Testing functions

As we start writing tests to be used by pytest we don’t actually need to use pytest in the code. We just need to create one or more files with a name starting with test_.

In those files one or more “test-functions”, that is functions that start with test_.

Inside those function we can call the functions of the AUT and compare the returned values to the expected values passing the result to the regular assert function of Python.

The assert function receives a boolean value. If it is True then assert does nothing. If it is False then assert raises an exception.

We should not call these test-functions ourselves! Pytest will do it for us.

import mymath

def test_math():
    assert mymath.add(2, 3)  == 5
    assert mymath.div(6, 3)  == 2
    assert mymath.div(42, 1) == 42
    assert mymath.add(-1, 1) == 0

Running the tests:

The place where we do need pytest is for running the tests.

pytest test_mymath_with_pytest.py

============================= test session starts ==============================
platform darwin -- Python 3.6.3, pytest-3.3.0, py-1.5.2, pluggy-0.6.0
rootdir: /Users/gabor/work/training/python, inifile:
collected 1 item

examples/pytest/test_mymath.py .                                         [100%]

=========================== 1 passed in 0.01 seconds ===========================

Pytest will read the content of the file(s), find the test_... functions and run each one of them. If any of them raises an exception (because of the assert or otherwise`), pytest will collect this data and create a report at the end.

In the above report you can see that pytest reported the name of the test-file. The single dot . following the filename indicates that there was one test function that passed.

After the test run we could also see the exit code of the program by typing in echo $? on Linux or Mac or echo %ERRORLEVEL% on Windows.

$ echo $?
0

> echo %ERRORLEVEL%
0