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If requests-html returns an empty JavaScript-driven section—or render() raises an error—first check whether you are using the right session for your execution context. In a plain Python script, fetch the page with HTMLSession and call response.html.render(). If an asyncio event loop is already running, switch to AsyncHTMLSession and await response.html.arender(). Rendering also depends on pyppeteer being able to download and launch Chromium.
Why the page has no JavaScript-created content
A normal HTTP request retrieves the server’s initial response; it does not run the page’s JavaScript. requests-html provides a browser-rendering path using Chromium through pyppeteer. Its documented render() method reloads the response in Chromium, executes JavaScript, and replaces the parsed HTML with the updated version. See the requests-html overview and API documentation.
Before changing selectors, inspect what the first request actually returned. If the expected text or elements are absent from the initial HTML but appear in a browser, rendering may be needed. If the elements are already in the HTML, the issue may instead be the selector, page structure, or response you fetched.
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Use the synchronous HTMLSession flow in a plain script
For a conventional script that is not running inside an asyncio event loop, use HTMLSession and call render() before querying rendered content:
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-
Create an
HTMLSession. -
Fetch the page with
session.get(url). -
Call
response.html.render()to let Chromium load and execute the page. -
Read the updated HTML or select the content you need.
from requests_html import HTMLSession
url = "https://example.com"
session = HTMLSession()
response = session.get(url)
response.html.render()
print(response.html.html)
print(response.html.find("title", first=True))This example demonstrates the documented flow; replace the URL and selectors with those appropriate to your page. The first call to render may trigger pyppeteer to download Chromium, so it can take longer than later calls in an environment where that download has completed.
Fix the existing-event-loop error with AsyncHTMLSession
If the traceback says Cannot use HTMLSession within an existing event loop. Use AsyncHTMLSession instead., the problem is the synchronous session being used where an asyncio loop is already active. This commonly matters in async applications and notebooks. Use the async session, await the request, and await arender():
from requests_html import AsyncHTMLSession
async def main():
url = "https://example.com"
session = AsyncHTMLSession()
response = await session.get(url)
await response.html.arender()
print(response.html.html)
# In an environment where no event loop is already running:
if __name__ == "__main__":
import asyncio
asyncio.run(main())In a notebook or framework that already runs the event loop, call await main() from its async context rather than starting a second loop with asyncio.run(). The package’s PyPI page shows the async-session and arender() pattern. Keep the surrounding application’s event-loop lifecycle in charge; do not mix synchronous HTMLSession rendering into a running loop.
| Execution context | Session and render call | What to watch |
|---|---|---|
| Plain synchronous script | HTMLSession(), then response.html.render() |
Use this only when there is no already-running event loop. |
| Async function, notebook, or async framework | AsyncHTMLSession(), await get(), then await response.html.arender() |
If the loop is already managed by the environment, do not start another one. |
Handle delayed content and page interaction
Some pages populate content after initial JavaScript execution, or only after scrolling or a runtime action. The API documents sleep, scrolldown, and script options for render(). These address page timing or interaction; they do not fix an event-loop mismatch or missing Chromium dependencies.
Wait for a page that updates after load
If the page needs additional time after rendering starts, try a short delay and inspect whether the content appears:
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Scroll to trigger lazy-loaded content
For pages that load additional material as the viewport moves, use the documented scrolling option:
response.html.render(scrolldown=2, sleep=1)Scrolling can prompt lazy-loaded sections to appear, but it is not a substitute for verifying the actual page behavior. Inspect the resulting HTML and adjust the interaction to the page rather than assuming a fixed number of scrolls will expose everything.
Run a page-specific JavaScript action
The script option accepts JavaScript to run in the rendered page. Use it only when the page requires a specific runtime action you understand, and check the updated HTML afterward:
response.html.render(script="document.querySelector('button')?.click()")A script can interact with the page, but it cannot compensate for Chromium failing to start or for rendering in the wrong event-loop context.
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Check Chromium installation and environment compatibility
On a first render, pyppeteer downloads Chromium into its home directory. An incomplete or blocked download can prevent the browser from starting. The documentation also warns that Linux environments may need additional system packages. Check whether the download completed and whether the runtime can launch the downloaded browser; consult the requirements for your particular operating system and deployment environment rather than applying one platform’s package list everywhere. See the package installation notes and project documentation.
Compatibility deserves special caution. The PyPI page says Python 3.6 is supported, while the stable documentation identifies version 0.3.4. These old project statements do not establish that the package works with every current Python release, Chromium build, or operating system. Verify compatibility in the environment where you intend to run it before depending on it in production.
Troubleshoot by the symptom, not by guesswork
| Symptom | Likely area to investigate | Next step |
|---|---|---|
| Expected content is missing, but no render error appears | The fetched HTML may be only the initial response, or the content may load later. | Inspect response.html.html before rendering, then render before selecting. If necessary, test an appropriate delay, scroll, or page script. |
Cannot use HTMLSession within an existing event loop |
A synchronous session is being used with an already-running asyncio loop. | Use AsyncHTMLSession; await both the request and response.html.arender(). |
| First render cannot start Chromium | The pyppeteer browser download may be incomplete, blocked, or unable to launch in the environment. | Check that the download finished and review the OS-specific browser requirements, including Linux package requirements where relevant. |
| Chromium closes unexpectedly or a protocol connection disappears | Possible causes include browser installation, platform libraries, runtime compatibility, or the target page. | Read the full traceback and isolate the environment and page involved. Historical issue reports show such failures but do not establish one universal cause or fix; see project issue reports. |
| Rendering completes, but a selector still finds nothing | The selector may not match the updated DOM, the site may render the data elsewhere, or additional interaction may be required. | Print the post-render HTML and verify the element’s actual tag, attributes, and timing before changing the selector. |
Reliability and maintenance considerations
Rendering adds a browser process and its installation requirements to what would otherwise be an HTTP-fetching workflow. Account for first-run browser download, runtime dependencies, and the added time required to launch a page and wait for its JavaScript. The cited project materials do not establish a current compatibility guarantee, universal performance figures, or a single repair for browser-launch failures. For a maintained application, test representative target pages in the exact Python and operating-system environment you deploy, record full tracebacks, and make the sync-versus-async choice based on how that application manages its event loop.
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Frequently asked questions
Does render() change the response HTML?
Yes. The documented behavior is to reload the response in Chromium, execute JavaScript, and replace the HTML content with an updated version.
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Will increasing sleep fix a Chromium startup error?
No. A delay is for page timing; it does not repair an incomplete browser download, missing system dependency, or event-loop mismatch.
Can I assume requests-html works on my current Python version?
No. The package page’s stated Python 3.6 support and the stable documentation’s v0.3.4 reference are old. Test the exact versions and operating system you plan to use.
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