SeleniumHQ/selenium · error · ValueError

should be of type

Error message

{self.name} should be of type {self.expected_type.__name__}

What it means

Raised as ValueError by the _IeOptionsDescriptor.__set__ descriptor when the assigned value does not match the expected_type declared for that option. Each IE option (browser_attach_timeout expects int, ensure_clean_session expects bool, etc.) is backed by a descriptor that validates type on assignment, preventing type mismatches from reaching the IE driver where they would cause confusing protocol errors.

Solutions

1. Convert the value to the expected type before assignment (int(value), bool(value), str(value))
2. Check the descriptor declaration in ie/options.py to see the expected_type for each option
3. Use native Python literals of the correct type directly

Example fix

// before
options.browser_attach_timeout = '30'

// after
options.browser_attach_timeout = 30

value = '30'
options.browser_attach_timeout = int(value)  # convert to expected type before assignment

def matches_expected_type(value, expected_type) -> bool:
    return isinstance(value, expected_type)

try:
    options.browser_attach_timeout = value
except ValueError:
    options.browser_attach_timeout = int(value)

When it happens

Trigger: Assigning options.browser_attach_timeout = '30' (str instead of int), options.ensure_clean_session = 1 (int instead of bool), or options.initial_browser_url = 123 (int instead of str) triggers the ValueError.

Common situations: Config values deserialized from JSON/YAML that arrive as strings. Integer 0/1 used for boolean options. Env var values that are always strings.

Related errors

• Element Scroll Behavior out of range.
• Binary Location Must be a String
• Only Proxy objects can be passed in.
• must be of type
• service_args must be a sequence

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/e3a57ddcbd4570ac. Report an issue: GitHub.

Appendix: source

Thrown at py/selenium/webdriver/ie/options.py:77

    When an attribute assignment happens:

    Example:
        `self.browser_attach_timeout` = 30
        `__set__` method sets/updates the value of the key `browserAttachTimeout` in `_options`
        dictionary in `Options` class.
    """

    def __init__(self, name, expected_type):
        self.name = name
        self.expected_type = expected_type

    def __get__(self, obj, cls):
        return obj._options.get(self.name)

    def __set__(self, obj, value) -> None:
        if not isinstance(value, self.expected_type):
            raise ValueError(f"{self.name} should be of type {self.expected_type.__name__}")

        if self.name == "elementScrollBehavior" and value not in [
            ElementScrollBehavior.TOP,
            ElementScrollBehavior.BOTTOM,
        ]:
            raise ValueError("Element Scroll Behavior out of range.")
        obj._options[self.name] = value


class Options(ArgOptions):
    KEY = "se:ieOptions"
    SWITCHES = "ie.browserCommandLineSwitches"

    BROWSER_ATTACH_TIMEOUT = "browserAttachTimeout"
    ELEMENT_SCROLL_BEHAVIOR = "elementScrollBehavior"
    ENSURE_CLEAN_SESSION = "ie.ensureCleanSession"
    FILE_UPLOAD_DIALOG_TIMEOUT = "ie.fileUploadDialogTimeout"
    FORCE_CREATE_PROCESS_API = "ie.forceCreateProcessApi"

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