Error with GDAL - python-2.7

Question

I have tried and run this script from Rutger Kassies.
import gdal
import matplotlib.pyplot as plt
ds = gdal.Open('HDF4_SDS:sample:"A2002037045000.L2_LAC.SAMPLE.hdf":01')
data = ds.ReadAsArray()
ds = None
fig, ax = plt.subplots(figsize=(6,6))
ax.imshow(data[0,:,:], cmap=plt.cm.Greys, vmin=1000, vmax=6000)
But then an error always occured:
Traceback (most recent call last):
File "D:\path\to\python\stackoverflow.py", line 5, in <module>
data = ds.ReadAsArray()
AttributeError: 'NoneType' object has no attribute 'ReadAsArray'
What's wrong with the script? Am I missing something? In installing GDAL I have followed this instruction http://pythongisandstuff.wordpress.com/2011/07/07/installing-gdal-and-ogr-for-python-on-windows/
Am using windows 7/32 bit/Python 2.7.
Thanks!

Answer 1

gdal.Open() is failing and returning 'None'. This produces the sometimes counterintuitive message "NoneType' object has no attribute ...". Quoting from Python: Attribute Error - 'NoneType' object has no attribute 'something', "NoneType means that instead of an instance of whatever Class or Object you think you're working with, you've actually got None. That usually means that an assignment or function call up above failed or returned an unexpected result."
Apparently GDAL is correctly installed. It could be that the file is not readable or that there is an issue with the HDF driver. Are you getting any error message like:
`HDF4_SDS:sample:"A2002037045000.L2_LAC.SAMPLE.hdf":01' does not
exist in the file system, and is not recognised as a supported dataset
name.
To get additional information you can try something like this instead of the gdal.Open() line in your script:
gdal.UseExceptions()
ds=None
try:
ds = gdal.Open('HDF4_SDS:sample:"A2002037045000.L2_LAC.SAMPLE.hdf":01')
except RuntimeError, err:
print "Exception: ", err
exit(1)
Also, there's an extra '}' at the end of the script.

Answer 2

By default, osgeo.gdal returns None on error, and does not normally raise informative exceptions. You can change this with gdal.UseExceptions().
Try something like this:
from osgeo import gdal
gdal.UseExceptions()
source_path = r'HDF4_SDS:sample:"D:\path\to\file\A2002037045000.L2_LAC.SAMPLE.hdf":01'
try:
ds = gdal.Open(source_path)
except RuntimeError as ex:
raise IOError(ex)
The last bit just re-raises the exception as an IOError rather than a RuntimeException.
The solution is to modify source_path to a working path to your data source, e.g., I see
IOError: `HDF4_SDS:sample:"A2002037045000.L2_LAC.SAMPLE.hdf":01' does not exist in the file system, and is not recognised as a supported dataset name.