Merge params with existing URL query parameters instead of replacing

[veeceey]

Summary

• Fixes When the URL contains request parameters and the params parameter is set, the request parameters in the URL will disappear unexpectedly. #3621: When a URL already contains query parameters (e.g. https://example.com/path?page=1&s=list) and the params argument is also provided, the existing query parameters were silently dropped and replaced by only the new params. This was inconsistent with the requests library and surprised users.
• Changed Request.__init__ to use URL(url).copy_merge_params(params) instead of URL(url, params=params), so existing URL query parameters are preserved and merged with the new ones.
• Updated test_request_params to reflect the new merge behavior and added an additional test case for overlapping keys.

Before (broken)

request = httpx.Request("GET", "https://example.com?page=1&s=list", params={"pid": 0})
print(request.url)
# https://example.com?pid=0   <-- page and s are lost!

After (fixed)

request = httpx.Request("GET", "https://example.com?page=1&s=list", params={"pid": 0})
print(request.url)
# https://example.com?page=1&s=list&pid=0   <-- all params preserved

Test plan

• Existing test_request_params updated and passes
• New test case for overlapping parameter keys added
• All 173 model and client tests pass
• Verified the exact reproduction case from the issue works correctly

[naman-netomi]

Hi team,
We’re extensively using httpx in our production setup, and some recent critical changes require us to upgrade from v0.27.2 → v0.28.1. However, this specific issue (where existing query parameters in the URL are lost when passing params) is breaking our flow.

Given the criticality of this change, deploying a custom patch isn’t an option for us at the moment.
Could you please share an estimated timeline for when this fix will be released?
It would be great if this could be prioritized, as it directly impacts our production rollout.

Thanks in advance!

[veeceey]

Hi @naman-netomi, thanks for sharing your use case — I appreciate hearing that this fix would unblock your production upgrade. Unfortunately, release timelines are up to the httpx maintainers, so I can't speak to when this will land in a release. Hopefully the maintainers can weigh in when they get a chance.