What causes "The size of fields must be greater than 0" in microsoft/qlib?

Provide at least one expression: fields=['$close'] or the (exprs, names) tuple form.

microsoft/qlib · error · ValueError

The size of fields must be greater than 0

Error message

The size of fields must be greater than 0

What it means

The data loaders (e.g. QlibDataLoader and friends) parse a `fields` config of the form [expr...] or ([expr...], [names...]). An empty fields list has no expressions to load, so `_parse_fields_info` raises ValueError immediately at loader construction.

Source

Thrown at qlib/data/dataset/loader.py:97

                    "group_name1": <fields_info1>
                    "group_name2": <fields_info2>
                }
                or
                <config> := <fields_info>

                <fields_info> := ["expr", ...] | (["expr", ...], ["col_name", ...])
                # NOTE: list or tuple will be treated as the things when parsing
        """
        self.is_group = isinstance(config, dict)

        if self.is_group:
            self.fields = {grp: self._parse_fields_info(fields_info) for grp, fields_info in config.items()}
        else:
            self.fields = self._parse_fields_info(config)

    def _parse_fields_info(self, fields_info: Union[list, tuple]) -> Tuple[list, list]:
        if len(fields_info) == 0:
            raise ValueError("The size of fields must be greater than 0")

        if not isinstance(fields_info, (list, tuple)):
            raise TypeError("Unsupported type")

        if isinstance(fields_info[0], str):
            exprs = names = fields_info
        elif isinstance(fields_info[0], (list, tuple)):
            exprs, names = fields_info
        else:
            raise NotImplementedError(f"This type of input is not supported")
        return exprs, names

    @abc.abstractmethod
    def load_group_df(
        self,
        instruments,
        exprs: list,
        names: list,

View on GitHub (pinned to 79633dd950)

Solutions

Provide at least one expression: fields=['$close'] or the (exprs, names) tuple form.
Guard feature-generation code: raise your own descriptive error when selection returns [].

Example fix

# before
loader_kwargs = {'fields': [], 'names': []}

# after
loader_kwargs = {'fields': ['$close', '$volume'], 'names': ['close', 'volume']}

Defensive patterns

Strategy: validation

Validate before calling

def valid_fields_config(fields_info) -> bool:
    return isinstance(fields_info, (list, tuple)) and len(fields_info) > 0

Type guard

def is_nonempty_fields(cfg) -> bool:
    return bool(cfg) if isinstance(cfg, (list, tuple)) else bool(cfg and all(bool(v) for v in cfg.values()))

Prevention

Assert non-empty fields when configs are generated dynamically.
Treat an empty feature-selection result as a pipeline error, not a pass-through.

When it happens

Trigger: Passing `data_loader={'class': 'QlibDataLoader', 'kwargs': {'fields': [], ...}}` to DatasetH, or a fields list that a generator/feature-selection step returned empty.

Common situations: Automated feature pruning / selection pipelines that can return zero surviving features; YAML template configs where the fields block was left empty as a placeholder.

Related errors

AI-assisted analysis of microsoft/qlib@79633dd950 (2026-08-15). Data as JSON: /api/errors/500abacd6c561504. Report an issue: GitHub.