Lazily initialize OpenAI client so local routers import without an API key

[zl190]

Problem

routellm/routers/similarity_weighted/utils.py instantiated the OpenAI client at module import time:

OPENAI_CLIENT = OpenAI()

Constructing OpenAI() requires OPENAI_API_KEY. Because this module is imported transitively by routers.py and matrix_factorization/model.py, importing any router forced a client to be built — even fully-local routers (bert, causal_llm) that never call OpenAI. So from routellm.controller import Controller crashed with openai.OpenAIError: Missing credentials for users who only wanted a local router.

This is the symptom behind issues like #21 and #38.

Reproduce (before this PR)

In a fresh env with no OPENAI_API_KEY:

pip install routellm
python -c "from routellm.controller import Controller"
# openai.OpenAIError: Missing credentials. Please pass an `api_key` ...

Fix

Make the client lazy — instantiate on first use, not at import. Replace the module-level OPENAI_CLIENT = OpenAI() with a cached accessor:

@functools.lru_cache(maxsize=1)
def get_openai_client():
    return OpenAI()

and update the two call sites (SWRankingRouter in routers.py, MFModel in matrix_factorization/model.py) to call get_openai_client(). Behavior is identical when a key is present — the sw_ranking / mf embedding path still gets a (now cached) client. No routing-logic or signature changes.

Scope note: openai_server.py (AsyncOpenAI()) and evals/find_contaminated.py (OpenAI()) also instantiate at import, but neither is on the router/Controller import path — they're a standalone server entrypoint and an eval script — so they don't affect importing routers. Left untouched to keep this diff minimal.

Result

• bert & causal_llm now import and construct keyless.
• With a key present, the embedding path still gets a client (verified without making real API calls).

# no OPENAI_API_KEY set
python -c "from routellm.controller import Controller; Controller(routers=['bert'], strong_model='x', weak_model='y', progress_bar=False)"
# succeeds

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