Modern Python Projects Transcripts
Chapter: Testing code
Lecture: Parametrized tests
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Another useful feature of by testes parameterization. You can use it when you want to test some code with different inputs.
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Let's continue our example with the online shop. You want to test that adding different numbers of item to the cart will result in
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the correct cart size. So we have a first test where we add one booked the cart and we want to make sure that the card size is equal to one
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Then we try to add 10 books, and then we would expect to have 10 items in the cart.
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And then we also want to check that when we have a malicious user that tries to add minus one item to the cart.
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Our cart is empty, so all those three test looks very similar. The only difference is the number of items and expected cart size.
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We can use pytest parameterization to extract those two variables into a fixture and turn those three tests into one test.
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So, let's do that. First we have to create parametrize Fixture. The first parameter is the number of arguments that we will use in the function.
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I will explain that a bit later, so we have two variable things, the number of items in the order and the cart size. Then we pass a list of tupples.
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Those tupples will be assigned to those parameters that we just specified. So in one test we want to see that adding one item to the cart results
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in a expected card size of one, the same for 10 items and adding -1 results in the current size of 0.
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So now let's just modify this test and let's call it add_items_to_cart and
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it should accept those two parameters as arguments and now we can take them and use them in our function. If we run pytest on this file,
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pytest will create three different tests and each time, It will replace the number of items and expected cart size with the values from the
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tupples in the list above. Since I don't have the code for cart written, I made another simple example with the calculator.
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So here we are, just making sure that adding numbers in Python works correctly. So we have three arguments left,
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right and output, and first we want to check that adding 1 + 1 = 2 that adding 10 + 100 is 110. and so on. So this file,
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we can actually run with pytest. And as you can see, we have four tests.
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The output from pytest looks different than before because we created this pyproject.toml there
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We have specified that we want to have a different output using this -array and -q parameters. So let's see what happens if one of those parameters is
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incorrect. As you can see, we have three tests that passed and one that failed,
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and pytest is actually adding the values from the parametrize fixture in the square brackets here
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so we can see that we have a failing test pytest other numbers with parameters -10 --10 --10. So we know it's this guy that's failing.