rohitg00/ai-engineering-from-scratch · error · ValueError

schema enum for {location} must be a non-empty list

Error message

schema enum for {location} must be a non-empty list

What it means

_validate_schema_value raises ValueError(f'schema enum for {location} must be a non-empty list') when a (sub)schema contains an "enum" keyword whose value is not a list or is an empty list. Like the other schema-shape errors this reports a malformed schema, not bad data: an enum constraint with no choices is meaningless, so the validator refuses it at schema-check time.

Source

Thrown at certifications/claude/lessons/10-tool-use-and-agentic-loops/code/main.py:124


def _validate_schema_value(value: Any, schema: Any, location: str) -> None:
    if not isinstance(schema, dict):
        raise ValueError(f"schema for {location} must be an object")

    declared_type = schema.get("type")
    if declared_type is not None:
        declared_types = declared_type if isinstance(declared_type, list) else [declared_type]
        if not declared_types or not all(isinstance(item, str) for item in declared_types):
            raise ValueError(f"schema type for {location} must be a string or non-empty string list")
        if not any(_matches_json_type(value, item) for item in declared_types):
            expected = " or ".join(declared_types)
            raise ValueError(f"invalid type for {location}: expected {expected}")

    if "enum" in schema:
        choices = schema["enum"]
        if not isinstance(choices, list) or not choices:
            raise ValueError(f"schema enum for {location} must be a non-empty list")
        if value not in choices:
            raise ValueError(f"invalid value for {location}: not in enum")

    if isinstance(value, (int, float)) and not isinstance(value, bool):
        for keyword, comparison, message in (
            ("minimum", lambda current, bound: current >= bound, "below minimum"),
            ("maximum", lambda current, bound: current <= bound, "above maximum"),
            ("exclusiveMinimum", lambda current, bound: current > bound, "at or below exclusive minimum"),
            ("exclusiveMaximum", lambda current, bound: current < bound, "at or above exclusive maximum"),
        ):
            if keyword not in schema:
                continue
            bound = schema[keyword]
            if not isinstance(bound, (int, float)) or isinstance(bound, bool):
                raise ValueError(f"schema {keyword} for {location} must be numeric")
            if not comparison(value, bound):
                raise ValueError(f"invalid value for {location}: {message} {bound}")

View on GitHub (pinned to 39ea8a1c6d)

Solutions

  1. Write the enum as a non-empty JSON array of allowed values: {"enum": ["low", "medium", "high"]}.
  2. When generating enums in code, fail loudly or fall back to a default if the source list is empty.
  3. In YAML configs, always use list syntax even for one item: enum: ["yes"].
  4. Read {location} in the message to locate the offending subschema.

Example fix

# before
{"type": "string", "enum": "low|medium|high"}
# ValueError: schema enum for $.priority must be a non-empty list

# after
{"type": "string", "enum": ["low", "medium", "high"]}
Defensive patterns

Strategy: validation

Validate before calling

def enum_keyword_ok(schema: dict) -> bool:
    return "enum" not in schema or (isinstance(schema["enum"], list) and len(schema["enum"]) > 0)

Type guard

def is_valid_enum(choices) -> bool:
    return isinstance(choices, list) and len(choices) > 0

Try / catch

try:
    validate_tool_input(value, schema)
except ValueError as exc:
    if "schema enum" in str(exc):
        raise SchemaError(f"fix the schema: {exc}") from exc  # schema bug, not data bug
    raise

Prevention

When it happens

Trigger: Schemas like {"type": "string", "enum": "abc"} (string instead of list), {"enum": []} (empty), or an enum built from a config/API source list that came back empty. Hit at any properties/items location once the type check passes.

Common situations: Generating enums dynamically from a listing that returns nothing; typos like enum: "low|medium|high"; YAML configs where a single-item enum collapses to a scalar.

Related errors

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/7305faa1586c5fa0. Report an issue: GitHub.