I'm not very familiar with pytest but try to incorporate it into my project. I already have some tests and understand main ideas.
But I got stuck with test for Excel output. I have a function that makes a report and saves it in Excel file (I use xlsxwriter to save in Excel format). It has some merged cells, different fonts and colors, but first of all I would like to be sure that values in cells are correct.
I would like to have a test that will automatically check content of this file to be sure that function logic isn't broken.
I'm not sure that binary comparison of generated excel file to the correct sample is a good idea (as excel format is rather complex and minor change of xlsxwriter library may make files completely different).
So, I seek an advice how to implement this kind of test. Had someone similar experience? May you give advice?
IMHO a unit test should not touch external things (like file system, database, or network). If your test does this, it is an integration test. These usually run much slower and tend to be brittle because of the external resources.
That said, you have 2 options: unit test it, mocking the xls writing or integration test it, reading the xls file again after writing.
When you mock the xlswriter, you can have your mock check that it receives what should be written. This assumes that you don't want to test the actual xlswriter, which makes sense cause it's not your code, and you usually just test your own code. This makes for a fast test.
In the other scenario you could open the excel file with xslsreader and compare the written file to what is expected. This is probably best if you can avoid the file system and write the xls data to a memory buffer from which you can read again. If you can't do that, try using a tempdir for your test, but with that you're already getting into integration test land. This makes for a slower, more complicated, but also more thorough test.
Personally, I'd write one integration test to see that it works in general, and then a lot of unit tests for the different things you want to write.
Python Unit Test for writing Excel file