I think using both 'params' and '_params'  might be confusing.  Many people like to copy one test to make a new one, so having names be very clear can help avoid problems in other tests too.  Can we change the <code>_params</code>name to make it more clear?  How about <code>test_config</code> or <code>test_options</code> or <code>test_dict</code> or something totally different.  This way there can be no mixup over names.