Handling HTTP/3 Requests with Scrapling's Fetcher Class: A Complete Guide

Scrapling enables HTTP/3 requests through the optional http3 parameter in the Fetcher and AsyncFetcher classes, which internally configures curl-cffi to use CurlHttpVersion.V3ONLY.

Scrapling is a Python web scraping library built on top of curl-cffi, providing high-level abstractions through its Fetcher and AsyncFetcher classes. When handling HTTP/3 requests with Scrapling's Fetcher class, developers can leverage the optional http3 flag to force HTTP/3-only connections, though this requires understanding the internal configuration flow from session initialization through to the actual curl execution.

How HTTP/3 Support Works in Scrapling

Scrapling's HTTP client architecture delegates protocol handling to curl-cffi, with HTTP/3 support controlled through a cascading configuration system.

The http3 Configuration Flag

The http3 parameter is defined as an optional boolean throughout the codebase. In scrapling/engines/_browsers/_types.py (line 32), the RequestsSession TypedDict declares http3: Optional[bool], making the flag part of the type-checked API. This parameter propagates from high-level fetcher classes down to the static engine that interfaces with curl-cffi.

Internal Implementation Details

The actual HTTP/3 activation occurs in scrapling/engines/static.py through three key phases:

1. Session Initialization (lines 71-84): The __init__ method stores self._default_http3 from the http3 kwarg, establishing the session-wide default.

2. Argument Merging (lines 104-108): The _merge_request_args method reads a request-level http3 flag, falling back to the session default if not provided.

3. Curl Version Switch (lines 156-162): When http3 is truthy, the code injects http_version = CurlHttpVersion.V3ONLY into the arguments passed to curl-cffi. This section also emits a warning if impersonate is active, as some sites reject HTTP/3 when custom browser headers are generated.

Enabling HTTP/3 in Your Code

Scrapling provides multiple patterns for activating HTTP/3 depending on your architecture needs.

Session-Level Configuration

For applications requiring HTTP/3 across multiple requests, configure the session once using FetcherSession:

from scrapling.fetchers.requests import FetcherSession

# All requests inside this context will use HTTP/3

with FetcherSession(http3=True) as sess:
    resp = sess.get("https://http3.tech")
    print(resp.status_code, resp.http_version)   # → 200, "HTTP/3"

This approach stores the http3=True default in scrapling/engines/static.py and applies it to every request made through that session instance.

Per-Request Overrides

When you need HTTP/3 only for specific calls while maintaining standard HTTP/1.1 or HTTP/2 for others:

from scrapling.fetchers.requests import FetcherSession

with FetcherSession() as sess:                     # default: HTTP/1.1/2

    # Normal request – no HTTP/3

    r1 = sess.get("https://example.com")
    print(r1.http_version)                       # e.g. "HTTP/2"

    # Force HTTP/3 just for this call

    r2 = sess.get("https://http3.tech", http3=True)
    print(r2.http_version)                       # "HTTP/3"

The _merge_request_args method in scrapling/engines/static.py handles this fallback logic, checking for a request-level http3 parameter before defaulting to the session configuration.

Using the Fetcher Facade

For quick scripts or single requests, use the low-level Fetcher class directly:

from scrapling.fetchers.requests import Fetcher

# Fetcher is a thin wrapper around a global SyncSession instance

resp = Fetcher.get("https://http3.tech", http3=True)
print(resp.http_version)      # "HTTP/3"

This approach bypasses explicit session management while still supporting the http3 parameter through the same underlying mechanism in scrapling/engines/static.py.

Important Considerations and Warnings

HTTP/3 and Browser Impersonation Conflicts

When combining http3=True with the impersonate parameter (which generates browser-like headers), Scrapling emits a warning from scrapling/engines/static.py (lines 156-162). Some servers reject HTTP/3 connections when they detect mismatched or custom browser headers, potentially causing connection errors.

import logging
logging.basicConfig(level=logging.WARNING)

from scrapling.fetchers.requests import FetcherSession

with FetcherSession(http3=True, impersonate="chrome") as sess:
    sess.get("https://http3.tech")

# → WARNING: The argument `http3` might cause errors if used with `impersonate` argument...

Recommendation: Disable impersonation (impersonate=False) when you need reliable HTTP/3 connectivity, or test thoroughly against your target site to verify compatibility.

Summary

• Scrapling supports HTTP/3 through the optional http3 boolean parameter in Fetcher, AsyncFetcher, and FetcherSession classes.
• The flag propagates through scrapling/engines/static.py, where _merge_request_args injects CurlHttpVersion.V3ONLY when HTTP/3 is enabled.
• Configure HTTP/3 at the session level for consistency across requests, or override per-request for selective usage.
• Avoid combining http3=True with impersonate unless you have verified server compatibility, as this triggers warnings and potential connection failures.

Frequently Asked Questions

Does Scrapling support HTTP/3 by default?

No, HTTP/3 is disabled by default in Scrapling. You must explicitly set http3=True either when creating a FetcherSession or on individual request methods. The default behavior uses HTTP/1.1 or HTTP/2 depending on server support and curl-cffi's negotiation.

Can I use HTTP/3 with browser impersonation?

While technically possible, combining http3=True with the impersonate parameter is not recommended. According to the implementation in scrapling/engines/static.py, this combination triggers a warning because some servers reject HTTP/3 connections when they detect custom browser headers. For reliable HTTP/3 requests, disable impersonation by setting impersonate=False.

How do I check if a response used HTTP/3?

The response object returned by Scrapling's fetcher methods includes an http_version attribute. After making a request with http3=True, inspect response.http_version to verify the protocol. A successful HTTP/3 connection will return the string "HTTP/3" or similar, depending on how curl-cffi reports the version.

Is HTTP/3 available in AsyncFetcher?

Yes, the AsyncFetcher class supports the same http3 parameter as the synchronous Fetcher. The underlying FetcherSession and AsyncFetcherSession classes in scrapling/fetchers/requests.py both expose the http3 argument, which propagates to the same _merge_request_args logic in scrapling/engines/static.py for protocol negotiation.

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