fix(python): honor explicit Cohere and VoyageAI reranker API keys (#4546)

`CohereReranker` and `VoyageAIReranker` read the environment variable
before the `api_key` argument. When `COHERE_API_KEY` or `VOYAGE_API_KEY`
is set, the reranker ignores an explicit `api_key` and uses the account
from the environment.

The docs say the opposite. `docs/web/integrations/reranking/cohere.mdx`
and `voyageai.mdx` use the environment variable only when `api_key` is
not provided. `JinaReranker` and `WatsonxReranker` already put the
explicit key first, and #4515 makes the same change for
`OpenaiReranker`.

## Fix

`CohereReranker._client` and `VoyageAIReranker._client` now use
`self.api_key` first and the environment variable as the fallback.

## Tests

`python/python/tests/test_reranker_credentials.py` replaces the SDK
module with a stub whose `Client` records the API key. It covers both
keys set, each key alone, and no key. With the stub, the test runs
without `cohere` or `voyageai` installed.

- On `main`, the 2 cases with both keys set fail and the other 6 pass.
With the fix, all 8 pass.
- `test_rerankers.py` with the new file: 24 passed, 13 skipped (the
skipped tests need real API keys).
- `ruff format --check` and `ruff check` (0.9.9) pass on the changed
files.
- I did not build the native extension. I ran the tests against the
lancedb 0.40.0 wheel with the changed files copied over it. In that
wheel, both reranker files are byte-identical to `main` before this
change.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

python/python/lancedb/rerankers/cohere.py

@@ -62,7 +62,7 @@ class CohereReranker(Reranker):
                 "COHERE_API_KEY not set. Either set it in your environment or \
                 pass it as `api_key` argument to the CohereReranker."
             )
-        return cohere.Client(os.environ.get("COHERE_API_KEY") or self.api_key)
+        return cohere.Client(self.api_key or os.environ.get("COHERE_API_KEY"))
 
     def _rerank(self, result_set: pa.Table, query: str):
         result_set = self._handle_empty_results(result_set)

python/python/lancedb/rerankers/voyageai.py

@@ -64,7 +64,7 @@ class VoyageAIReranker(Reranker):
                 pass it as `api_key` argument to the VoyageAIReranker."
             )
         return voyageai.Client(
-            api_key=os.environ.get("VOYAGE_API_KEY") or self.api_key,
+            api_key=self.api_key or os.environ.get("VOYAGE_API_KEY"),
         )
 
     def _rerank(self, result_set: pa.Table, query: str):

python/python/tests/test_reranker_credentials.py

@@ -0,0 +1,74 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import sys
+import types
+
+import pytest
+
+from lancedb.rerankers import CohereReranker, VoyageAIReranker
+
+RERANKERS = [
+    pytest.param("cohere", "COHERE_API_KEY", CohereReranker, {}, id="cohere"),
+    pytest.param(
+        "voyageai",
+        "VOYAGE_API_KEY",
+        VoyageAIReranker,
+        {"model_name": "rerank-2.5"},
+        id="voyageai",
+    ),
+]
+
+
+def fake_sdk(monkeypatch, module_name):
+    """Replace the SDK module with one whose Client records the API key."""
+    keys = []
+
+    def client(api_key=None):
+        keys.append(api_key)
+        return object()
+
+    module = types.ModuleType(module_name)
+    module.Client = client
+    monkeypatch.setitem(sys.modules, module_name, module)
+    return keys
+
+
+@pytest.mark.parametrize("module_name, env_var, reranker_cls, kwargs", RERANKERS)
+@pytest.mark.parametrize(
+    "explicit_key, environment_key, expected_key",
+    [
+        ("explicit-test-key", "environment-test-key", "explicit-test-key"),
+        ("explicit-test-key", None, "explicit-test-key"),
+        (None, "environment-test-key", "environment-test-key"),
+    ],
+)
+def test_reranker_api_key_precedence(
+    monkeypatch,
+    module_name,
+    env_var,
+    reranker_cls,
+    kwargs,
+    explicit_key,
+    environment_key,
+    expected_key,
+):
+    keys = fake_sdk(monkeypatch, module_name)
+    if environment_key is None:
+        monkeypatch.delenv(env_var, raising=False)
+    else:
+        monkeypatch.setenv(env_var, environment_key)
+
+    reranker_cls(api_key=explicit_key, **kwargs)._client
+    assert keys == [expected_key]
+
+
+@pytest.mark.parametrize("module_name, env_var, reranker_cls, kwargs", RERANKERS)
+def test_reranker_requires_api_key(
+    monkeypatch, module_name, env_var, reranker_cls, kwargs
+):
+    keys = fake_sdk(monkeypatch, module_name)
+    monkeypatch.delenv(env_var, raising=False)
+    with pytest.raises(ValueError, match=f"{env_var} not set"):
+        reranker_cls(**kwargs)._client
+    assert keys == []