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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsKeep your tests as unittest.TestCase classes and run them with pytest plus pytest-xdist. Pytest can discover and execute unittest tests, while xdist distributes work among separate worker processes. Start with a fixed, modest worker count, give each test its own WebDriver session, and tune concurrency to the CPU, memory, and browser capacity available.
Install the runner and run unittest tests in parallel
Install pytest, its xdist plugin, and Selenium in the environment used for the suite:
python -m pip install pytest pytest-xdist selenium
pytest -n 4The -n 4 option asks xdist to use four worker processes. Pytest supports tests written with Python’s unittest framework, so you do not need to rewrite the test classes as pytest functions. See pytest’s unittest support and the pytest-xdist distribution guide.
Make each test own and close its browser
Create a driver for the test and register cleanup immediately. addCleanup ensures quit() is called even if a later assertion fails.
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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)
This follows the lifecycle pattern in Selenium’s Python API example. Do not share one WebDriver instance across concurrently running tests: each test needs an isolated browser session and cleanup path.
Choose a worker count that your environment can support
xdist accepts an explicit count with -n or --numprocesses. Its documentation also describes -n auto, which selects a count based on detected physical CPU cores. For browser tests, that detection is only a starting point: each worker may launch a browser process, and remote Grid capacity can be a tighter limit than local CPU count.
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- Begin with a small fixed value such as
-n 2or-n 4, then compare representative suite runs in the actual CI or developer environment. - Watch CPU and memory use as well as elapsed time. More workers can increase contention rather than reduce total time.
- If workers request remote sessions from Grid, keep concurrent demand within the available browser-session capacity.
- Use
-n autoonly when the automatically detected core count is a sensible limit for the machine and browser infrastructure.
xdist schedules tests among workers; it does not guarantee a particular speedup. Selenium’s Grid applicability page includes an illustrative calculation of test count, average test time, and node count, not a measured performance guarantee: When to Use Grid.
Decide whether execution should be local or remote
| Need | Approach | What it does |
|---|---|---|
| Parallel processes on one machine for a unittest suite | pytest with pytest-xdist | Pytest discovers unittest cases; xdist schedules tests among worker processes. |
| Remote machines or coverage across browser types, versions, or operating systems | Selenium Grid | Routes WebDriver commands to remote browser instances. See Grid documentation and Grid applicability. |
| Local scheduling combined with remote browser capacity | pytest-xdist with tests configured for Grid | xdist distributes tests while Grid provides remote sessions. Align worker concurrency with the sessions available. |
Use local workers when the goal is simply to make a suite run concurrently on one host. Use Grid when browser placement or browser/platform variety is part of the requirement. Selenium describes Grid as enabling WebDriver scripts to run on remote machines by routing client commands to remote browser instances.
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Make tests safe to run concurrently
Parallel execution exposes hidden dependencies that may not appear in a serial run. Before increasing the worker count, check that tests can run in any order and at the same time.
- Isolate mutable data. Avoid having workers edit the same account, database record, file, or other shared resource. Create unique test data or arrange safe cleanup.
- Do not rely on execution order. A test should establish its own preconditions rather than depend on another test having run first.
- Keep collection deterministic. xdist workers collect tests independently and check that they collected the same tests in the same order. Avoid collection logic whose results vary by worker or environment; see How pytest-xdist works.
- Put browser and infrastructure configuration in the test setup. Do not use mutable global browser state as an implicit coordination mechanism.
- Match concurrency to capacity. Consider both local machine resources and remote browser slots when setting the worker count.
Troubleshoot common failures
Pytest reports that no tests were collected
Check that the test files and test methods follow pytest’s discovery conventions, and run pytest from the project directory containing the suite. Confirm that the unittest classes inherit from unittest.TestCase. Pytest’s documented unittest support does not require converting those cases to pytest-style functions.
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Tests fail only when run with multiple workers
Look for shared accounts, records, files, ports, or other mutable state, along with tests that depend on order. Isolate those resources and ensure each test creates and closes its own driver. Re-run with a lower worker count to distinguish concurrency-sensitive test design from simple resource pressure.
Browser sessions fail to start or disappear under load
Reduce -n and inspect available CPU and memory. When using Grid, check that requested concurrency does not exceed the browser sessions the Grid can provide. A worker count is not a guarantee that the machine or Grid has matching capacity.
Best Value
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Workers disagree about collected tests
Make test collection stable across processes. Avoid collection-time decisions based on changing external data or process-specific state, then consult the xdist worker and collection description.
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Frequently Asked Questions
Do I have to rewrite unittest tests to use pytest-xdist?
No. Pytest supports unittest test cases, and xdist supplies the worker-process distribution.
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No. Browser resource use, test duration, and available Grid sessions constrain the result; measure in the target environment.
Quick Recap
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