@classmethod
def validate_environment_variables(cls, use_client_cert, use_mtls_endpoint):
    if use_client_cert not in ("true", "false"):
        raise ValueError("Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` must be either `true` or `false`")
    if use_mtls_endpoint not in ("auto", "never", "always"):
        raise auth_exceptions.MutualTLSChannelError("Environment variable `GOOGLE_API_USE_MTLS_ENDPOINT` must be `never`, `auto` or `always`")

@classmethod
def get_client_cert_source(cls, client_options, use_client_cert):
    client_cert_source = None
    if use_client_cert == "true":
        if client_options.client_cert_source:
            client_cert_source = client_options.client_cert_source
        elif mtls.has_default_client_cert_source():
            client_cert_source = mtls.default_client_cert_source()
    return client_cert_source

@classmethod
def get_api_endpoint(cls, api_override, use_mtls_endpoint, client_cert_source):
    if api_override is not None:
        api_endpoint = api_override
    elif use_mtls_endpoint == "always" or (use_mtls_endpoint == "auto" and client_cert_source):
        api_endpoint = cls.DEFAULT_MTLS_ENDPOINT
    else:
       # return the default api endpoint
    return api_endpoint

@classmethod
def get_mtls_endpoint_and_cert_source(cls, client_options: Optional[GoogleClientOptions] = None) -> Tuple[str, Callable[[], Tuple[bytes, bytes]], Optional[str]]:
    if client_options is None:
        client_options = GoogleClientOptions()

    use_client_cert = os.getenv("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false")
    use_mtls_endpoint = os.getenv("GOOGLE_API_USE_MTLS_ENDPOINT", "auto")

    cls.validate_environment_variables(use_client_cert, use_mtls_endpoint)

    client_cert_source = cls.get_client_cert_source(client_options, use_client_cert)
    api_endpoint = cls.get_api_endpoint(client_options.api_override, use_mtls_endpoint, client_cert_source)

    return api_endpoint, client_cert_source