Use aiohttp to download the HTML, then hand the result to a renderer. For static, print-oriented markup, WeasyPrint is the smallest solution. For pages that depend on JavaScript, browser layout, or print behavior, use Playwright instead. aiohttp performs the asynchronous fetch; it does not itself convert HTML into a PDF.
What the pipeline does
The conversion has two separate stages:
- Fetch: an
aiohttp.ClientSessionrequests the source, checks the status, and decodes the response. - Render: WeasyPrint or a browser engine turns HTML, CSS, images, and fonts into a PDF.
Keeping those stages separate makes failures easier to diagnose. A 404 is a fetch problem; missing JavaScript content is a renderer choice; an absent logo may be a relative-URL or authentication problem.
Static HTML: aiohttp plus WeasyPrint
Install the Python packages and the native libraries required by WeasyPrint for your operating system. Then run this complete example:
import asyncio
from pathlib import Path
import aiohttp
from weasyprint import HTML
async def html_to_pdf(url: str, output_path: str) -> None:
timeout = aiohttp.ClientTimeout(total=30)
async with aiohttp.ClientSession(timeout=timeout) as session:
async with session.get(url, allow_redirects=True) as response:
response.raise_for_status()
content_type = response.headers.get("Content-Type", "")
if "text/html" not in content_type and "application/xhtml+xml" not in content_type:
raise ValueError(f"Expected HTML, got {content_type!r}")
html = await response.text()
final_url = str(response.url)
Path(output_path).parent.mkdir(parents=True, exist_ok=True)
HTML(string=html, base_url=final_url).write_pdf(output_path)
if __name__ == "__main__":
asyncio.run(html_to_pdf("https://example.com", "out.pdf"))response.raise_for_status() stops a failed HTTP request before rendering. base_url is essential: it lets relative links such as /styles/site.css and images/logo.png resolve against the page’s final URL after redirects.
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Why this works well for static pages
WeasyPrint accepts an HTML string and exposes HTML.write_pdf(). It handles HTML and print-oriented CSS without starting a full browser, so the program is straightforward and often has a lower startup cost. It does not execute page JavaScript. A script that inserts the article after load will therefore produce an incomplete document.
Large responses without loading everything
response.text(), response.read(), and response.json() load the complete response into memory. For a known small document that is convenient. For a large or user-selected URL, stream chunks and enforce a limit:
import aiohttp
async def download_html(session: aiohttp.ClientSession, url: str,
max_bytes: int = 10_000_000) -> tuple[str, str]:
async with session.get(url, allow_redirects=False) as response:
response.raise_for_status()
content_type = response.headers.get("Content-Type", "")
if "text/html" not in content_type and "application/xhtml+xml" not in content_type:
raise ValueError("The response is not HTML")
chunks: list[bytes] = []
total = 0
async for chunk in response.content.iter_chunked(64 * 1024):
total += len(chunk)
if total > max_bytes:
raise ValueError("HTML response exceeds the configured limit")
chunks.append(chunk)
raw = b"".join(chunks)
encoding = response.charset or "utf-8"
return raw.decode(encoding, errors="strict"), str(response.url)This still assembles the final string because WeasyPrint needs the document, but it prevents an unbounded download. For very large jobs, put a queue in front of the renderer and cap concurrent conversions.
When JavaScript requires Playwright
Choose Playwright when the page needs JavaScript execution, browser layout, client-side data fetching, or browser print behavior. The following example fetches and renders in one browser context:
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import asyncio
from playwright.async_api import async_playwright
async def page_to_pdf(url: str, output_path: str) -> None:
async with async_playwright() as pw:
browser = await pw.chromium.launch()
context = await browser.new_context()
page = await context.new_page()
try:
response = await page.goto(url, wait_until="networkidle", timeout=30_000)
if response is None or not response.ok:
status = response.status if response else "no response"
raise RuntimeError(f"Navigation failed: {status}")
await page.pdf(path=output_path, format="A4", print_background=True)
finally:
await context.close()
await browser.close()
asyncio.run(page_to_pdf("https://example.com", "out.pdf"))Playwright’s page.pdf() uses print CSS media by default. If the design is written for screens, call await page.emulate_media(media="screen") before generating the PDF. Replace networkidle with a specific selector wait when an application keeps long-lived connections open:
await page.goto(url, wait_until="domcontentloaded", timeout=30_000)
await page.wait_for_selector("main.article", state="visible", timeout=15_000)
await page.pdf(path="out.pdf", format="A4")WeasyPrint or Playwright?
| Requirement | WeasyPrint | Playwright |
|---|---|---|
| Already-rendered HTML and print CSS | Usually the simpler choice | Works, but starts a browser |
| JavaScript-generated content | Not executed | Supported |
| Exact browser layout and print behavior | Different rendering engine | Strongest match to Chromium |
| Startup and resource footprint | Generally lighter | Browser processes require more memory |
| Authenticated subresources | Requires a custom URL fetcher | Use browser context cookies, headers, or authentication |
These are capability-based trade-offs, not a performance benchmark. Pick the renderer that matches the page rather than converting every URL with a browser by default.
