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pytest

pytest#

pytest is a unit testing framework for python code.

Basic elements:

  • Discoverability: it will find the tests

  • Automation

  • Fixtures (setup and teardown)

Installing#

You can install pytest for a single user as:

pip install pytest

This should put pytest in your search path, likely in ~/.local/bin.

If you want to generate coverage reports, you should also install pytest-cov:

pip install pytest-cov

Test discovery#

Adhering to these naming conventions will ensure that your tests are automatically found:

  • File names should start or end with “test”:

    • test_example.py

    • example_test.py

  • For tests in a class, the class name should begin with Test

    • e.g., TestExample

    • There should be no __init__()

  • Test method / function names should start with test_

    • e.g., test_example()

Assertions#

Tests use assertions (via python’s assert statement) to check behavior at runtime

  • https://docs.python.org/3/reference/simple_stmts.html#assert

  • Basic usage: assert expression

    • Raises AssertionError if expression is not true

    • e.g., assert 1 == 0 will fail with an exception

  • pytest does some magic under the hood to add more details about what exactly went wrong, which we will see below

Simple pytest example#

Create a file named test_simple.py with the following content:

def multiply(a, b):
    return a*b

def test_multiply():
    assert multiply(4, 6) == 24

def test_multiply2():
    assert multiply(5, 6) == 2

then we can run the tests as:

pytest -v

and we get the output:

============================= test session starts ==============================
platform linux -- Python 3.11.3, pytest-7.2.2, pluggy-1.0.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/zingale/temp/pytest
plugins: anyio-3.6.2
collected 2 items                                                              

test_simple.py::test_multiply PASSED                                     [ 50%]
test_simple.py::test_multiply2 FAILED                                    [100%]

=================================== FAILURES ===================================
________________________________ test_multiply2 ________________________________

    def test_multiply2():
>       assert multiply(5, 6) == 2
E       assert 30 == 2
E        +  where 30 = multiply(5, 6)

test_simple.py:8: AssertionError
=========================== short test summary info ============================
FAILED test_simple.py::test_multiply2 - assert 30 == 2
========================= 1 failed, 1 passed in 0.04s ==========================

this is telling us that one of our tests has failed.

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