Pytest Built-In Fixtures
Pytest fixtures are a powerful tool for writing and organizing tests in Python. They allow you to set up test dependencies and control the execution of your tests in a consistent and predictable way. Pytest provides a number of built-in fixtures that can be used for a variety of testing needs, including creating temporary directories, modifying and restoring attribute values, capturing output streams, and more. In this article, we will explore some of the most commonly used built-in pytest fixtures and provide examples of how to use them in your tests.
This fixture creates a temporary directory and returns its path, which can be used for testing file system operations.
import os def test_tmpdir(tmpdir): # create a file in the temporary directory p = tmpdir.mkdir("sub").join("hello.txt") p.write("content") # check that the file was created assert os.path.exists(p.strpath)
This fixture allows you to create multiple temporary directories and return their paths.
def test_tmpdir_factory(tmpdir_factory): # create a temporary directory tmpdir = tmpdir_factory.mktemp("temp") # create a file in the temporary directory p = tmpdir.join("hello.txt") p.write("content") # check that the file was created assert os.path.exists(p.strpath)
This fixture allows you to modify the values of certain attributes or functions during the test, and then restore them to their original values after the test has completed.
import math def test_monkeypatch(monkeypatch): def mock_sqrt(x): return 4 # replace the math.sqrt function with the mock function monkeypatch.setattr(math, "sqrt", mock_sqrt) # the math.sqrt function should now return 4 assert math.sqrt(16) == 4
This fixture captures the standard output and error streams (e.g., print statements) during the test, and allows you to assert on the captured output.
def test_capsys(capsys): print("Hello, world!") # capture the standard output and error streams captured = capsys.readouterr() # check that the output was as expected assert captured.out == "Hello, world!\n"
This fixture is similar to capsys, but captures the output of sys.stdout and sys.stderr to file descriptors, rather than strings.
import sys def test_capfd(capfd): # redirect the standard output stream to a file descriptor sys.stdout = capfd.out print("Hello, world!") # capture the standard output stream out, _ = capfd.readouterr() # check that the output was as expected assert out == "Hello, world!\n"
This fixture captures warning messages issued during the test, and allows you to assert on the captured warnings.
import warnings def test_recwarn(recwarn): warnings.warn("This is a warning") # capture the warning messages w = recwarn.pop() # check that the warning message was as expected assert str(w.message) == "This is a warning"
client (for Flask application)
This fixture is useful for testing web applications and returns a FlaskClient object, which can be used to make HTTP requests to the application.
django_client (for Django application)
This fixture is similar to client, but is specific to Django applications and returns a Client object.
def test_django_client(django_client): # send an HTTP GET request to the application response = django_client.get("/") # check that the response status code was 200 (OK)