Django tests are run in a transaction by default. Thus, any changed made at the database level in one test will not affect another tests
However, if you create an object in setUpTestData, modify one of the object properties (e.g. obj.name='new_name'), and then call save() on the object, other tests will still have obj.name of 'new_name'.
This is because while the database transaction of updating name to be new_name was walked back, you cannot walk back the in-memory setting of obj.name to new_name.
If you refresh_from_db, it will no longer have a name of new_name (because the name update was never committed). But if you refresh_from_db on a class object in the middle of a test after a database action has been committed, you are overwriting the self.<object> in memory and thus it will persist.
However, it is better practice to avoid modifying in memory attributes at all in your tests. Within views, they get different copies of the data so the in-memory updates there are fine.
If you are using a tests folder you must include an __init__.py file so the test runner can find and import the tests from that directory
