pytest-dev/pytest · error · TypeError

pytest.approx() does not support nan_ok for…

Error message

pytest.approx() does not support nan_ok for datetime/timedelta comparisons.

What it means

ApproxTimedelta.__init__ rejects nan_ok=True for any datetime or timedelta comparison because NaN has no representation in datetime/timedelta arithmetic (you cannot have a 'NaN datetime'), so allowing NaN-equality would be meaningless. The check runs after the datetime+rel check and before the explicit-tolerance check.

Solutions

Drop nan_ok for datetime/timedelta comparisons: `approx(dt, abs=timedelta(seconds=1))`.
Filter NaN values out of the data before comparison instead of using nan_ok.
Branch helper logic: only pass nan_ok when the expected value is numeric.

Example fix

# before
approx(datetime.now(), abs=timedelta(seconds=1), nan_ok=True)
# after
approx(datetime.now(), abs=timedelta(seconds=1))

Defensive patterns

Strategy: validation

Validate before calling

from datetime import datetime, timedelta
def safe_nan_ok(expected, nan_ok) -> bool:
    if isinstance(expected, (datetime, timedelta)):
        return False  # nan_ok never valid for datetime/timedelta
    return nan_ok
# usage
assert actual == approx(expected, abs=tol, nan_ok=safe_nan_ok(expected, nan_ok))

Type guard

from datetime import datetime, timedelta
def supports_nan_ok(expected) -> bool:
    return not isinstance(expected, (datetime, timedelta))

Prevention

Drop nan_ok for any datetime/timedelta approx.
In generic helpers, only set nan_ok for numeric expected.
Filter NaN rows from data instead of relying on nan_ok for time types.

When it happens

Trigger: Calling approx(datetime.now(), abs=timedelta(seconds=1), nan_ok=True); approx(timedelta(seconds=5), rel=0.01, nan_ok=True). Triggered at ApproxTimedelta.__init__ (src/_pytest/approx.py:651-655).

Common situations: Copy-pasting nan_ok=True from a numeric array test into a datetime test; using a generic helper that always sets nan_ok.

Related errors

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/6d4d170f628221b6. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/approx.py:652

    Requires an explicit tolerance as a timedelta for abs, or a float for rel.
    Relative tolerance is not supported for datetime comparisons.
    """

    def __init__(
        self,
        expected: datetime | timedelta,
        rel: float | Decimal | timedelta | None,
        abs: float | Decimal | timedelta | None,
        nan_ok: bool,
    ) -> None:
        __tracebackhide__ = True
        if isinstance(expected, datetime) and rel is not None:
            raise TypeError(
                "pytest.approx() does not support relative tolerance for "
                "datetime comparisons. Use abs=timedelta(...) instead."
            )
        if nan_ok:
            raise TypeError(
                "pytest.approx() does not support nan_ok for "
                "datetime/timedelta comparisons."
            )
        if abs is None and rel is None:
            raise TypeError(
                "pytest.approx() requires an explicit tolerance for "
                "datetime/timedelta comparisons: "
                "e.g. approx(expected, abs=timedelta(seconds=1)) "
                "or approx(expected, rel=0.01)"
            )
        if abs is not None and not isinstance(abs, timedelta):
            raise TypeError(
                f"absolute tolerance for datetime/timedelta must be a "
                f"timedelta, got {type(abs).__name__}"
            )
        if abs is not None and abs < timedelta(0):
            raise ValueError(f"absolute tolerance can't be negative: {abs}")
        if rel is not None:

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