Authentication, assets, and URL handling
WeasyPrint resources
WeasyPrint’s default fetcher can retrieve HTTP and file resources. Cookies, authorization headers, and other advanced controls require a custom URL fetcher. If the source is private, downloading HTML with aiohttp does not automatically authenticate every CSS, image, or font request made later by WeasyPrint.
Playwright resources
Create a context with the required cookies, extra headers, user agent, timezone, or geolocation before navigation. Wait for the actual content selector rather than assuming that a network event means the page is visually complete.
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Encoding and redirects
Use the response’s declared charset when decoding. If server metadata is wrong, supply an explicit encoding appropriate to your input. Record the final URL after redirects and use it as base_url. For user-controlled URLs, consider disabling redirects or validating every redirect destination.
Security and reliability checklist
- Set both connect and total timeouts; a remote server can otherwise hold a worker indefinitely.
- Validate status and content type before invoking a renderer.
- Limit response bytes and PDF job duration.
- Restrict outbound requests, redirects, private-network access, and local-file access when URLs are supplied by users.
- Treat HTML, CSS, images, fonts, cookies, and redirects as untrusted. WeasyPrint warns that untrusted HTML or CSS can create security problems.
- Run rendering in an isolated worker with a process or container limit, especially for public conversion endpoints.
- Reuse one
ClientSessionfor batches instead of creating a session per URL. - Log the source URL, final URL, HTTP status, renderer, elapsed time, and failure class without logging secrets.
Common failures and fixes
“ClientResponseError” or an HTTP 4xx/5xx
The server rejected the request or returned an error page. Inspect the status, response headers, and (for diagnostics) a bounded portion of the body. Supply required authentication or a realistic user agent only when you are authorized to access the page.
The PDF is blank or missing the article
The content is probably inserted by JavaScript, hidden until interaction, or behind a consent screen. Switch to Playwright, wait for the content selector, and perform the required interaction before page.pdf().
Images, CSS, or fonts are missing
Check the document’s relative URLs and the renderer’s base URL. Confirm that resources are publicly reachable or pass their credentials. A successful HTML download does not prove that every subresource can be fetched.
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Playwright times out at networkidle
Analytics, websockets, and other persistent connections can prevent network idle. Use domcontentloaded followed by wait_for_selector or a carefully bounded delay.
WeasyPrint cannot handle a private asset
Implement a custom URL fetcher that adds the required cookies or authorization and restricts which hosts it may contact. Do not expose unrestricted file or network access to untrusted input.
Memory usage grows during batch jobs
Stream and cap downloads, limit concurrent renderers, close browser contexts, and recycle workers after a bounded number of jobs. Avoid calling response.text() on unbounded input.
Or skip the browser setup
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For a PDF, use the API documented at ScreenshotNeo’s documentation. The same endpoint supports waits, custom headers and cookies, user-agent, authorization, timezone, geolocation, CSS or JavaScript, blocking rules, page ranges, paper size, margins, and landscape output.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.pdfIn Python:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
timeout=90,
)
r.raise_for_status()
open("shot.pdf", "wb").write(r.content)In Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const fs = await import('node:fs/promises');
await fs.writeFile('shot.pdf', Buffer.from(await res.arrayBuffer()));ScreenshotNeo also provides an MCP server with take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients. The Free plan includes 1,000 shots each month without a card; paid plans start at $5 for 3,000 shots. Create a free ScreenshotNeo account.
Operational and cost decisions
- Single conversion: WeasyPrint is convenient for static HTML; Playwright is appropriate when the page is dynamic.
- Many conversions: reuse aiohttp sessions, queue rendering, cap concurrency, and cache stable inputs.
- Untrusted URLs: isolate the renderer and enforce network, size, redirect, and time limits.
- Hosted capture: ScreenshotNeo removes browser installation and offers asynchronous jobs, signed webhooks, bulk capture of up to 100 URLs per call, caching with a chosen TTL, and a usage API.
FAQ
Can aiohttp convert HTML to PDF by itself?
No. aiohttp is the asynchronous HTTP client/server layer; a PDF engine such as WeasyPrint or Playwright performs rendering.
Should I save the fetched HTML to disk first?
Not necessarily. Passing the string directly to WeasyPrint avoids a temporary file. Save it when you need reproducible debugging or an audit trail.
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WeasyPrint and Chromium implement different layout engines. Even with Playwright, PDF generation uses print media unless you explicitly emulate screen media.
Frequently Asked Questions
Can aiohttp convert HTML to PDF by itself?
No. aiohttp fetches the document; WeasyPrint or Playwright renders it.
Should I save fetched HTML before rendering?
No. You can pass the string directly, although saving a bounded copy can help debugging.
Why does the PDF differ from the browser view?
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