ing.  Modifying dirnames when
    topdown is false has no effect on the behavior of os.walk(), since the
    directories in dirnames have already been generated by the time dirnames
    itself is generated. No matter the value of topdown, the list of
    subdirectories is retrieved before the tuples for the directory and its
    subdirectories are generated.

    By default errors from the os.scandir() call are ignored.  If
    optional arg 'onerror' is specified, it should be a function; it
    will be called with one argument, an OSError instance.  It can
    report the error to continue with the walk, or raise the exception
    to abort the walk.  Note that the filename is available as the
    filename attribute of the exception object.

    By default, os.walk does not follow symbolic links to subdirectories on
    systems that support them.  In order to get this functionality, set the
    optional argument 'followlinks' to true.

    Caution:  if you pass a relative pathname for top, don't change the
    current working directory between resumptions of walk.  walk never
    changes the current directory, and assumes that the client doesn't
    either.

    Example:

    import os
    from os.path import join, getsize
    for root, dirs, files in os.walk('python/Lib/email'):
        print(root, "consumes", end="")
        print(sum(getsize(join(root, name)) for name in files), end="")
        print("bytes in", len(files), "non-directory files")
        if 'CVS' in dirs:
            dirs.remove('CVS')  # don't visit CVS directories

    z