callback
should complete immediately since otherwise the thread which
handles the results will get blocked.'

def apply_async(self, func, args=(), kwds=dict(), callback=None):

    assert (not self._closed)
    apply_result = ApplyResult(callback=callback)
    job = Job(func, args, kwds, apply_result)
    self._workq.put(job)
    return apply_result
