Python – Maintaining log stream after file name change

I think this is a great question. Unit testing, micro-testing, TDD, and BDD are difficult, sometimes confusing, and often not taken seriously. So kudos for trying and asking for help!

I am by no means an expert, but I did just take a formal class and am also part of a small team that is leading an initiative to roll out unit testing and TDD within our organization.

Based on my experience and what I learned in my class, here is what I think you need to know in order to start:

1. Consider that there are different types of testing and they accomplish different goals. There are lots of things that unit testing is simply a poor or incomplete solution for, which can lead one to think that unit testing is a waste of time. However, I view it as complementary to integration testing and functional testing.
2. Don't stress out about coverage. There is a time and place to think about code coverage, but if you worry about that right now, you will waste time unit testing accessors and mutating, which provides no business value.
3. Don't think about testing classes or methods, but think about testing behaviors. What does this class do? What does it not do? (This is part of why the term BDD is used these days)
4. Test interesting behaviors. What qualifies as interesting is up to you. Many experts argue that a bug is a missing test. Therefore, anything you feel could reasonably break is worth testing. Would you write a test for i++? Probably not. Would you write a test for incrementI()? Maybe. If it's a calculator app, I probably would. If it's a web app, then I wouldn't sweat something that simple.
5. Tests are not just about making sure your code works. They serve as an executable specification. Meaning, if somebody needs to understand the requirements of a class, they can look at the test.
6. It is ok for a test to assume that a dependency works. If that worries you, then maybe you should unit test the dependency as well. But keep it separate. For example, one could argue that testing the data processing without a real file is bad because you don't know if the file parsing works. But if you have a unit test which verifies that file parsing works, and a unit test that verifies the processing of the file's data works, then clearly both work. Granted, this does not prove that they work together. But that's ok. That is integration testing.
7. Unit tests should be repeatable and isolated. A unit test that makes a web service call, then writes to a database, and then prints some result to the console is not a unit test. It's a test, sure, but not a unit test. Don't get so hung up on testing everything that you write fragile tests which do a ton of work. So if a method does those things, how do you unit test it? This leads to the next topic...
8. Separating out dependencies - Write small units which can be called independently, with dependencies passed in. If you design so that the processing is independent of the source file itself, you can test the processing logic without needing files. For example, instead of the file processing taking a java.io.File object, parsing it, and doing all the work, perhaps it could take a java.io.InputStream, parse it, and do all the work? Or a method which reads the file's contents into an array or other data structure, then passes that into the processing?
9. Mocking out dependencies - ultimately, somebody needs to talk to the external world. Ideally, you will separate your dependencies, but sometimes you reach points where the complexity introduced by doing so is counter-productive. For example, a method which several levels down the call stack needs to talk to the database and separating that out is unreasonable, then mock it out with a fake implementation that returns test data. I've gone as far as creating a MockDatabase class before, which lets me map stored procedure names to hard-coded, in-memory result sets.
10. Realize when you have mocked so much that unit testing is pointless. If you have mocked out everything a method does, then you are left testing an empty method. If that is the case, maybe that method is not interesting to test? Trust that an integration test, automated functional test, or even manual testing will catch any problems with it, and don't worry about unit testing it.

I could go on forever. Hopefully that helps.

Remember, this is not easy. If it was easy, everybody would do it.

It's not uncommon to do what you described. In fact, when you create a new database for your user, you basically creating a new file for that user. So, it just adding a file to the set of per user files.

The choice of when to do the work begs explanation though. Doing the sign up process in batch periodically can create load spikes on the servers. It's better and simpler to copy all the files for new users straight away. If the process is long enough (e.g. creating a vm for user), you show them an "setup in progress" page.