Parameterizing Tests with Pytest

What is a parameterization?

Roughly speaking, parametrization is a process of varying (changing) one or more coefficients in a mathematical equation. In the context of testing, parametrization is a process of running the same test with varying sets of data. Each combination of a test and data is counted as a new test case.

The most simple form of parameterization:

@pytest.mark.parametrize("number", [10, 20, 30, 0, 42])
def test_big(number):
    assert number > 0 

In this case we are getting five tests: for number 10, 20, 30, 0 and 42. Each of those tests can fail independently of one another (if in this example the test with 0 will fail, and four others will pass). This approach is much more convenient for debugging and development compared with a simple loop with an assert in it.

Parameterizing of a test is done to run the test against multiple sets of inputs. We can do this by using the following marker −


Write the following code in your file:

def add(a, b):
    return a + b

Now, the file is as follows:

import sample
import pytest

@pytest.mark.parametrize('num1, num2, result',
                             (90, 3, 93),
                             ('Hello', ' World', 'Hello World'),
                             (15.5, 25.5, 41)
def test_add(num1, num2, result):
    assert sample.add(num1, num2) == result
  • Pass the values according to the parametrize argument i.e. on adding 90 and 3 we will get 93, so we have passed num1=90, num2=3, result=93. If you want to fail any test case simply change the result which will give wrong output.
  • In your terminal type the following command:
pytest -v

  • Now let’s change the value 93 to 90 and perform testing:

  • We can see two test cases passed and one failed.