def determine_image(py_version):
    """
    Determines which Docker image to use for CodeBuild based on the Python
    version being used on AWS Lambda.

    Parameters
    ----------
    py_version : str
        Python version being used. Must be one of "2.7", "3.6", or "3.7"

    Returns
    -------
    str
        URI of the Docker image.
    """
    if py_version == "2.7":
        return "irlrobot/alppb-python27"
    if py_version == ("3.6" or "None"):
        return "irlrobot/alppb-python36"
    if py_version == "3.7":
        return "irlrobot/alppb-python37"

    # We should never get here, but fallback to Python 3.6.
    return "irlrobot/alppb-python36"