What causes "Unsupported comparison operator: {op}" in huggingface/smolagents?

If building AST manually, use only standard ast comparison nodes (ast.Eq, ast.Lt, ast.In, etc.)

huggingface/smolagents · error · InterpreterError

Unsupported comparison operator: {op}

Error message

Unsupported comparison operator: {op}

What it means

A comparison chain used a comparator node the interpreter does not handle. It supports Is, IsNot, In, NotIn, Eq, NotEq, Lt/LtE/Gt/GtE — anything else in an ast.Compare triggers this error.

Source

Thrown at src/smolagents/local_python_executor.py:995

            current_result = left != right
        elif op == ast.Lt:
            current_result = left < right
        elif op == ast.LtE:
            current_result = left <= right
        elif op == ast.Gt:
            current_result = left > right
        elif op == ast.GtE:
            current_result = left >= right
        elif op == ast.Is:
            current_result = left is right
        elif op == ast.IsNot:
            current_result = left is not right
        elif op == ast.In:
            current_result = left in right
        elif op == ast.NotIn:
            current_result = left not in right
        else:
            raise InterpreterError(f"Unsupported comparison operator: {op}")

        if current_result is False:
            return False
        result = current_result if i == 0 else (result and current_result)
        left = right
    return result


def evaluate_if(
    if_statement: ast.If,
    state: dict[str, Any],
    static_tools: dict[str, Callable],
    custom_tools: dict[str, Callable],
    authorized_imports: list[str],
) -> Any:
    result = None
    test_result = evaluate_ast(if_statement.test, state, static_tools, custom_tools, authorized_imports)
    if test_result:

View on GitHub (pinned to 30bb116109)

Solutions

If building AST manually, use only standard ast comparison nodes (ast.Eq, ast.Lt, ast.In, etc.)
Re-run with code parsed via ast.parse rather than hand-built nodes
Update smolagents if you're on an old version that misses newly supported operators

Defensive patterns

Strategy: validation

Validate before calling

# only build AST via ast.parse on real source, never hand-constructed Compare nodes
import ast
tree = ast.parse(source_text)

Try / catch

try:
    evaluate_ast(tree, state, static_tools, custom_tools, authorized_imports)
except InterpreterError as e:
    if 'Unsupported comparison operator' in str(e):
        # rebuild tree from source with ast.parse and retry

Prevention

Never construct comparison nodes manually
Keep smolagents updated alongside your Python version

When it happens

Trigger: Effectively unreachable with standard Python source (the ast module only produces the supported ops), but can occur with manually constructed or patched AST nodes, or after a library version changes AST node types.

Common situations: AST being built programmatically and passed to evaluate_ast; version mismatch between Python's ast node classes and the interpreter's if-elif chain; custom AST transformations injecting exotic operators.

Related errors

AI-assisted analysis of huggingface/smolagents@30bb116109 (2026-08-28). Data as JSON: /api/errors/c393ce30b76f2b20. Report an issue: GitHub.