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Assertions

00:00 Now that you’ve seen the flexibility of Mock objects, let’s start to see how we can use its built-in methods to gain deeper insight into our program. I’ve cleaned up this mock.py file.

00:12 You can clear yours out or make a new file, and let’s import json again, and let’s go under the assumption that we want to patch the json module and some of its methods.

00:23 So we’ll say json is a Mock object, and then let’s pretend that somewhere in our program, we use the json.dumps() method.

00:34 So we’ll say json.dumps() and we’ll just put in a very simple dictionary here with 'a' as the key and 1 as the value. And let’s remind ourself what is available, what methods come with this Mock object.

00:49 So we’ll print the dir() representation of json, which is now just an instance of the Mock class, and go down into the terminal and see what we can use.

01:00 I see a bunch of assert methods and some other methods. Let’s go ahead and play with some of these and see what they do. Let’s start with 'assert_called'.

01:11 I’m going to delete this print(dir()) representation, and we’ll say json.dumps.assert_called(). When you originally printed this dir() representation, it was the representation of json.

01:30 But remember that when you call a method on a Mock object, it creates an instance of another Mock object, so the dir() representation of json.dumps will also have this .assert_called() method because it is a Mock object as well.

01:49 Let’s save this program and run it—mock.py.

01:56 And it doesn’t print anything because I didn’t print it. So we’ll just wrap a print() around there and run the program again, and we’ll see a return value of None.

02:08 So this return value of None is essentially saying that “Yes, json.dumps() was called,” because on line 5 we called the method json.dumps(). Let’s try a different one.

02:21 So I’m going to copy this, paste it. And another method we have here is 'assert_called_once'. So we’re going to change this to .assert_called_once(), save it, and let’s clear the screen, and we’ll run the program again.

02:39 And we get another None. So I interpret this as basically, like, true. So it was called once because on line 5, we called it once. Now let’s try to break this and we’ll repeat this line here, so we’re actually calling json.dumps() twice.

02:59 Now we’ll save this and run the program again, and this time we get an AssertionError. It says Expected 'dumps' to have been called once. But it’s Called 2 times. Now you may start to see why this is really valuable and useful in your tests, because sometimes you will want to test that your methods or your functions are only called a certain number of times.

03:23 I’m going to clear the screen, and let’s look at another assertion method. We’ll copy line 9 and we’ll change this to .assert_called_with(). This assertion method allows you to check and verify that your functions are called with the parameters that you expect them to be called with. So first, let’s try to make this pass the test, so to speak.

03:46 So we originally called this json.dumps() with this parameter. Let’s say we want to assert that it was called with this dictionary of {'a': 1}.

03:59 Let’s go ahead and just—well, we’ll delete one of these json.dumps(), just so we don’t get that exception from the .assert_called_once().

04:07 And let’s test out this new method here, .assert_called_with(), with these specific parameters.

04:14 Let’s run our program and we get a None, so that seems to be true. And then let’s try to break this, and we’ll just change the value to 2.

04:25 We’ll go down and clear the screen, run the program again,

04:29 and there we get another AssertionError. So, Expected call to be with the value of 2, and we actually got a value of 1.

04:38 This is very helpful information when you run your tests to see that you’re using your functions as you should be. You’ve looked at some of the assertion methods associated with Mock objects.

04:53 In the next video, you’ll see and play with some of the other attributes that come with Mock objects.

Steve Wehba on Aug. 16, 2020

I don’t understand why you’re printing the return value of your assertions. Never seen anybody do that in a test case before. Is there some reason you’re doing that?

abdelmalek13 on Aug. 5, 2021

I think it’s just to show us, but nah, no one prints an assert statement

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