B.1 Tutorial: Writing Unit Tests with Pytest

Python has a native unit testing module that you can readily use (unittest). Other third-party unit testing packages exist.

In this tutorial, we will use pytest.

We can install it with pip install pytest.

Pytest Syntax

• File names should start or end with “test”, as in test_example.py or example_test.py.
• If tests are defined as methods on a class, the class name should start with “Test”, as in TestExample. The class should not have an __init__method.
• File names should start or end with “test”, as in test_example.py or example_test.py.
• If tests are defined as methods on a class, the class name should start with “Test”, as inTestExample. The class should not have an __init__method.

Write a Basic Test with Pytest

In file test_example.py, let’s write a simple function sum that takes two arguments num1 and num2 and returns their sum.

def sum(num1, num2):
    """It returns sum of two numbers"""
    return num1 + num2

Now we will write tests to test our sum function. To test an expected result we use an assertion. Tests typically involve many assertions. With pytest, we can simply use the built-in assert keyword. Further convenient assertion functions are provided by NumPy (see http://docs.scipy.org/doc/numpy/reference/routines.testing.html Links to an external site.). They are especially useful when working with arrays. For example, np.testing.assert_allclose(x, y) asserts that the x and y arrays are almost equal, up to a given precision that can be specified.

import pytest #make sure to start function name with test
def test_sum():
    assert sum(1, 2) == 3

To run this test,  run the command:

#It will find all tests inside file test_example.py and run them
pytest test_example.py

or 

python -m pytest test_example.py

To get more info about the test run, use the above command with -v (verbose) option.

python -m pytest test_example.py

When running, we will get:

Add one more test in file test_example.py, which will test the type of output which function sum gives i.e integer.

def test_sum_output_type():
    assert type(sum(1, 2)) is int

Now, if we run the tests again, we will get the output of two tests and test_sum and test_sum_output_type

let’s change the assertion of test_sumto make it fail:

def test_sum():
    assert sum(1, 2) == 4

This will give a result along with failure reason that we expected sum(1, 2) which is 3 to equal 4.

Parametrizing Tests

import pytest@pytest
Links to an external site..mark.parametrize('num1, num2, expected', [(3,5,8)])
def test_sum(num1, num2, expected):
        assert sum(num1, num2) == expected

import pytest
@pytest
Links to an external site..mark.parametrize('num1, num2, expected',[(3,5,8),              
(-2,-2,-4), (-1,5,4), (3,-5,-2), (0,5,5)])
def test_sum(num1, num2, expected):
        assert sum(num1, num2) == expected

def get_sum_test_data():
        return [(3,5,8), (-2,-2,-4), (-1,5,4), (3,-5,-2), (0,5,5)]@pytest
Links to an external site..mark.parametrize('num1, num2, expected',get_sum_test_data())
def test_sum(num1, num2, expected):
        assert sum(num1, num2) == expected

import pytest
@pytest
Links to an external site..fixture
def get_sum_test_data():
        return [(3,5,8), (-2,-2,-4), (-1,5,4), (3,-5,-2), (0,5,5)]def test_sum(get_sum_test_data):
        for data in get_sum_test_data:
                num1 = data[0]
                num2 = data[1]
                expected = data[2]
                assert sum(num1, num2) == expected

@pytest
Links to an external site..fixture(scope='session')
def get_sum_test_data():
        return [(3,5,8), (-2,-2,-4), (-1,5,4), (3,-5,-2), (0,5,5)]@pytest
Links to an external site..fixture(autouse=True)
def setup_and_teardown():
        print('\nFetching data from db')
        yield
        print('\nSaving test run data in db')
def test_sum(get_sum_test_data):
        for data in get_sum_test_data:
                num1 = data[0]
                num2 = data[1]
                expected = data[2]
                assert sum(num1, num2) == expected

pytest test_example.py -v -s