pydantic/monty · error · TypeError

invalid mode: binary mode specified twice

Error message

invalid mode: binary mode specified twice

What it means

The mode string contained 'b' more than once (e.g. 'rbb' or 'wbb'), which is ambiguous/meaningless. canonicalFileMode validates each character and rejects duplicate binary flags with a TypeError, mirroring CPython's ValueError for invalid mode.

Source

Thrown at crates/monty-js/ts/types.ts:135

}

/** Canonicalizes the subset of Python file modes Monty supports. */
export function canonicalFileMode(mode: string): string {
  if (mode.length === 0) {
    throw new TypeError('Must have exactly one of create/read/write/append mode and at most one plus')
  }

  let action: string | undefined
  let binary = false
  let text = false
  for (const char of mode) {
    if (char === 'r' || char === 'w' || char === 'a') {
      if (action !== undefined) throw new TypeError('must have exactly one of create/read/write/append mode')
      action = char
    } else if (char === 'x') {
      throw new TypeError('exclusive creation mode is not supported')
    } else if (char === 'b') {
      if (binary) throw new TypeError('invalid mode: binary mode specified twice')
      binary = true
    } else if (char === 't') {
      if (text) throw new TypeError('invalid mode: text mode specified twice')
      text = true
    } else if (char === '+') {
      throw new TypeError("update modes ('+') are not yet supported")
    } else {
      throw new TypeError(`invalid mode: '${char}'`)
    }
  }
  if (binary && text) throw new TypeError("can't have text and binary mode at once")
  if (action === undefined) {
    throw new TypeError('Must have exactly one of create/read/write/append mode and at most one plus')
  }
  return `${action}${binary ? 'b' : ''}`
}

/** Validates that a file position can cross the JavaScript boundary exactly. */

View on GitHub (pinned to adc986b362)

Solutions

  1. Fix the mode construction so 'b' appears at most once, e.g. 'rb' not 'rbb'
  2. Deduplicate the mode characters before passing: `Array.from(new Set(mode)).join('')` (careful to preserve order)
  3. Log/inspect the mode string in the TypeError-adjacent call site to find the duplicate append

Example fix

// before
const mode = base + 'b' // base already 'rb'
// after
const mode = base.endsWith('b') ? base : base + 'b'
Defensive patterns

Strategy: validation

Validate before calling

if (typeof mode === 'string' && (mode.match(/b/g) || []).length > 1) throw new Error('mode has duplicate b')

Type guard

function hasSingleB(mode) { return typeof mode === 'string' && (mode.match(/b/g) || []).length <= 1 }

Try / catch

try { fh = new MontyFileHandle(path, mode) } catch (e) { if (e instanceof TypeError && e.message.includes("binary mode specified twice")) fh = new MontyFileHandle(path, Array.from(new Set(mode)).join('')) ; else throw e }

Prevention

When it happens

Trigger: Passing a mode like 'rbb', 'wb+', 'bwb' (any string where a second 'b' is scanned) to MontyFileHandle's constructor or pushFileHandle.

Common situations: Programmatic mode-string assembly where 'b' is appended twice (e.g. base mode + flags both adding 'b'); typo'd concatenated constants.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/6e79d03d56b269b9. Report an issue: GitHub.