What causes "Cannot add non-list value {value_to_add} to a list." in huggingface/smolagents?

Use .append(item) for single elements

huggingface/smolagents · error · InterpreterError

Cannot add non-list value {value_to_add} to a list.

Error message

Cannot add non-list value {value_to_add} to a list.

What it means

For `list += value` inside the sandbox, the executor requires the right-hand side to also be a list; adding anything else raises InterpreterError. This deliberately diverges from CPython (where list += iterable does in-place extend) to keep semantics predictable.

Source

Thrown at src/smolagents/local_python_executor.py:668

            key = evaluate_ast(target.slice, state, static_tools, custom_tools, authorized_imports)
            return obj[key]
        elif isinstance(target, ast.Attribute):
            obj = evaluate_ast(target.value, state, static_tools, custom_tools, authorized_imports)
            return getattr(obj, target.attr)
        elif isinstance(target, ast.Tuple):
            return tuple(get_current_value(elt) for elt in target.elts)
        elif isinstance(target, ast.List):
            return [get_current_value(elt) for elt in target.elts]
        else:
            raise InterpreterError("AugAssign not supported for {type(target)} targets.")

    current_value = get_current_value(expression.target)
    value_to_add = evaluate_ast(expression.value, state, static_tools, custom_tools, authorized_imports)

    if isinstance(expression.op, ast.Add):
        if isinstance(current_value, list):
            if not isinstance(value_to_add, list):
                raise InterpreterError(f"Cannot add non-list value {value_to_add} to a list.")
            current_value += value_to_add
        else:
            current_value += value_to_add
    elif isinstance(expression.op, ast.Sub):
        current_value -= value_to_add
    elif isinstance(expression.op, ast.Mult):
        current_value *= value_to_add
    elif isinstance(expression.op, ast.Div):
        current_value /= value_to_add
    elif isinstance(expression.op, ast.Mod):
        current_value %= value_to_add
    elif isinstance(expression.op, ast.Pow):
        current_value **= value_to_add
    elif isinstance(expression.op, ast.FloorDiv):
        current_value //= value_to_add
    elif isinstance(expression.op, ast.BitAnd):
        current_value &= value_to_add
    elif isinstance(expression.op, ast.BitOr):

View on GitHub (pinned to 30bb116109)

Solutions

Use .append(item) for single elements
Use .extend(iterable) or list += list(iterable) for tuples/generators/strings-as-list
Wrap scalars: items += [item]

Example fix

# before
code = "items = [1, 2]\nitems += 3"

# after
code = "items = [1, 2]\nitems.append(3)"

Defensive patterns

Strategy: type-guard

Validate before calling

def safe_list_add(lst, other):
    if isinstance(other, list):
        return lst + other
    if hasattr(other, '__iter__'):
        return lst + list(other)
    return lst + [other]  # append semantics

Type guard

def is_list_compatible_extender(v) -> bool:
    return isinstance(v, list)

Try / catch

from smolagents.local_python_executor import InterpreterError
try:
    evaluate_python(code)
except InterpreterError as e:
    if 'Cannot add non-list value' in str(e):
        code = code.replace('items +=', 'items.extend([')  # or rewrite to .append

Prevention

Use .append(x) for single elements, .extend(iter) for collections
Never rely on CPython's list += str/generator behavior inside the sandbox
Lint generated code for `+=` on lists with non-list right-hand sides

When it happens

Trigger: Executed code does `my_list += x` (ast.AugAssign with Add) where my_list is a list and x is a non-list: e.g. `items += 'abc'`, `items += 1`, `items += (1, 2)`, `items += generator()`.

Common situations: LLM uses += to append a single element (`lst += item`); string concatenation habits carried to lists; extending with a tuple or generator which CPython would accept.

Related errors

AI-assisted analysis of huggingface/smolagents@30bb116109 (2026-08-28). Data as JSON: /api/errors/6b5f60f5e66a9bbe. Report an issue: GitHub.