Test Automation Lesson 4 of 6 ⏱️ 4 min read

Data-Driven & BDD Testing

Running one test against many inputs with pytest parametrize, and writing Gherkin scenarios with pytest-bdd.

Updated Oct 30, 2026

Data-driven testing

Data-driven testing separates test logic from test data: one test function runs once for every row of inputs and expected results. It's how you turn the test design techniques — equivalence partitions, boundary values, decision tables — into automated checks without copy-pasting tests.

pytest parametrize

Using the page objects from the previous lesson:

Python
# test_login_data.py — run with: pytest -v
import pytest

from pages.login_page import LoginPage

LOGIN_CASES = [
    # username,   password,                expected message
    ("tomsmith", "SuperSecretPassword!",  "You logged into a secure area!"),
    ("tomsmith", "wrong-password",        "Your password is invalid!"),
    ("someone",  "SuperSecretPassword!",  "Your username is invalid!"),
    ("",         "",                      "Your username is invalid!"),
]


@pytest.mark.parametrize("username, password, expected", LOGIN_CASES,
                         ids=["valid", "wrong-password", "unknown-user", "empty"])
def test_login_messages(driver, username, password, expected):
    login = LoginPage(driver).open()
    login.login(username, password)

    assert expected in login.flash_message()

pytest reports each row as its own test, so a failure tells you exactly which input broke:

Text

test_login_data.py::test_login_messages[valid] PASSED
test_login_data.py::test_login_messages[wrong-password] PASSED
test_login_data.py::test_login_messages[unknown-user] PASSED
test_login_data.py::test_login_messages[empty] PASSED

Loading data from a CSV file

When testers or business analysts maintain the data, keep it outside the code:

Text

# login_cases.csv
username,password,expected
tomsmith,SuperSecretPassword!,You logged into a secure area!
tomsmith,wrong-password,Your password is invalid!
someone,SuperSecretPassword!,Your username is invalid!

Python
# test_login_csv.py
import csv
from pathlib import Path

import pytest

from pages.login_page import LoginPage


def load_cases():
    path = Path(__file__).parent / "login_cases.csv"
    with path.open(newline="", encoding="utf-8") as f:
        return [(row["username"], row["password"], row["expected"]) for row in csv.DictReader(f)]


@pytest.mark.parametrize("username, password, expected", load_cases())
def test_login_from_csv(driver, username, password, expected):
    login = LoginPage(driver).open()
    login.login(username, password)
    assert expected in login.flash_message()

Adding a test case is now a one-line CSV change.

Behaviour-Driven Development (BDD)

BDD describes behaviour in plain-language scenarios that business people, developers and testers all understand. Scenarios are written in Gherkin:

Feature — the capability being described.
Scenario — one example of behaviour.
Given (context), When (action), Then (outcome), And/But (more of the same).

Gherkin
# features/login.feature
Feature: Login
  Registered users can sign in to reach the secure area.

  Scenario: Successful login
    Given I am on the login page
    When I log in as "tomsmith" with password "SuperSecretPassword!"
    Then I see the message "You logged into a secure area!"

  Scenario Outline: Rejected login
    Given I am on the login page
    When I log in as "<username>" with password "<password>"
    Then I see the message "<message>"

    Examples:
      | username | password             | message                   |
      | tomsmith | wrong-password       | Your password is invalid! |
      | someone  | SuperSecretPassword! | Your username is invalid! |

Step definitions with pytest-bdd

Bash

pip install pytest-bdd

Python
# test_login_bdd.py — run with: pytest -v
from pytest_bdd import given, parsers, scenarios, then, when

from pages.login_page import LoginPage

scenarios("features/login.feature")


@given("I am on the login page", target_fixture="login_page")
def open_login_page(driver):
    return LoginPage(driver).open()     # returned value becomes the `login_page` fixture


@when(parsers.parse('I log in as "{username}" with password "{password}"'))
def log_in(login_page, username, password):
    login_page.login(username, password)


@then(parsers.parse('I see the message "{message}"'))
def see_message(login_page, message):
    assert message in login_page.flash_message()

Each Gherkin step maps to one Python function, and the page objects do the real work. Other popular BDD tools: Cucumber (Java/JS), SpecFlow/Reqnroll (.NET) and Behave (Python).

When BDD helps — and when it doesn't

BDD shines when product owners actually read and help write the scenarios: the .feature files become living documentation and acceptance criteria. If only testers ever read them, Gherkin adds a translation layer without much benefit — plain parametrized tests are simpler.

Next, run all of this automatically on every change with CI integration.

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Next lesson

Data-driven testing#

pytest parametrize#

Loading data from a CSV file#

Behaviour-Driven Development (BDD)#

Step definitions with pytest-bdd#

When BDD helps — and when it doesn't#