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Pytest Basic Usage

Assert that something is true

# content of test_sample.py
def func(x):
    return x + 1


def test_answer():
    assert func(3) == 5, "some msgto be printed in the traceback if fails"

Assert that a certain exception is raised

# content of test_sysexit.py
import pytest


def f():
    raise SystemExit(1)


def test_mytest():
    with pytest.raises(SystemExit):
        f()

Matching Exception messages

import pytest


def myfunc():
    raise ValueError("Exception 123 raised")


def test_match():
    with pytest.raises(ValueError, match=r".* 123 .*"):
        myfunc()

Using context provided by raises

def test_foo_not_implemented():
    def foo():
        raise NotImplementedError

    with pytest.raises(RuntimeError) as excinfo:
        foo()
    assert excinfo.type is RuntimeError

Handling ExceptionGroup in pytest

  • You can also use the context provided by raises to assert that an expected exception is part of a raised ExceptionGroup:
# content of test_exceptiongroup.py
import pytest


def f():
    raise ExceptionGroup(
        "Group message",
        [
            RuntimeError(),
        ],
    )


def test_exception_in_group():
    with pytest.raises(ExceptionGroup) as excinfo:
        f()
    assert excinfo.group_contains(RuntimeError)
    assert not excinfo.group_contains(TypeError)

Group multiple tests in a class

  • Class name should be prefixed with Test, and method names should be prefixed with test.
  • No __init__ method should be there.
# content of test_class_demo.py
class TestClassDemoInstance:
    value = 0

    def test_one(self):
        self.value = 1
        assert self.value == 1

    def test_two(self):
        assert self.value == 1 # will fail, as each test run has unique instance

Grouping tests in class

Inside classes, each test has a unique instance of the class.

xfail mark and pytest.raises

def f():
    raise IndexError()


@pytest.mark.xfail(raises=IndexError)
def test_f():
    f()
  • This will only “xfail” if the test fails by raising IndexError or subclasses.

xfail v/s raises So when to use which one?

  • If there's an existing bug that is yet to be fixed, use xfail. It will help you in documenting unfixed bugs when you decide to fix.
  • pytest.raises is likely to be better for cases where you are testing exceptions your own code is deliberately raising, which is the majority of cases.