How to parametrize test in pytest

PytestHow-ToBeginner · 3 min read

How to Parametrize Tests in pytest for Efficient Testing

In pytest, you can parametrize tests using the @pytest.mark.parametrize decorator to run the same test function with different sets of inputs. This helps you test multiple cases without writing separate test functions.

📐

Syntax

The @pytest.mark.parametrize decorator takes two main arguments: a string of comma-separated parameter names, and a list of tuples with values for those parameters. Each tuple represents one test case.

Example: @pytest.mark.parametrize('input,expected', [(1,2), (3,4)]) runs the test twice with input=1, expected=2 and input=3, expected=4.

python

@pytest.mark.parametrize('param1,param2', [(value1, value2), (value3, value4)])
def test_example(param1, param2):
    # test code using param1 and param2
    pass

💻

Example

This example shows a test for a simple function that adds 1 to a number. The test runs with multiple inputs and checks the expected output.

python

import pytest

def add_one(x):
    return x + 1

@pytest.mark.parametrize('input,expected', [
    (1, 2),
    (5, 6),
    (0, 1),
    (-1, 0)
])
def test_add_one(input, expected):
    assert add_one(input) == expected

Output

==== test session starts ==== collected 4 items test_sample.py .... [100%] ==== 4 passed in 0.01s ====

⚠️

Common Pitfalls

Forgetting to separate parameter names by commas inside the string (e.g., use 'input,expected' not 'input expected').
Passing a list of values instead of a list of tuples when multiple parameters are used.
Mismatch between the number of parameters and the tuple length.
Using mutable default arguments inside test functions can cause unexpected behavior.

python

import pytest

# Wrong: parameter names missing comma
# @pytest.mark.parametrize('input expected', [(1,2), (3,4)])

# Wrong: values not tuples for multiple params
# @pytest.mark.parametrize('input,expected', [1, 2, 3])

# Correct usage:
@pytest.mark.parametrize('input,expected', [(1,2), (3,4)])
def test_correct(input, expected):
    assert input < expected

📊

Quick Reference

Use @pytest.mark.parametrize to run tests with different inputs easily. Always match parameter names and tuple values. Use tuples for multiple parameters. This reduces code duplication and improves test coverage.

Feature	Usage	Example
Decorator	@pytest.mark.parametrize	@pytest.mark.parametrize('x,y', [(1,2), (3,4)])
Parameter names	Comma-separated string	'input,expected'
Values	List of tuples	[(1,2), (3,4)]
Test function args	Match parameter names	def test_func(input, expected):
Multiple tests	Runs once per tuple	4 tuples = 4 test runs

✅

Key Takeaways

Use @pytest.mark.parametrize to run one test with many input sets.

Parameter names must be a comma-separated string matching test function arguments.

Provide a list of tuples with values for each test case.

Ensure tuple length matches the number of parameters exactly.

Parametrization reduces code duplication and improves test clarity.