def max_shear_stress(V, A, shape):
    """Returns The maximum shear stress for known shapes

    :param float V: Shear Stress
    :param float A: Cross section area
    :param str shape: Cross section shape

    :returns: Maximum shear Stress
    :rtype: float
    """
    if shape == 'circle':
        return (4 * V) / (3 * A)
    elif shape == ' rectangle':
        return (3 * V) / (2 * A)
    else:
        raise ValueError(f"shape = {shape} is unknown")