pytest-dev/pytest · error · TypeError

expected string as 'msg' parameter, got ' '…

Error message

{} expected string as 'msg' parameter, got '{}' instead.
Perhaps you meant to use a mark?

What it means

Raised by OutcomeException.__init__ (parent of Skipped, Failed, XFailed) when msg is not None and not a str. The constructor at lines 18-23 enforces that msg be a string because outcome messages are rendered into reports and tracebacks; a non-string msg usually signals a misuse where a mark or object was passed instead. The error is raised eagerly as TypeError.

Solutions

1. Pass a string reason: pytest.skip('reason text').
2. If propagating an exception, convert it: pytest.skip(str(exc)).
3. Use pytest.fail or pytest.raises appropriately rather than stuffing objects into msg.

Example fix

// before
pytest.skip(some_exception)
// after
pytest.skip(str(some_exception))

if msg is not None and not isinstance(msg, str):
    msg = str(msg)

def _outcome_msg_ok(msg) -> bool:
    return msg is None or isinstance(msg, str)

When it happens

Trigger: pytest.skip(some_object); pytest.fail(an_exception); pytest.xfail(123); passing a Mark or a non-string payload to an outcome function.

Common situations: Developers call pytest.skip(exception) intending to re-raise an exception; passing a mark object; passing a dict of context as the reason.

Related errors

• use delattr(target, name) or delattr(target) with target…
• config is not a valid argument for from_parent
• config or parent must be provided
• object (type: ) must be callable
• module has __version__ , required is

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/9e8a88e10b26884f. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/outcomes.py:23

import importlib
import sys
from typing import Any
from typing import ClassVar
from typing import NoReturn


class OutcomeException(BaseException):
    """OutcomeException and its subclass instances indicate and contain info
    about test and collection outcomes."""

    def __init__(self, msg: str | None = None, pytrace: bool = True) -> None:
        if msg is not None and not isinstance(msg, str):
            error_msg = (  # type: ignore[unreachable]
                "{} expected string as 'msg' parameter, got '{}' instead.\n"
                "Perhaps you meant to use a mark?"
            )
            raise TypeError(error_msg.format(type(self).__name__, type(msg).__name__))
        super().__init__(msg)
        self.msg = msg
        self.pytrace = pytrace

    def __repr__(self) -> str:
        if self.msg is not None:
            return self.msg
        return f"<{self.__class__.__name__} instance>"

    __str__ = __repr__


TEST_OUTCOME = (OutcomeException, Exception)


class Skipped(OutcomeException):
    # XXX hackish: on 3k we fake to live in the builtins
    # in order to have Skipped exception printing shorter/nicer

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