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Use Playwright for Python to capture both page states under the same browser and viewport conditions, then compare the resulting images with a Python pixel-diff tool such as pixelmatch. Save a difference image as well as a numeric result: the image helps you judge what changed, while the number lets your test apply an explicit tolerance. Pixel differences are sensitive to rendering variation, so control the environment and review proposed baseline updates.
How the comparison works
A visual comparison has two separate jobs: capturing equivalent renders and deciding which pixel changes matter. Playwright’s Python API handles capture; an image-diff stage compares the captures. The Python package pixelmatch on PyPI describes pixel-level comparison, PIL.Image support, anti-aliased-pixel detection, and perceptual colour-difference metrics. Check its current compatibility and maintenance before adopting it for a long-lived project; those details are not established here.
Playwright’s toHaveScreenshot() assertion is documented for Playwright Test and uses pixelmatch. It is not an automatic Python assertion: in Python, capture with Playwright and call an image-comparison implementation separately. See the Playwright visual comparisons guide and Playwright for Python screenshot documentation.
Install the Python tools
Install Playwright for Python and its browser binaries, plus Pillow and pixelmatch. The code below uses Playwright’s async Python API and the pixelmatch Python package interface that accepts PIL images.
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Create and activate a virtual environment for the project.
-
Install the packages:
python -m pip install playwright pillow pixelmatch. -
Install a Playwright browser:
python -m playwright install chromium.
Package APIs can change. If the pixelmatch import or call signature differs in the version you install, consult the package’s current PyPI documentation rather than assuming the JavaScript package interface applies unchanged.
Capture and compare two page states
This example captures a reference and a current version at the same viewport, waits for the page to reach a usable state, loads each PNG with Pillow, and writes a visual diff. It assumes the page content is stable between captures; for changing sites, control test data or use a fixed test state before relying on the result.
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import asyncio
from pathlib import Path
from PIL import Image
from pixelmatch import pixelmatch
from playwright.async_api import async_playwright
URL = "https://example.com"
REFERENCE_PATH = Path("reference.png")
CURRENT_PATH = Path("current.png")
DIFF_PATH = Path("diff.png")
async def capture(url: str, output_path: Path) -> None:
async with async_playwright() as p:
browser = await p.chromium.launch()
page = await browser.new_page(
viewport={"width": 1280, "height": 800},
device_scale_factor=1,
)
await page.goto(url, wait_until="networkidle")
await page.screenshot(path=str(output_path), full_page=True)
await browser.close()
async def main() -> None:
await capture(URL, REFERENCE_PATH)
await capture(URL, CURRENT_PATH)
reference = Image.open(REFERENCE_PATH).convert("RGBA")
current = Image.open(CURRENT_PATH).convert("RGBA")
if reference.size != current.size:
raise ValueError(
f"Image sizes differ: {reference.size} vs {current.size}. "
"Capture both pages with the same viewport and page state."
)
diff = Image.new("RGBA", reference.size)
changed_pixels = pixelmatch(
reference,
current,
diff,
includeAA=False,
)
diff.save(DIFF_PATH)
total_pixels = reference.width * reference.height
print(f"Changed pixels: {changed_pixels} / {total_pixels}")
print(f"Diff image: {DIFF_PATH}")
# Example policy only: choose a threshold appropriate for your application.
allowed_changed_pixels = 100
if changed_pixels > allowed_changed_pixels:
raise AssertionError(
f"Visual difference exceeds policy: {changed_pixels} > "
f"{allowed_changed_pixels}; inspect {DIFF_PATH}"
)
asyncio.run(main())The example treats the first capture as the reference, but a repeatable test should normally capture a known-good baseline once, store it under version control or an intentional artifact-retention policy, and compare new runs against it. Do not overwrite that baseline automatically when a test fails: inspect the difference, confirm whether the change is intended, then update the accepted reference deliberately.
Capture a viewport, full page, or one element
Playwright can write a screenshot to a file, return screenshot bytes for further processing, capture the full page, or target an element. Keep the selected scope identical on both sides of a comparison.
