def parse_and_validate_radius(request):
    """Extract and Check the validity of the radius.

    Args:
        request: HTTP request.

    Returns:
        radius(float) if valid.
        default 5(km).

    Raises:
        ValueError: if page is not float.
    """
    radius = float(request.rel_url.query.get("radius", 5))  # default 5
    return radius