Using io.StringIO in a Unit Test

00:00 I’ve mentioned that these StringIO objects could be useful for writing unit tests for some of your functions. So let’s go ahead and write a unit test for the upcase_file_content() function and mimic the file object using a StringIO object so that you don’t actually need to work with a real file on disk.

00:19 So create a new file that I call test.py

00:23 and then I will import unittest and from process import upcase_file_content. It’s called upcase_file_content().

00:40 I’ll create a class.

00:59 Okay, so I have the basic structure for writing a unit test for this upcase_file_content() function that lives in my process module.

01:06 And now instead of needing to work with a real file, I want to create a StringIO object that mimics a file and lives inside of memory just to make it easier for this test to run.

01:16 And also quicker than if I would have to deal with input-output operations on my disk.

01:22 So to do this, I will create, call it fake_file again, because we’ve been working with that before and I’ll say StringIO. Well, I need to first import the io module.

01:34 So I’ll say import io, and then here I can use io.StringIO and pass it some text.

01:44 hello, world in lowercase.

01:48 And now I want to say that I would expect, expect that string would be HELLO, WORLD,

01:59 because I want my upcase_file_content() function to upcase all the characters that are in that file.

02:07 And now I can self.assertEqual() the output of upcase_file_content() when passing it the fake_file object should be the same as the expected uppercase HELLO, WORLD. Alright, so this would be a quick test that uses io.StringIO to mimic a file on disk that, let’s assume that usually upcase_file_content() would work with files on disk, but you want to write a test for it that can run in all sorts of environments that maybe don’t necessarily have disk access.

02:43 Then you could do it like this. Let’s go ahead and check whether it works. python -m unittest test.py and you can see that it ran one test and the test passed.

02:58 So that’s great. You’ve created a unit test for your upcase_file_content() function that works just in memory and doesn’t actually need to have access to any sort of file on disk.

03:11 Of course, this only works if your function expects a file object as an input and then only operates on that file object. If your function would take a path as an input and then do the file handling inside of the body, then this would work in that same way.

03:28 And in the next lesson, we’ll take a quick look at what you can do if you’re working with a function like that.