Add ArgumentParser to FlexibleArgumentParser conversion

General
1

I noticed an update in vllm==0.9 about integrating the deprecated option in FlexibleArgumentParser. See PR#17426 for related changes.
This change caused an error in my code like this

    parser = argparse.ArgumentParser()
    parser.add_argument("--host", type=str, default="0.0.0.0")
    parser.add_argument("--port", type=int, default=5000)
    parser.add_argument("--ssl-keyfile", type=str, default=None)
    parser.add_argument("--ssl-certfile", type=str, default=None)
    parser.add_argument(
        "--root-path",
        type=str,
        default=None,
        help="FastAPI root_path when app is behind a path based routing proxy")
    parser = AsyncEngineArgs.add_cli_args(parser)
    args = parser.parse_args()

    engine_args = AsyncEngineArgs.from_cli_args(args)
    engine = AsyncLLMEngine.from_engine_args(engine_args)

and it reported some error look like this:

File "/home/aiscuser/.conda/envs/ruler/lib/python3.10/argparse.py", line 1430, in add_argument action = action_class(**kwargs) TypeError: _StoreAction.__init__() got an unexpected keyword argument 'deprecated'

The cause of this error is a FlexibleArgumentParser should be passed into AsyncEngineArgs.add_cli_args(parser), not a ArgumentParser. I have been using this function wrong for a long time. (Of course modern day editors and typecheckers should be able to spot this, but I didn’t pay much attention to this since the original code is copied from NVIDIA/RULER, a popular repo with more than 1.2k star, and it’s been working normal for pass vllm versions until v0.9).

What changed in vllm==0.9 is that in order to support the deprecated option in python version lower than 3.13, the FlexibleArgumentParser overrided the parse_known_args function. Since the ArgumentParser does not have a overrided function that deals with the deprecated option, it caused an error like above.

But this error message does not obvious tell me what went wrong is actually the parser.
This got me thinking if we should let the AsyncEngineArgs.add_cli_args raise a warning if the input parser is not a FlexibleArgumentParser rather a normal parser, and convert the input parser to FlexibleArgumentParser.

I believe this is very helpful for the user experience and debugging if the other people has similar mistakes. If this is worth looking into, I’m very glad to contribute.

2

You are correct: starting with vLLM v0.9, the CLI argument deprecation feature requires that you use vllm.utils.FlexibleArgumentParser (not argparse.ArgumentParser) when calling AsyncEngineArgs.add_cli_args. This is because FlexibleArgumentParser supports the deprecated argument and related logic, as introduced in PR#17426. Passing a standard ArgumentParser will cause errors like the one you saw, since it cannot handle the deprecated kwarg or the overridden methods required for vLLM’s CLI extensions (source, source).

Your suggestion to add a warning or auto-convert to FlexibleArgumentParser in add_cli_args is reasonable and would improve user experience, as the current error is not very clear. This is not currently implemented, but your contribution would be welcome (source). Would you like more detail on how to implement this or on the relevant code paths?

Sources: