problem when using the command rospack depends1

edit

hello everybody, I'm a newbie to ROS groovy as well as Ubuntu. I'm trying to follow up the ROS tutorials, and when I've com to this ROS/Tutorials/CreatingPackage when trying to find out the dependency of the package I've just created using command rospack depends1 Beginner std_msgs roscpp rospy which Beginner is my pack name, it showed the following error:

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/rosdep2/rospack.py", line 55, in init_rospack_interface
    lookup = _get_default_RosdepLookup(Options())
  File "/usr/lib/pymodules/python2.7/rosdep2/main.py", line 107, in _get_default_RosdepLookup
    verbose=options.verbose)
  File "/usr/lib/pymodules/python2.7/rosdep2/sources_list.py", line 501, in create_default
    sources = load_cached_sources_list(sources_cache_dir=sources_cache_dir, verbose=verbose)
  File "/usr/lib/pymodules/python2.7/rosdep2/sources_list.py", line 419, in load_cached_sources_list
    with open(cache_index, 'r') as f:
IOError: [Errno 13] Permission denied: '/home/edmond/.ros/rosdep/sources.cache/index'
Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/rosdep2/rospack.py", line 55, in init_rospack_interface
    lookup = _get_default_RosdepLookup(Options())
  File "/usr/lib/pymodules/python2.7/rosdep2/main.py", line 107, in _get_default_RosdepLookup
    verbose=options.verbose)
  File "/usr/lib/pymodules/python2.7/rosdep2/sources_list.py", line 501, in create_default
    sources = load_cached_sources_list(sources_cache_dir=sources_cache_dir, verbose=verbose)
  File "/usr/lib/pymodules/python2.7/rosdep2/sources_list.py", line 419, in load_cached_sources_list
    with open(cache_index, 'r') as f:
IOError: [Errno 13] Permission denied: '/home/edmond/.ros/rosdep/sources.cache/index'
[rospack] Error: could not call python function 'rosdep2.rospack.init_rospack_interface'

I think the problem goes with some python related files, but I've also tried not including rospy for my package, it remains the same error. Does anyone know what could possibly be the reason? Thanks a lot

EDIT: Forgot to mention, I'm using Ubuntu 12.04 right now