<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _sumImages(self,numarrayObjectList):
 """ Sum a list of numarray objects. """

        if numarrayObjectList in [None, []]:
            return None

        tsum = np.zeros(numarrayObjectList[0].shape, dtype=numarrayObjectList[0].dtype)

        for image in numarrayObjectList:
            tsum += image

        return tsum