"Permission denied" when importing tf

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Hi All,

I am using ros kinetic on ubuntu 16.04 xenial. while trying to import tf library in python code I get following error

Traceback (most recent call last):
  File "pose_visualizer.py", line 13, in <module>
    import  tf
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/tf/__init__.py", line 28, in <module>
    from tf2_ros import TransformException as Exception, ConnectivityException, LookupException, ExtrapolationException
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/tf2_ros/__init__.py", line 39, in <module>
    from .buffer_interface import *
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/tf2_ros/buffer_interface.py", line 32, in <module>
    import roslib; roslib.load_manifest('tf2_ros')
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslib/launcher

.py", line 62, in load_manifest
    sys.path = _generate_python_path(package_name, _rospack) + sys.path
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslib/launcher.py", line 93, in _generate_python_path
    m = rospack.get_manifest(pkg)
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 167, in get_manifest
    return self._load_manifest(name)
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 211, in _load_manifest
    retval = self._manifests[name] = parse_manifest_file(self.get_path(name), self._manifest_name, rospack=self)
  File "/usr/lib/python2.7/dist-packages/rospkg/manifest.py", line 414, in parse_manifest_file
    _static_rosdep_view = init_rospack_interface()

  File "/usr/lib/python2.7/dist-packages/rosdep2/rospack.py", line 59, in init_rospack_interface
    lookup = _get_default_RosdepLookup(Options())
  File "/usr/lib/python2.7/dist-packages/rosdep2/main.py", line 134, in _get_default_RosdepLookup
    verbose=options.verbose)
  File "/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py", line 585, in create_default
    sources = load_cached_sources_list(sources_cache_dir=sources_cache_dir, verbose=verbose)
  File "/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py", line 543, in load_cached_sources_list
    with open(cache_index, 'r') as f:
IOError: [Errno 13] Permission denied: '/home/rnd424/.ros/rosdep/sources.cache/index'

How to resolve this error ?