def METHOD_NAME(self, mock_boto3, aws_region):
    self.app.conf.s3_access_key_id = 'somekeyid'
    self.app.conf.s3_secret_access_key = 'somesecret'
    self.app.conf.s3_bucket = 'bucket'
    self.app.conf.s3_region = aws_region
    S3Backend(app=self.app)
    mock_boto3.Session.assert_called_once_with(
        aws_access_key_id='somekeyid',
        aws_secret_access_key='somesecret',
        region_name=aws_region)