tests/test_exception_custom_html.py

import pytest


class CustomHtmlThatRaises:
    def __html__(self):
        raise ValueError(123)


def test_exception_custom_html(escape):
    """Checks whether exceptions in custom __html__ implementations are
    propagated correctly.

    There was a bug in the native implementation at some point:
    https://github.com/pallets/markupsafe/issues/108
    """
    obj = CustomHtmlThatRaises()
    with pytest.raises(ValueError):
        escape(obj)