pytest-dev/pytest · error · AssertionError

{fail_reason}

Error message

{fail_reason}

What it means

The AssertionError surfaced when an exception WAS raised and its type matched, but it failed the match regex or the check predicate. The message is the dynamically-built fail_reason explaining exactly why the match failed (regex mismatch, check returned False, etc.).

Source

Thrown at src/_pytest/raises.py:711

    ) -> bool:
        __tracebackhide__ = True
        if exc_type is None:
            if not self.expected_exceptions:
                fail("DID NOT RAISE any exception")
            if len(self.expected_exceptions) == 1:
                fail(f"DID NOT RAISE {self.expected_exceptions[0].__name__}")
            else:
                names = ", ".join(x.__name__ for x in self.expected_exceptions)
                fail(f"DID NOT RAISE any of ({names})")

        assert self.excinfo is not None, (
            "Internal error - should have been constructed in __enter__"
        )

        if not self.matches(exc_val):
            if self._just_propagate:
                return False
            raise AssertionError(self._fail_reason) from None

        # Cast to narrow the exception type now that it's verified....
        # even though the TypeGuard in self.matches should be narrowing
        exc_info = cast(
            "tuple[type[BaseExcT_co_default], BaseExcT_co_default, types.TracebackType]",
            (exc_type, exc_val, exc_tb),
        )
        self.excinfo.fill_unfilled(exc_info)
        return True


@final
class RaisesGroup(AbstractRaises[BaseExceptionGroup[BaseExcT_co]]):
    """
    .. versionadded:: 8.4

    Contextmanager for checking for an expected :exc:`ExceptionGroup`.
    This works similar to :func:`pytest.raises`, but allows for specifying the structure of an :exc:`ExceptionGroup`.

View on GitHub (pinned to 98b357f69e)

Solutions

1. Read the fail_reason — it shows the expected regex and the actual message; align the regex to reality.
2. Use re.escape(literal_text) when matching literal strings that contain regex metacharacters.
3. Debug the check predicate in isolation to confirm it returns True for the actual exception.
4. Loosen or correct the match pattern to reflect the current exception message format.

Example fix

// before
with pytest.raises(ValueError, match='^foo$'):
    raise ValueError('bar')
// after
with pytest.raises(ValueError, match='bar'):
    raise ValueError('bar')

try:
    with pytest.raises(ValueError, match=pattern):
        func()
except AssertionError as e:
    # inspect e.args[0] for the fail_reason, then fix pattern or check
    raise

When it happens

Trigger: with pytest.raises(ValueError, match='foo'): raise ValueError('bar') — type matches but the regex does not. Or a check=lambda e: e.code==5 that returns False. Hits raises.py:711.

Common situations: Regex is too strict or wrong; expected message wording changed in the code under test; the check predicate has a logic bug; special regex characters in the expected literal text are not escaped.

Related errors

• Invalid regex {e.pattern!r}: {e}
• Expected an exception type or a tuple of exception types, bu
• {func!r} object (type: {type(func)}) must be callable
• Only `ExceptionGroup[Exception]` or `BaseExceptionGroup[Base
• Expected {expected}, but got {exc.__name__!r}

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/951c302d74f09e9c.json. Report an issue: GitHub.