pytest-dev/pytest · error · TypeError

relative tolerance for timedelta must be a number, got {type

Error message

relative tolerance for timedelta must be a number, got {type(rel).__name__}

What it means

ApproxTimedelta.__init__ requires the relative tolerance for a timedelta comparison to be a number (int or float), because rel is multiplied by abs(expected) (a timedelta) to produce a timedelta tolerance. Non-numeric rel values cannot be multiplied.

Source

Thrown at src/_pytest/approx.py:672

                "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:
            if not isinstance(rel, (int, float)):
                raise TypeError(
                    f"relative tolerance for timedelta must be a "
                    f"number, got {type(rel).__name__}"
                )
            if rel < 0:
                raise ValueError(f"relative tolerance can't be negative: {rel}")
            if math.isnan(rel):
                raise ValueError("relative tolerance can't be NaN.")
            if math.isinf(rel):
                raise ValueError("relative tolerance can't be infinite.")
        # Compute the effective tolerance. abs_tolerance is a timedelta, rel * expected
        # gives a timedelta (timedelta * float works in Python).
        abs_tolerance = abs
        if rel is None:
            rel_tolerance = None
        else:
            # Checked above.
            assert not isinstance(expected, datetime)
            rel_tolerance = rel * builtins.abs(expected)

View on GitHub (pinned to 98b357f69e)

Solutions

Pass rel as a float: approx(td, rel=0.01).
If you have a string tolerance, cast it: rel=float(my_str).
Use abs=timedelta(...) instead if you actually have a duration tolerance.

Example fix

// before
assert got == approx(td, rel='0.01')
// after
assert got == approx(td, rel=0.01)

Defensive patterns

Strategy: type-guard

Validate before calling

def approx_td_rel(value, rel):
    if not isinstance(rel, (int, float)):
        raise TypeError(f'rel must be a number, got {type(rel).__name__}')
    return pytest.approx(value, rel=rel)

Type guard

def is_number(v) -> bool:
    return isinstance(v, (int, float)) and not isinstance(v, bool)

Prevention

Cast string tolerances with float() at the config boundary.
Keep abs=timedelta and rel=float distinct in your helpers; do not let callers swap them.

When it happens

Trigger: Call pytest.approx(some_timedelta, rel='0.01') or rel=timedelta(...) — any non-int/float value for rel when expected is a timedelta.

Common situations: Reading rel from a config file as a string; accidentally passing a timedelta as rel instead of abs; copy-paste errors.

Related errors

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/a44456c59550bba0.json. Report an issue: GitHub.