Python 3.6.4: os.walk (lines 378-394 in os.py)

os.walk.py

            if not topdown and is_dir:
                # Bottom-up: recurse into sub-directory, but exclude symlinks to
                # directories if followlinks is False
                if followlinks:
                    walk_into = True
                else:
                    try:
                        is_symlink = entry.is_symlink()
                    except OSError:
                        # If is_symlink() raises an OSError, consider that the
                        # entry is not a symbolic link, same behaviour than
                        # os.path.islink().
                        is_symlink = False
                    walk_into = not is_symlink

                if walk_into:
                    walk_dirs.append(entry.path)