Debugging vLLM with debugpy on WSL fails: AttributeError: module 'ray' has no attribute 'is_initialized'

Question

I’m trying to debug my Python code with debugpy in VS Code on WSL (Ubuntu).

Environment:

Python: 3.12.3 (venv)
vLLM: 0.10.1.1+cpu
debugpy: ms-python.debugpy-2025.10.0 (from VS Code extension)
Model: Tiny LLM downloaded from Hugging Face
Running inside WSL2 (Ubuntu)

Code (run-model.py):

from vllm import LLM, SamplingParams

def main():
    llm = LLM(model="./Tiny-LLM")
    params = SamplingParams(temperature=0.7, max_tokens=200)
    prompt = "Once upon a time in a magical forest,"

    for output in llm.generate([prompt], sampling_params=params):
        print(output.outputs[0].text)

if __name__ == "__main__":
    main()

When I run this script normally in the terminal, it works and generates text.

But when I run it with debugpy in VS Code, I get:

AttributeError: module 'ray' has no attribute 'is_initialized'

Traceback (excerpt):

  File ".../vllm/engine/arg_utils.py", line 1127, in create_engine_config
if is_ray_initialized():
  File ".../vllm/ray/lazy_utils.py", line 9, in is_ray_initialized
return ray.is_initialized()
AttributeError: module 'ray' has no attribute 'is_initialized'

launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Debug run-model.py with vllm",
            "type": "debugpy",
            "request": "launch",
            "program": "${workspaceFolder}/run-model.py",
            "console": "integratedTerminal",
            "cwd": "${workspaceFolder}",
            "justMyCode": false,
            "env": {
                "VLLM_TARGET_DEVICE": "cpu",
                "PYTHONPATH": "${workspaceFolder}/../vllm"
            }
        }
    ]
}

What I’ve tried so far:

Verified that the model loads and runs outside of debugpy (works fine).
Installed and reinstalled different Ray versions (2.9.3, 2.49.1, etc.) — same error.
Checked that ray.is_initialized() exists when I import ray directly in Python (works outside vLLM).
Tried running debugpy both in integratedTerminal and externalTerminal.
Explicitly set environment variables (VLLM_TARGET_DEVICE=cpu, PYTHONPATH=...) in launch.json.

Questions:

Why does vLLM call ray.is_initialized() only when running under debugpy?
Is this a Ray version mismatch, or something about vLLM’s CPU mode with debugpy?
What’s the correct way to set up debugpy with vLLM on CPU in WSL?

jei · Accepted Answer · 2025-09-10 12:07:55Z

1

The traceback shows the failure at vllm/ray/lazy_utils.py on line 9: return ray.is_initialized().

A simple workaround would be to wrap the problematic line in a try...except block to handle the AttributeError. Something like:

Python

# File: .../vllm/ray/lazy_utils.py

import ray

def is_ray_initialized():
    try:
        return ray.is_initialized()
    except AttributeError:
        return False

answered Sep 10 at 12:07

jei

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