First find out whether Selenium returned screenshot bytes or whether those bytes were lost while writing the file. Call get_screenshot_as_png(), check its length and PNG signature, then compare the bytes with the file at an absolute path. A successful save_screenshot() return value alone does not prove that the result is a valid PNG.
Separate screenshot capture from file writing
Selenium’s Python implementation requests screenshot data, decodes it, and then writes the resulting bytes to the filename. That gives you two distinct places to investigate: the WebDriver/browser response and the filesystem write. The method returns False if the file operation raises an OSError; it does not validate PNG dimensions or check that the payload is a valid PNG before returning True. Selenium Python WebDriver implementation
Inspect the returned screenshot bytes
png = driver.get_screenshot_as_png()
print("screenshot bytes:", len(png))
print("PNG signature:", png[:8])
with open("/absolute/path/screenshot.png", "wb") as f:
f.write(png)A PNG normally starts with b'x89PNGrnx1an'. A zero length or different signature means the captured payload is empty or does not look like a PNG, regardless of the destination file. If the bytes look right but the saved file does not, focus on the destination and subsequent file operations.
Check the file Selenium actually wrote
from pathlib import Path
path = Path("screenshots/page.png").resolve()
path.parent.mkdir(parents=True, exist_ok=True)
ok = driver.save_screenshot(str(path))
print("saved:", ok)
print("path:", path)
if path.exists():
data = path.read_bytes()
print("file bytes:", len(data))
print("PNG signature:", data[:8])
else:
print("file does not exist")The Selenium API documents that save_screenshot() saves the current window as PNG, returns False for an IOError, recommends a full path, and expects a .png extension. The implementation warns when the name does not end in .png. Check both the boolean and the resulting file: True is not a content-validation result. Selenium Python WebDriver API
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Follow the evidence to the failing layer
| What you observe | What it points to | What to check next |
|---|---|---|
| Returned bytes are empty or lack the PNG signature | The issue is before your explicit file write: the screenshot response or its decoding. | Log the exception; inspect the active WebDriver session, selected window, browser and driver logs, and version details. |
| Returned bytes have a PNG signature, but the target file is missing or empty | The issue is likely in the path or file-writing stage, or a later operation. | Print the resolved path; check directory existence and write permissions; look for code or another process that truncates or replaces the file. |
| The screenshot call raises a WebDriver exception | The command did not complete successfully; this is distinct from a successful write of an empty payload. | Keep the full traceback and inspect browser/driver logs for session or renderer problems. |
| The file has PNG bytes but the page looks incomplete | Capture timing or page state may be the issue rather than an empty file. | Wait for the specific content you need and confirm the intended window is selected. |
To confirm whether a later operation changes the file, compare its size immediately after the capture and again after the rest of the test runs.
Confirm Selenium is capturing the intended window
The WebDriver Take Screenshot command captures the visual viewport of the current top-level browsing context. A closed top-level context is an error. Confirm the intended tab or window is selected and still open. If your code has switched into an iframe but intends to capture the page window, switch back to the default content before taking the screenshot. W3C WebDriver specification: Screen capture
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Wait for the content you need to render
driver.get() waits for the page’s onload event, but asynchronous application work can continue afterward. The screenshot command’s animation-frame behavior does not guarantee that a site’s own data fetching, transitions, or client-side rendering has finished. Wait for an application-specific condition—such as the target element becoming visible or a loading indicator disappearing—instead of treating navigation completion as proof that the page is ready. Selenium waits documentation W3C WebDriver specification: Screen capture
A fixed sleep may occasionally mask a timing issue, but it does not establish that the required content is ready. An explicit wait tied to the content is more diagnostic and resilient.
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Handle browser and renderer failures separately
If the screenshot call raises WebDriverException, capture the complete exception and record the Selenium, browser, and driver versions, operating system or container, whether execution is local or through Grid, and the headless configuration. Check browser and driver logs for a renderer disconnection or session loss before changing screenshot code.
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Practical troubleshooting checklist
- Print the byte count and first eight bytes from
get_screenshot_as_png(). - Write to a resolved absolute path with a
.pngextension, then print the save result, file size, and file signature. - If returned bytes are valid but the file changes later, find code or processes that overwrite or truncate it.
- If the capture command fails, keep the traceback and relevant browser, driver, Selenium, and environment details.
- Verify that the target top-level window is open and selected, then wait for the page content you actually need.
Frequently Asked Questions
Does save_screenshot() returning True mean the PNG is valid?
No. It indicates the write did not raise the handled file error; check the file’s size and PNG signature separately.
What details should I include when asking for help with an empty screenshot?
Include the byte count and signature, save return value, resolved output path and file size, full exception if any, and Selenium, browser, driver, OS/container, and execution-mode details.
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