`test_round_trip_floating_point` vector checks never assert

[nikagra]

Problem

test_round_trip_floating_point in TypeTestsVector never asserts, so the float, double and decimal vector round trips pass even when the values read back are wrong.

The test passes partial(pytest.approx, abs=1e-5) as the comparison callback:

python-driver/tests/integration/standard/test_types.py

Lines 1053 to 1062 in fe0f0c9

	def test_round_trip_floating_point(self):
	_almost_equal_test_fn = partial(pytest.approx, abs=1e-5)
	def _random_decimal():
	return Decimal(random.uniform(0.0, 100.0))
	# Max value here isn't really connected to max value for floating point nums in IEEE 754... it's used here
	# mainly as a convenient benchmark
	self._round_trip_test("float", partial(random.uniform, 0.0, 100.0), _almost_equal_test_fn)
	self._round_trip_test("double", partial(random.uniform, 0.0, 100.0), _almost_equal_test_fn)
	self._round_trip_test("decimal", _random_decimal, _almost_equal_test_fn)

_round_trip_test calls that callback as test_fn(observed, expected) and discards its return value:

python-driver/tests/integration/standard/test_types.py

Lines 1037 to 1044 in fe0f0c9

	if use_positional_parameters:
	observed1 = self._get_row_prepared(2, table_name)
	for idx in range(0, 3):
	test_fn(observed1[idx], expected1[idx])
	observed2 = self._get_row_simple(5, table_name)
	for idx in range(0, 3):
	test_fn(observed2[idx], expected2[idx])

The call becomes pytest.approx(observed, expected, abs=1e-5): expected is taken as rel, and the returned matcher is never compared with anything.

#1043 fixed the same pattern for timestamp only, because pytest 9 started rejecting relative tolerance for datetimes:

python-driver/tests/integration/standard/test_types.py

Lines 1072 to 1074 in fe0f0c9

	def _assert_almost_equal(observed, expected):
	# Datetimes support an absolute timedelta, not relative tolerance.
	assert observed == pytest.approx(expected, abs=timedelta(seconds=1), rel=None)

Impact

A regression in float, double or decimal serialization inside a vector passes CI. This is the only integration test that round-trips these vector subtypes through a server.

Reproduction

With pytest 9.1.1:

from functools import partial
from decimal import Decimal
import pytest
f = partial(pytest.approx, abs=1e-5)
f(1.0, 50.0)                      # ApproxScalar, no AssertionError
f(Decimal("1"), Decimal("50"))    # ApproxDecimal, no AssertionError

Expected behavior

The floating-point vector round trips fail when a value read back differs from the one written by more than the 1e-5 absolute tolerance. That tolerance holds for vector<float, 3>: the worst float32 rounding error for values in [0, 100) was about 3.8e-6 over 200,000 samples.

Acceptance criteria

• Replacing an expected value with a wrong one makes each of the float, double and decimal round trips fail.

Notes

• The other _round_trip_test callers pass assertEqual or, since build(deps): remove vulnerable Python 3.9 resolutions #1043, an asserting helper; only the floating-point partial is affected.

Related

• build(deps): remove vulnerable Python 3.9 resolutions #1043 made the timestamp round trip a real assertion.