How do you take a screenshot on test failure?

Screenshots-on-failure are the cheapest, highest-leverage debug aid in Selenium tests. The mechanism is simple; the integration with your test runner is what makes it useful.

The capture call (Java):

File src = ((TakesScreenshot) driver).getScreenshotAs(OutputType.FILE);
Files.copy(src.toPath(), Paths.get("screenshots/" + testName + ".png"));

getScreenshotAs returns whatever you asked for: OutputType.FILE, OutputType.BYTES, or OutputType.BASE64. BASE64 is handy for embedding directly in HTML reports.

Wiring to failures (TestNG):

@AfterMethod(alwaysRun = true)
public void onTestEnd(ITestResult result) {
    if (result.getStatus() == ITestResult.FAILURE) {
        captureScreenshot(result.getName());
    }
}

pytest equivalent uses a hook in conftest.py:

@pytest.hookimpl(hookwrapper=True)
def pytest_runtest_makereport(item, call):
    outcome = yield
    rep = outcome.get_result()
    if rep.when == "call" and rep.failed:
        driver = item.funcargs["driver"]
        driver.save_screenshot(f"screenshots/{item.name}.png")

Two refinements that separate juniors from mids:

1. Attach to the report, not just save to disk. Allure / Extent / pytest-html can embed the image.
2. Capture page source and console logs too — a screenshot tells you what's visible, not why.