the package which was being imported) if any exception
    occurs while trying to import a package.  If no onerror function is
    supplied, ImportErrors are caught and ignored, while all other
    exceptions are propagated, terminating the search.

    Examples:

    # list all modules python can access
    walk_packages()

    # list all submodules of ctypes
    walk_packages(ctypes.__path__, ctypes.__name__+'.')
    c