pytest-dev/pytest · error · TypeError

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

Error message

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

What it means

ApproxTimedelta.__init__ rejects nan_ok=True for datetime/timedelta comparisons. NaN datetimes/timedeltas are not a supported concept, so allowing them would be meaningless.

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:

View on GitHub (pinned to 98b357f69e)

Solutions

Drop the nan_ok=True argument for datetime/timedelta comparisons.
If you genuinely need NaN handling, you are comparing the wrong type — restructure the test.

Example fix

// before
assert got == approx(dt, abs=timedelta(seconds=1), nan_ok=True)
// after
assert got == approx(dt, abs=timedelta(seconds=1))

Defensive patterns

Strategy: validation

Validate before calling

def approx_dt(value, abs_td):
    return pytest.approx(value, abs=abs_td, nan_ok=False)  # never pass nan_ok for datetime

Type guard

from datetime import datetime, timedelta

def is_datetime_like(v) -> bool:
    return isinstance(v, (datetime, timedelta))

Prevention

Do not blanket-default nan_ok=True across parametrized tests; set it per-type.
Audit copy-pasted approx() calls when changing the expected value's type.

When it happens

Trigger: Call pytest.approx(dt_or_timedelta, abs=timedelta(seconds=1), nan_ok=True) (or with rel= for a timedelta).

Common situations: Copy-pasting a numeric approx() call (that used nan_ok=True) when migrating to datetime/timedelta values; blanket defaulting nan_ok=True across a parametrized test matrix.

Related errors

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/6d4d170f628221b6.json. Report an issue: GitHub.