Exercise

Completed

In this exercise, you'll use pytest with parametrize to test a function. Then, you update a test class to use a fixture instead of a setup() and teardown() method. Using parametrize and working with fixtures allows you to become more flexible when creating or updating tests.

Step 1 - Add a file with tests for this exercise

1. Create a new test file called test_advanced.py and add the following code:

   def str_to_bool(string):
       if string.lower() in ['yes', 'y', '1']:
           return True
       elif string.lower() in ['no', 'n', '0']:
           return False
   
   

   The function str_to_bool() accepts a string as input and depending on its contents it returns a True or False value.

2. In the same file, append the tests for the str_to_bool() function. Use pytest.mark.parametrize() to test all the true values first:

   import pytest 
   
   @pytest.mark.parametrize("string", ['Y', 'y', '1', 'YES'])
   def test_str_to_bool_true(string):
       assert str_to_bool(string) is True
   

3. Next, append another test with the false values:

   @pytest.mark.parametrize("string", ['N', 'n', '0', 'NO'])
   def test_str_to_bool_false(string):
       assert str_to_bool(string) is False
   

   There are now two tests covering all the possible inputs for both the True and False return values.

Note

It isn't common to have tests within the same file as actual code. For simplicity, the examples in this exercise will have actual code in the same file. In real-world Python projects you'll find that tests are separated by files and directories from the code it's testing.

Step 2 - Run the tests and explore the report

After you add the tests, the next step is to run pytest and inspect the output. Use the increased verbosity flag (-v) so that you can see all input values being treated as a separate test.

$ pytest -v test_avanced.py
============================= test session starts ==============================
Python 3.9.6, pytest-6.2.5, py-1.11.0, pluggy-1.0.0 
rootdir: /private
collected 8 items

test_advanced.py::test_str_to_bool_true[Y] PASSED                        [ 12%]
test_advanced.py::test_str_to_bool_true[y] PASSED                        [ 25%]
test_advanced.py::test_str_to_bool_true[1] PASSED                        [ 37%]
test_advanced.py::test_str_to_bool_true[YES] PASSED                      [ 50%]
test_advanced.py::test_str_to_bool_false[N] PASSED                       [ 62%]
test_advanced.py::test_str_to_bool_false[n] PASSED                       [ 75%]
test_advanced.py::test_str_to_bool_false[0] PASSED                       [ 87%]
test_advanced.py::test_str_to_bool_false[NO] PASSED                      [100%]

============================== 8 passed in 0.01s ===============================

Although you wrote only two test functions, pytest was able to create eight tests in total thanks to the parametrize() function.

Step 3 - Port an existing test to a fixture

1. Add a new class-based test to the test_advanced.py file. This test should use a setup() and teardown() function that creates a temporary file with some text on it. After each test, the file gets removed. It should look like this:

   import os
   
   
   class TestFile:
   
       def setup(self):
           with open("/tmp/done", 'w') as _f:
               _f.write("1")
   
       def teardown(self):
           try:
               os.remove("/tmp/done")
           except OSError:
               pass
   
       def test_done_file(self):
           with open("/tmp/done") as _f:
               contents = _f.read()
           assert contents == "1"
   

   This test class creates a file, but is problematic because the /tmp/ path isn’t guaranteed to be present on every system.

2. Create a fixture that uses the pytest tmpdir() fixture to write to the file and return the path:

   import pytest
   
   @pytest.fixture
   def tmpfile(tmpdir):
       def write():
           file = tmpdir.join("done")
           file.write("1")
           return file.strpath
       return write
   

   The tmpfile() fixture uses pytest's tmpdir() fixture, which guarantees a valid temporary file that gets cleaned after tests are done.

3. Update the TestFile class so that it uses the fixture instead of the helper methods:

   class TestFile:
   
       def test_f(self, tmpfile):
           path = tmpfile()
           with open(path) as _f:
               contents = _f.read()
           assert contents == "1"
   

   This test class can now ensure that a temporary file gets created and has the proper contents for the assertion to work.

Check your work

By now you should have a Python file named test_advanced.py with the following code:

• A str_to_bool() function that accepts a string and returns a boolean value depending on the contents of the string.
• Two parametrized tests for the str_to_bool() function, one that tests the True values and the other one that tests the False values.
• A custom pytest fixture that uses the tmpdir() fixture to create a temporary done file with some contents.
• A test class that uses the custom tmpfile() fixture to create the file.

All tests should pass when running them in the terminal, with no errors.