pydantic/monty · error · TypeError

ClassInstance attr name must be a string

Error message

ClassInstance attr name must be a string

What it means

Validation guard in the wasm-worker `pushClassInstance` converter: fires when a `ClassInstance` marker's `attrs` entry contains a first element (the attribute name) that is not a string. Attribute names must be Python identifier strings, matching the napi transport's encoding, so the host passed a malformed marker and gets a TypeError before serialization.

Source

Thrown at crates/monty-js/ts/worker/value.ts:190

 * path produces: `attrs` as ordered `[name, value]` pairs, uuids as
 * strings). Validation messages mirror napi's so both transports fail
 * malformed markers alike.
 */
function pushClassInstance(object: Record<string, unknown>, nodes: ValueNode[]): ValueNode {
  if (typeof object.type !== 'object' || object.type === null) {
    throw new TypeError(
      `Object property 'type' type mismatch. Expect value to be Object, but received ${jsType(object.type)}`,
    )
  }
  if (!Array.isArray(object.attrs)) {
    throw new TypeError(
      `Object property 'attrs' type mismatch. Expect value to be Array, but received ${jsType(object.attrs)}`,
    )
  }
  const pairs: [unknown, unknown][] = []
  for (const pair of object.attrs as unknown[]) {
    if (!Array.isArray(pair)) throw new TypeError('ClassInstance attrs entries must be [name, value] pairs')
    if (typeof pair[0] !== 'string') throw new TypeError('ClassInstance attr name must be a string')
    if (!(1 in pair)) throw new TypeError('ClassInstance attr value missing')
    pairs.push([pair[0], pair[1]])
  }
  const classTypeNode = pushClassType(object.type as Record<string, unknown>, nodes)
  const classTypeIndex = nodes.length
  nodes.push({ tag: 'class-type', val: classTypeNode })
  return {
    tag: 'class-instance',
    val: {
      classType: classTypeIndex,
      instanceId: uuidString(object.instanceId, 'ClassInstance instanceId'),
      attrs: pushPairs(pairs, nodes),
    },
  }
}

/** Builds a class-type node from the plain `classType` marker object,
 *  appending its eager attr nodes to the arena. */

View on GitHub (pinned to adc986b362)

Solutions

1. Convert the name element with String(name) before building the pair
2. Validate typeof pair[0] === 'string' for every pair before passing the marker
3. Keep attribute names as strings end to end

Example fix

// before
attrs: [[42, 'answer']]
// after
attrs: [['answer', 42]]

const namesAreStrings = (attrs: unknown[]): boolean => attrs.every(a => Array.isArray(a) && typeof a[0] === 'string')

const hasStringName = (p: unknown): p is [string, unknown] => Array.isArray(p) && typeof p[0] === 'string'

try {
  return pushClassInstance(object, nodes)
} catch (e) {
  if (e instanceof TypeError && e.message === 'ClassInstance attr name must be a string') {
    object.attrs = (object.attrs as unknown[]).map(([n, v]) => [String(n), v]) // coerce then retry
    return pushClassInstance(object, nodes)
  }
  throw e
}

When it happens

Trigger: attrs pairs like [1, 'one'] using numeric keys; attribute names derived from symbols or numbers without conversion; pairs built as [key, value] where key came from a numeric source.

Common situations: Converting a Map with non-string keys to attrs pairs; indexing artifacts where attribute positions were used instead of names.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• ClassInstance marker instanceId must be a uuid string
• Object property 'type' type mismatch. Expect value to be Obj
• Object property 'attrs' type mismatch. Expect value to be Ar
• ClassInstance attrs entries must be [name, value] pairs
• ClassInstance attr value missing

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/b02632e33b587fe8. Report an issue: GitHub.