# Viewport screenshot saved to a file
await page.screenshot(path="viewport.png")
# Entire scrollable page
await page.screenshot(path="full-page.png", full_page=True)
# Screenshot as bytes, useful when passing data to another image pipeline
png_bytes = await page.screenshot()
# A particular element
await page.locator("main article").screenshot(path="article.png")When you capture an element, make sure the locator identifies the same intended element in both runs. Full-page screenshots can expose lazy-loaded content or page sections that are not present in a viewport capture; wait for the relevant content to load before taking either screenshot.
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Exact equality is appropriate only when the rendering environment and page state are deterministic enough that identical output is expected. Otherwise, define and justify tolerance rather than treating every changed pixel as a defect or ignoring all small changes.
-
Changed-pixel allowance: Set a maximum count that fits the captured region and the kinds of harmless variation your application permits. A count that works for a small element capture may be too strict or too lax for a full-page image.
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Per-pixel colour tolerance: A perceptual comparison can treat small colour differences differently from large ones. The threshold is a policy choice; a more permissive setting can conceal a genuine visual regression.
-
Anti-aliasing: Text and edges can differ slightly across render environments. Pixelmatch advertises anti-aliased-pixel handling, but decide whether excluding those pixels is suitable for your UI and review the resulting diff.
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Diff image review: A count cannot tell you whether a change is a broken layout, an intentional content update, or harmless rendering noise. Inspect the highlighted image when a comparison fails.
Playwright’s visual-comparison documentation describes threshold as a perceived colour-difference tolerance and maxDiffPixels as an allowed count of differing pixels. It gives an example maxDiffPixels: 100 and documents a threshold scale from 0 (strict) to 1 (lax), with a JavaScript documentation default of 0.2. Those are Playwright Test settings, not recommended defaults for a separate Python pixel-diff implementation.
Make captures reproducible
Screenshot output can vary with the host operating system, browser version, browser settings, hardware, power source, and headless mode. Run reference and current captures in a consistent environment, and keep capture settings the same.
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Pin or otherwise control the browser version used for both captures.
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Use the same operating system, viewport dimensions, device scale factor, and headless configuration.
-
Use fixed test data and a consistent logged-in or anonymous state.
-
Wait for the specific content your test needs. A fixed delay can help with a known transition, but it is not a substitute for waiting for the relevant selector or state.
-
Prevent clocks, randomized records, rotating promotions, animations, and asynchronous content from changing unpredictably. Prefer controlled test state; where appropriate, hide known volatile regions in the capture workflow.
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The Playwright visual-comparison guide notes that rendering varies across environments and documents a stylePath option for hiding volatile regions in its test workflow. If using Python with a separate diff step, apply equivalent control in your own capture or test setup; do not assume that Playwright Test options are available directly in Python.
Common problems and fixes
The images have different dimensions
The captures used different viewport settings, device scale factors, page lengths, or target elements. Match those settings and make sure both pages reach equivalent content before capture. If the page content legitimately differs in length, decide whether full-page comparison is the right scope or whether a stable element is more meaningful.
The test reports changes on an unchanged page
Check for changing content and environment drift first: clocks, asynchronous updates, animation, randomized data, browser version, operating system, or headless configuration. Stabilize page state and capture conditions before increasing tolerance; a broader tolerance can hide actual regressions.
The diff is noisy around text or edges
Small rasterization differences can affect anti-aliased edges. Confirm that both captures use the same environment, then evaluate the diff implementation’s anti-alias handling and perceptual threshold. Review the changed image before accepting an exception.
The screenshot is blank or incomplete
Verify navigation completed and that the page reached the state your test expects. Wait for a meaningful selector or application-ready condition when possible; a generic network-idle condition may not represent readiness for every site. For lazy-loaded content, ensure the relevant region has been brought into view or otherwise loaded before capturing.
The baseline update makes the failure disappear
An updated reference can hide a real defect if it is accepted without review. Keep baseline updates deliberate: inspect the diff, confirm the intended UI change, and only then replace the known-good image. Playwright Test documents a separate snapshot-update operation in its baseline workflow; a Python project should likewise make baseline changes explicit.
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