code: def apply_async(self, func, args=(), kwds=dict(), callback=None):
          
          assert not self._closed  # No lock here. We assume it's atomic...
          apply_result = ApplyResult(callback=callback)
          job = Job(func, args, kwds, apply_result)
          self._workq.put(job)
          return apply_result
docs: 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.