def set_relative_directory() -> None:
    """Set the directory that `relative_filename` will be relative to."""
    global RELATIVE_DIR, CANONICAL_FILENAME_CACHE

    # The current directory
    abs_curdir = abs_file(os.curdir)
    if not abs_curdir.endswith(os.sep):
        # Suffix with separator only if not at the system root
        abs_curdir = abs_curdir + os.sep

    # The absolute path to our current directory.
    RELATIVE_DIR = os.path.normcase(abs_curdir)

    # Cache of results of calling the canonical_filename() method, to
    # avoid duplicating work.
    CANONICAL_FILENAME_CACHE = {}
Set the directory that `relative_filename` will be relative to.
coverage/files.py