Breaking Change in httpx 0.28.1: Handling of Empty params in URL

[to-bee]

Issue Description

After upgrading httpx from version 0.27.2 to 0.28.1, tests using the FastAPI test client are failing due to changes in how the URL object handles params during construction.

The issue was traced to the following code:

Version 0.27.2:

self.url = URL(url)
if params is not None:
    self.url = self.url.copy_merge_params(params=params)

Version 0.28.1:

self.url = URL(url) if params is None else URL(url, params=params)

In httpx 0.28.1, passing params=None or params=[] to the URL constructor causes query parameters to be removed from the URL.

[YuriiMotov]

Here is a MRE:

from starlette.applications import Starlette
from starlette.responses import JSONResponse
from starlette.routing import Route
from starlette.testclient import TestClient


async def get_index(request):
    query_params = request.query_params
    a = query_params.get("a")
    b = query_params.get("b")
    b = b.lower() == "true" if b is not None else None
    return JSONResponse({"a": a, "b": b})


app = Starlette(routes=[Route("/index", get_index)])


def test_():
    client = TestClient(app)
    resp = client.get("/index?a=123", params={})
    assert resp.status_code == 200
    resp_json = resp.json()
    assert resp_json["a"] == "123"
    assert resp_json["b"] is None

The test above fails after you upgrade to httpx 0.28.1

[Kludex]

Duplicated of #3433