Use Selenium WebDriver to resize the browser, capture a PNG at each target width, and save each image under a filename that records that width. In Python, the core calls are driver.set_window_size(width, height) and driver.save_screenshot(path). Check the actual viewport in your browser: a requested window width is not necessarily the same as the page’s CSS viewport width.
Capture a screenshot at each width
Choose the widths and a consistent height you want to inspect. The Selenium Python API accepts window dimensions in pixels. This loop navigates once, resizes the browser for each width, and saves a separate PNG for every capture.
from selenium import webdriver
url = "https://example.com"
widths = [375, 768, 1280]
height = 900
driver = webdriver.Chrome()
try:
driver.get(url)
for width in widths:
driver.set_window_size(width, height)
driver.save_screenshot(f"screenshot-{width}.png")
finally:
driver.quit()
The resulting files are screenshot-375.png, screenshot-768.png, and screenshot-1280.png. Replace the example URL and widths with your page and the sizes relevant to your test. The snippet illustrates the documented API calls; page-specific waiting may be needed if resizing changes content asynchronously.
Make the captures reflect the intended viewport
Window size is not a guaranteed CSS viewport size
set_window_size(width, height) sets browser window dimensions. Browser chrome, the driver, and the execution environment can affect the relationship between those dimensions and the page’s CSS viewport. If you need to test a breakpoint precisely, inspect window.innerWidth in the page after resizing and compare it with the target breakpoint. Do not assume that a requested outer window width proves the CSS viewport has that width.
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Wait for page changes after resizing
Some pages recalculate layout immediately; others load images, fonts, or data after a resize. There is no universal settling delay that suits every site. Prefer a condition tied to the page under test, such as waiting for a particular element or expected layout state, rather than adding an arbitrary sleep. If a capture is inconsistent, inspect the page state immediately before the screenshot.
Confirm what the screenshot includes
Python’s save_screenshot(filename) saves the current window as a PNG. Selenium’s JavaScript API describes screenshot capture as a best effort that may return, in order, the entire page, current window, visible portion of the current frame, or the display containing the browser. Capture scope can depend on the binding and current context, so verify whether your result is a viewport or full-page image in your browser and language binding.
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Choose widths and interpret the results
Selenium provides the mechanics for setting dimensions and capturing images; it does not define the responsive breakpoints your site should use or prescribe a universal device-emulation setup. Select widths based on the layouts you need to check, then inspect the saved images for overflow, collapsed navigation, clipped content, and other page-specific issues.
- Use a distinct filename for every width so images are easy to identify and compare.
- Keep height consistent when you want to compare layouts at different widths without also changing the vertical viewing area.
- Record or inspect the effective CSS viewport when the precise breakpoint matters.
- For full-page output or device emulation, verify the behavior supported by your chosen browser, driver, and Selenium binding rather than inferring it from the requested window dimensions.
Troubleshooting
The image has the wrong dimensions
Check the saved PNG’s pixel dimensions and inspect the page’s window.innerWidth after calling set_window_size. The requested browser window dimensions do not guarantee a matching CSS viewport or screenshot size across environments. Adjust the setup based on the actual browser output.
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The screenshot shows the old layout
The page may not have finished responding to the resize. Wait for an application-specific signal that the expected layout or content has appeared, then capture. A fixed delay can be a fallback, but it may be too short on a slow run and unnecessarily long on a fast one.
The file is missing or overwritten
Ensure each iteration constructs a different path, as in screenshot-{width}.png, and that the process can write to the current working directory. If the width list contains duplicates, those entries will target the same filename unless you add another distinguishing value.
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The image does not include the whole page
save_screenshot captures the current window in the Python API description. Screenshot scope can differ by binding and context; if you require full-page output, verify the behavior available in your specific setup instead of assuming viewport capture will expand automatically.
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Documentation
- Selenium: Working with windows and tabs
- Selenium 4.50.0 Python WebDriver API
- Selenium JavaScript WebDriver API
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