sgl-project/sglang · error · ValueError

must provide either a lambda or static kwargs

Error message

must provide either a lambda or static kwargs

What it means

The dumper.ctx decorator requires exactly one context source: a lambda extractor or static keyword arguments. If neither is supplied, the wrapper would have no context to register, so the decorator fails fast at decoration time rather than silently doing nothing.

Source

Thrown at python/sglang/srt/debug_utils/dumper.py:377

    def ctx(
        self,
        _extractor: Optional[Callable[..., dict]] = None,
        **static_ctx: Any,
    ) -> Callable:
        """Decorator that sets context before calling the wrapped function and clears it after.

        Two forms:
            @dumper.ctx(lambda self: dict(layer_id=self.layer_id))
            def forward(self, x): ...

            @dumper.ctx(phase="decode")
            def decode_step(self, x): ...
        """
        if _extractor is not None and static_ctx:
            raise ValueError("cannot mix lambda extractor with static kwargs")
        if _extractor is None and not static_ctx:
            raise ValueError("must provide either a lambda or static kwargs")

        def decorator(fn: Callable) -> Callable:
            @functools.wraps(fn)
            def wrapper(*args: Any, **kwargs: Any) -> Any:
                ctx_dict: dict = _extractor(args[0]) if _extractor else static_ctx
                self.set_ctx(**ctx_dict)
                try:
                    return fn(*args, **kwargs)
                finally:
                    self.set_ctx(**{k: None for k in ctx_dict})

            return wrapper

        return decorator

    def apply_source_patches(self) -> None:
        """Apply source patches from DUMPER_SOURCE_PATCHER_CONFIG if set.

View on GitHub (pinned to 0132848349)

Solutions

  1. Pass a lambda extractor: @dumper.ctx(lambda self: {...})
  2. Or pass at least one static kwarg: @dumper.ctx(phase='decode')

Example fix

# before
@dumper.ctx
def step(self, x): ...
# after
@dumper.ctx(phase="decode")
def step(self, x): ...
Defensive patterns

Strategy: validation

Validate before calling

assert extractor is not None or bool(static_kwargs), "ctx needs a lambda or static kwargs"

Prevention

When it happens

Trigger: @dumper.ctx() with no arguments decorating a method; calling dumper.ctx with no positional callable and no kwargs.

Common situations: Decorating a method intending to use a default context, or forgetting to pass the extractor lambda during a refactor.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/c76a4334c0b2e504. Report an issue: GitHub.