def build_tree_file_pathname(filename, directory_depth=8, pathname_separator_character=os.sep):
    """
    Return a file pathname which pathname is built of the specified
    number of sub-directories, and where each directory is named after the
    nth letter of the filename corresponding to the directory depth.

    Examples::

        >>> build_tree_file_pathname('foo.txt', 2, '/')
        'f/o/foo.txt'
        >>> build_tree_file_pathname('0123456789abcdef')
        '0/1/2/3/4/5/6/7/0123456789abcdef'

    @param filename: name of a file, with or without extension.

    @param directory_depth: number of sub-directories to be generated.

    @param pathname_separator_character: character to be used to separate
           pathname components, such as '/' for POSIX and '\\' for
           Windows.  If not defined, the default is the character used by
           the operating system ``os.sep``.

    @return: a file pathname.
    """
    return build_tree_pathname(filename, directory_depth, pathname_separator_character) + filename
Return a file pathname which pathname is built of the specified
    number of sub-directories, and where each directory is named after the
    nth letter of the filename corresponding to the directory depth.

    Examples::

        >>> build_tree_file_pathname('foo.txt', 2, '/')
        'f/o/foo.txt'
        >>> build_tree_file_pathname('0123456789abcdef')
        '0/1/2/3/4/5/6/7/0123456789abcdef'

    @param filename: name of a file, with or without extension.

    @param directory_depth: number of sub-directories to be generated.

    @param pathname_separator_character: character to be used to separate
           pathname components, such as '/' for POSIX and '\\' for
           Windows.  If not defined, the default is the character used by
           the operating system ``os.sep``.

    @return: a file pathname.