Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    gt = ds.GetGeoTransform()
AttributeError: 'NoneType' object has no attribute 'GetGeoTransform'

from osgeo import gdal
driver = gdal.GetDriverByName( ’DTED’)
driver.Register()
file = gdal.Open( ’path/to/file’)