SeleniumHQ/selenium · error · TypeError

service_args must be a sequence

Error message

service_args must be a sequence

What it means

Raised as TypeError by the FirefoxService.service_args setter when the value is a bare str or not a Sequence type. The check explicitly rejects str because a string is technically a Sequence of characters, which would be incorrectly unpacked into individual character arguments. Only list or tuple (or other non-str Sequence) of argument strings is valid.

Source

Thrown at py/selenium/webdriver/firefox/service.py:95

        )

        # Set a port for CDP
        if "--connect-existing" not in self._service_args:
            self._service_args.append("--websocket-port")
            self._service_args.append(f"{utils.free_port()}")

    def command_line_args(self) -> list[str]:
        return ["--port", f"{self.port}"] + self._service_args

    @property
    def service_args(self) -> Sequence[str]:
        """Returns the sequence of service arguments."""
        return self._service_args

    @service_args.setter
    def service_args(self, value: Sequence[str]):
        if isinstance(value, str) or not isinstance(value, Sequence):
            raise TypeError("service_args must be a sequence")
        self._service_args = list(value)

View on GitHub (pinned to aa36b38e69)

Solutions

1. Wrap arguments in a list: service_args=['--verbose']
2. Use a tuple if preferred: service_args=('--verbose',)
3. Split a space-separated string with shlex.split() before passing

Example fix

// before
service = Service(service_args='--verbose --marionette-port 2828')

// after
service = Service(service_args=['--verbose', '--marionette-port', '2828'])

args = '--verbose'
if isinstance(args, str):
    import shlex
    args = shlex.split(args)
service = Service(service_args=args)

from collections.abc import Sequence
def is_arg_sequence(v) -> bool:
    return not isinstance(v, str) and isinstance(v, Sequence)

try:
    service.service_args = args
except TypeError:
    service.service_args = [args] if isinstance(args, str) else list(args)

When it happens

Trigger: Passing service_args='--verbose' (a single string instead of a list), service_args=42 (an int), or service_args=None triggers the TypeError. The value must be a list/tuple of strings.

Common situations: Developers who pass a single CLI argument as a string rather than wrapping it in a list. Passing a space-separated argument string expecting it to be split.

Related errors

• service_args must be a sequence
• service_args must be a sequence
• Port needs to be an integer
• Please pass in a Boolean to this call
• Binary Location Must be a String

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/803990bbc78f30b1. Report an issue: GitHub.