pytest-dev/pytest · error · RuntimeError

Cannot enter twice

Error message

Cannot enter {self!r} twice

What it means

Raised by WarningsRecorder.__enter__ when the context manager is entered a second time without an intervening __exit__. The recorder tracks an _entered flag (reset to False on exit) to mirror correct context-manager lifecycle; double-entry would corrupt the recorded list and the warnings filter state. The RuntimeError protects against misuse such as manual re-entry.

Solutions

1. Always pair __enter__ with __exit__; use the with statement so Python manages lifecycle.
2. Create a fresh WarningsRecorder per use rather than reusing one.
3. If reusing programmatically, assert `not recorder._entered` before calling __enter__.

Example fix

// before
recorder = WarningsRecorder(_ispytest=True)
recorder.__enter__()
recorder.__enter__()  # RuntimeError
// after
with WarningsRecorder(_ispytest=True) as recorder:
    ...
// for reuse across tests, create a new instance each time

assert not recorder._entered, f'cannot re-enter {recorder!r}; create a fresh recorder'

def recorder_enterable(recorder) -> bool:
    return not getattr(recorder, '_entered', False)

When it happens

Trigger: Calling recorder.__enter__() twice; a with-block that re-enters the same recorder via __enter__ inside the body; using a recorder in a custom harness that calls __enter__ explicitly then `with recorder:` again.

Common situations: Wrapping WarningsRecorder in a custom context manager that forwards __enter__ twice; fixtures that cache and re-enter a recorder across tests; misuse by internal tooling.

Related errors

• Cannot exit without entering first
• not found in warning list
• exceptions must be derived from Warning, not
• object (type: ) must be callable
• Unexpected keyword arguments passed to pytest.warns

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/ea0c21a54548e9d0. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/recwarn.py:239

                best_idx is None
                or not issubclass(w.category, self._list[best_idx].category)
            ):
                best_idx = i
        if best_idx is not None:
            return self._list.pop(best_idx)
        __tracebackhide__ = True
        raise AssertionError(f"{cls!r} not found in warning list")

    def clear(self) -> None:
        """Clear the list of recorded warnings."""
        self._list[:] = []

    # Type ignored because we basically want the `catch_warnings` generic type
    # parameter to be ourselves but that is not possible(?).
    def __enter__(self) -> Self:  # type: ignore[override]
        if self._entered:
            __tracebackhide__ = True
            raise RuntimeError(f"Cannot enter {self!r} twice")
        _list = super().__enter__()
        # record=True means it's None.
        assert _list is not None
        self._list = _list
        warnings.simplefilter("always")
        return self

    def __exit__(
        self,
        exc_type: type[BaseException] | None,
        exc_val: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> None:
        if not self._entered:
            __tracebackhide__ = True
            raise RuntimeError(f"Cannot exit {self!r} without entering first")

        super().__exit__(exc_type, exc_val, exc_tb)

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