为什么下面的代码不起作用?
new_type = sa.Enum('nonexistent_executable', 'output_limit_exceeded',
'signal', 'success', 'timed_out', name='status')
old_type = sa.Enum('nonexistent_executable', 'signal', 'success', 'timed_out',
name='status')
op.alter_column('testcaseresult', u'status', type_=new_type,
existing_type=old_type)