"""Stdlib assertion helpers for the beam lane's bare-style test files.

Why this file exists (Q128, 2026-10-30): four of this lane's test files
imported `pytest` only for `pytest.raises` and one `pytest.fail`.  On an
interpreter without pytest those four modules fail at import, so the lane's
own gate could not grade them at all -- `beam/CHECK` exited 2 with
"pytest not importable -- cannot grade" and ran zero of the lane's cells.
The surface actually used is two names, so it lives here on the standard
library instead of behind a third-party package.

`raises` is a context manager that fails unless the expected exception is
raised, matching `pytest.raises(X)` for the way these files use it (no
`match=`, no `as`).  `fail` raises AssertionError with the message, matching
`pytest.fail`.  Both work under `unittest`, under `pytest`, and as bare
`assert`-style modules run by `run_lane_tests.py`.
"""

import contextlib


@contextlib.contextmanager
def raises(exc_type):
    """Fail unless the block raises ``exc_type`` (or a subclass)."""
    try:
        yield
    except exc_type:
        return
    raise AssertionError(
        "expected %s to be raised; nothing was raised" % (exc_type.__name__,)
    )


def fail(msg=""):
    """Fail the current test with ``msg``."""
    raise AssertionError(msg)