def get_existing_path(path, topmost_path=None):
    """Get the longest parent path in `path` that exists.

    If `path` exists, it is returned.

    Args:
        path (str): Path to test
        topmost_path (str): Do not test this path or above

    Returns:
        str: Existing path, or None if no path was found.
    """
    prev_path = None

    if topmost_path:
        topmost_path = os.path.normpath(topmost_path)

    while True:
        if os.path.exists(path):
            return path

        path = os.path.dirname(path)
        if path == prev_path:
            return None

        if topmost_path and os.path.normpath(path) == topmost_path:
            return None

        prev_path = path