pytest-dev/pytest · error · TypeError

pytest.approx() does not support relative tolerance for…

Error message

pytest.approx() does not support relative tolerance for datetime comparisons. Use abs=timedelta(...) instead.

What it means

ApproxTimedelta.__init__ explicitly rejects a relative tolerance (rel) when the expected value is a datetime. Relative tolerance for an absolute timestamp is undefined (a 'fraction of' a datetime has no useful meaning and would yield a float, not a datetime), so pytest requires an absolute timedelta tolerance instead. The error message suggests the correct abs=timedelta(...) form.

Solutions

Use an absolute timedelta tolerance: `approx(dt, abs=timedelta(seconds=1))`.
If comparing durations (timedelta expected), rel is fine: `approx(td, rel=0.01)`.
Convert datetimes to a common reference and compare the deltas with abs tolerance.

Example fix

# before
approx(datetime.now(), rel=0.01)
# after
approx(datetime.now(), abs=timedelta(seconds=1))

Defensive patterns

Strategy: type-guard

Validate before calling

from datetime import datetime, timedelta
def datetime_approx_kwargs(expected):
    if isinstance(expected, datetime):
        return {'abs': timedelta(seconds=1)}  # never rel for datetime
    return {'rel': 0.01}  # rel ok for timedelta
# usage
assert actual == approx(expected, **datetime_approx_kwargs(expected))

Type guard

from datetime import datetime, timedelta
def is_datetime(v) -> bool:
    return isinstance(v, datetime)

Prevention

Never pass rel for datetime expected; use abs=timedelta(...).
In helpers, branch on isinstance(expected, datetime) before choosing tolerance.
Remember rel IS valid for timedelta (durations), just not datetime.

When it happens

Trigger: Calling approx(datetime.now(), rel=0.01); approx(datetime(2024,1,1), rel=0.001). Triggered at ApproxTimedelta.__init__ (src/_pytest/approx.py:646-650). Note: rel IS supported for timedelta expected values (where it represents a fraction of the duration), just not for datetime.

Common situations: Generalizing a timedelta test to datetime without changing the tolerance style; copy-pasting numeric rel syntax into datetime comparisons.

Related errors

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/326a947ddbbead2b. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/approx.py:647

class ApproxTimedelta(Approx[datetime | timedelta]):
    """Perform approximate comparisons where the expected value is a
    datetime or timedelta.

    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 "

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