huggingface/transformers · error · TypeError

`skip` must contain only strings.

Error message

`skip` must contain only strings.

What it means

Second-stage validation of the heterogeneity 'skip' entry: after confirming skip is a non-string Iterable, each element must be a string. Mixed lists like ["mlp", 2] or [None] fail the all(isinstance(item, str)) check and raise TypeError with the message "`skip` must contain only strings.".

Source

Thrown at src/transformers/integrations/heterogeneity/configuration_utils.py:56

@dataclass
class _HeterogeneitySpec:
    per_layer_overrides: dict[int, dict[str, Any]]
    per_layer_attributes: set[str]
    explicit_per_layer_attributes: set[str]


def _normalize_layer_overrides(layer_overrides: dict[str, Any]) -> dict[str, Any]:
    normalized = copy.deepcopy(layer_overrides)

    if "skip" in normalized:
        skip = normalized.pop("skip")
        if isinstance(skip, str) or not isinstance(skip, Iterable):
            raise TypeError("`skip` must be an iterable of strings.")

        skip = set(skip)
        if not all(isinstance(item, str) for item in skip):
            raise TypeError("`skip` must contain only strings.")

        if skip:
            normalized["skip"] = sorted(skip)

    return normalized


def _validate_layer_indices(config: PreTrainedConfig, per_layer_overrides: dict[int, dict[str, Any]]) -> None:
    if not per_layer_overrides:
        return

    num_hidden_layers = config.num_hidden_layers
    invalid_layer_indices = [
        layer_idx for layer_idx in per_layer_overrides if layer_idx < 0 or layer_idx >= num_hidden_layers
    ]
    if invalid_layer_indices:
        raise ValueError(
            f"`per_layer_config` keys must be integer layer indices in the range [0, {num_hidden_layers}); "

View on GitHub (pinned to a597f97485)

Solutions

1. Make every skip entry a string: skip=["mlp", "attn"]
2. Coerce programmatically built lists: [str(s) for s in skip]

Example fix

# before
config.per_layer_config = {0: {"skip": ["mlp", 2]}}

# after
config.per_layer_config = {0: {"skip": ["mlp", "2"]}}  # all strings

assert all(isinstance(x, str) for x in skip_list), "every skip entry must be a string"

def skip_entries_all_strings(skip) -> bool:
    return all(isinstance(item, str) for item in skip)

When it happens

Trigger: per_layer_config with skip=["mlp", 3] or skip=[None, "attn"] — any iterable containing at least one non-string element.

Common situations: Programmatically building skip from config values that include ints/None; YAML/JSON coercion turning intended strings into numbers; copy-pasting a skip list schema from elsewhere.

Related errors

• `skip` must be an iterable of strings.
• `per_layer_config` keys must be integer layer indices in the
• The following attributes are missing: {sorted(missing_requir
• The following layers have the mutually exclusive `sliding_wi
• Layer type '{layer_idx}' requested, but config.layer_types i

AI-assisted analysis of huggingface/transformers@a597f97485 (2026-08-14). Data as JSON: /api/errors/fd2f80957225c758. Report an issue: GitHub.