SeleniumHQ/selenium · error · ValueError

Behavior can only be one of the following: dismiss, accept,

Error message

Behavior can only be one of the following: dismiss, accept, dismiss and notify, accept and notify, ignore

What it means

The unhandled_prompt_behavior property validates against the W3C-defined alert-handling behaviors: 'dismiss', 'accept', 'dismiss and notify', 'accept and notify', and 'ignore'. Any other value raises ValueError. Note the values are lowercase, space-separated, using 'and' (not '&').

Source

Thrown at py/selenium/webdriver/common/options.py:112

    Args:
        behavior: behavior to use when an alert is encountered

    Returns:
        Values for implicit timeout, pageLoad timeout and script timeout if set (in milliseconds)
    """

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

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

    def __set__(self, obj, value):
        if value in ("dismiss", "accept", "dismiss and notify", "accept and notify", "ignore"):
            obj.set_capability(self.name, value)
        else:
            raise ValueError(
                "Behavior can only be one of the following: dismiss, accept, dismiss and notify, "
                "accept and notify, ignore"
            )


class _TimeoutsDescriptor:
    """How long the driver should wait for actions to complete before returning an error.

    See:
      - https://w3c.github.io/webdriver/#timeouts

    Args:
        timeouts: values in milliseconds for implicit wait, page load and script timeout

    Returns:
        Values for implicit timeout, pageLoad timeout and script timeout if set (in milliseconds)
    """

View on GitHub (pinned to aa36b38e69)

Solutions

Use one of the exact lowercase strings with spaces: 'dismiss', 'accept', 'dismiss and notify', 'accept and notify', 'ignore'.
Replace underscores with spaces and use 'and' not '&'.

Example fix

# before
options.unhandled_prompt_behavior = 'dismiss_and_notify'  # raises

# after
options.unhandled_prompt_behavior = 'dismiss and notify'

Defensive patterns

Strategy: validation

Validate before calling

ALLOWED = {'dismiss','accept','dismiss and notify','accept and notify','ignore'}
if behavior not in ALLOWED:
    raise ValueError(f'Invalid unhandled_prompt_behavior {behavior!r}')
options.unhandled_prompt_behavior = behavior

Type guard

ALLOWED = {'dismiss','accept','dismiss and notify','accept and notify','ignore'}
def is_valid_prompt_behavior(v: str) -> bool:
    return v in ALLOWED

Try / catch

try:
    options.unhandled_prompt_behavior = behavior
except ValueError:
    options.unhandled_prompt_behavior = 'dismiss and notify'

Prevention

Remember values use spaces and 'and', not underscores or '&'.
Validate against the constant set before assignment.

When it happens

Trigger: Setting options.unhandled_prompt_behavior to a value not in the allowed set, e.g. 'dismiss_and_notify', 'Accept', 'accept-and-notify', 'accept & notify', or 'default'.

Common situations: Using underscores instead of spaces ('dismiss_and_notify'). Using ampersands ('accept & notify'). Wrong casing. Confusing with a different driver's prompt-behavior vocabulary.

Related errors

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/6126bb0b5157ce37. Report an issue: GitHub.