You can keep your tests as unittest.TestCase classes and run them in parallel by using pytest with the pytest-xdist plugin. Install the packages, then start with a small worker count:
python -m pip install pytest pytest-xdist selenium
pytest -n 4pytest can run unittest tests, while xdist distributes test work across worker processes. Give each test its own WebDriver session and independent test data; increase worker count only as your machine and any Selenium Grid can handle it.
Keep unittest tests and run them through pytest
You do not need to rewrite your test cases in pytest style for this approach. pytest supports unittest-based tests, and pytest-xdist adds process-based parallel execution with -n or --numprocesses. See pytest’s unittest support and the pytest-xdist distribution guide.
Install the packages
python -m pip install pytest pytest-xdist seleniumRun the command in the Python environment where your test dependencies are installed. If your project uses a virtual environment, activate it first.
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Write a unittest case with reliable browser cleanup
import unittest
from selenium import webdriver
class SearchTests(unittest.TestCase):
def setUp(self):
self.driver = webdriver.Chrome()
self.addCleanup(self.driver.quit)
def test_search_page(self):
self.driver.get("https://example.com")
self.assertIn("Example", self.driver.title)addCleanup registers browser shutdown even if a test assertion fails. Selenium’s Python API documentation demonstrates this unittest pattern.
Start with a fixed number of workers
pytest -n 4Replace 4 with a conservative number appropriate to your environment. xdist also documents -n auto, which chooses a count based on detected physical CPU cores, but browser suites may be constrained by memory, CPU, or remote browser-session capacity rather than CPU count alone. Treat worker count as a capacity setting, not a promise of linear speedup.
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Make tests safe to run concurrently
Parallel workers can run tests in an order and at a time that differs from a serial run. Tests should not rely on another test having created state first, and simultaneous tests should not mutate the same account, record, file, or other shared resource unless the application and test setup explicitly make that safe.
- Create a separate WebDriver session for each test and close it during cleanup.
- Use independent or uniquely identified mutable test data so concurrent cases do not overwrite one another.
- Keep browser and infrastructure setup in test configuration rather than depending on execution order or mutable global state.
- Make test collection deterministic. xdist’s worker flow has workers collect tests and checks that they collected the same tests in the same order; see how pytest-xdist works.
- Choose concurrency based on representative runs in the environment where the suite will execute.
Choose local workers, Selenium Grid, or both
| Need | Approach | What it provides |
|---|---|---|
| Run a unittest suite in multiple local processes | pytest with pytest-xdist | pytest discovers unittest tests; xdist schedules work among worker processes. |
| Run browsers on remote machines or across browser and operating-system configurations | Selenium Grid | Remote WebDriver sessions and browser execution across machines. See the Grid overview. |
| Schedule tests locally while using remote browser sessions | pytest-xdist with tests configured for Grid | xdist distributes tests; Grid supplies remote sessions. Keep worker concurrency within the available Grid session capacity. |
Selenium describes Grid as routing WebDriver commands to remote browser instances, and says it is useful for parallel tests across browser types, versions, and operating systems. Its Grid applicability guide includes a formula and illustrative arithmetic for execution time; those examples are not measured speedup guarantees.
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Troubleshoot common parallel-run failures
Tests pass alone but fail in a parallel run
Look for shared mutable data, fixed usernames or record IDs, order-dependent setup, and shared files. Isolate or uniquely name the data each case changes, and ensure each test establishes its own prerequisites.
Browser sessions remain open after a failure
Register driver.quit with self.addCleanup when the driver is created. This ensures cleanup is scheduled even if a test assertion fails.
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Check that test discovery does not depend on mutable state, timing, or worker-specific side effects. xdist expects workers to collect the same tests in the same order; consult its worker and collection documentation.
More workers do not make the suite faster or stable
Reduce -n and observe CPU, memory, and browser-session use. For Grid runs, also check the number of sessions the Grid can serve. A higher worker count can increase contention rather than reduce elapsed time.
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Tests cannot reach the expected remote browser
Confirm the test configuration points to the intended Grid endpoint and that the Grid has available browser capacity. Grid is the execution layer for remote sessions; xdist does not create remote machines or browser slots.
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Frequently Asked Questions
Can I keep using unittest assertions and fixtures?
Yes. This approach runs unittest test cases through pytest; it does not require rewriting assertions as pytest functions.
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xdist distributes tests among worker processes; the worker count is controlled with -n or --numprocesses.
Quick Recap
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