pytest-dev/pytest · error · ValueError

relative tolerance can't be infinite.

Error message

relative tolerance can't be infinite.

What it means

ApproxTimedelta.__init__ rejects an infinite relative tolerance for timedelta comparisons. An infinite tolerance would make every value match, defeating the purpose of the assertion.

Source

Thrown at src/_pytest/approx.py:681

        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)
        if abs_tolerance is not None and rel_tolerance is not None:
            tolerance: timedelta | None = max(abs_tolerance, rel_tolerance)
        else:
            tolerance = abs_tolerance if abs_tolerance is not None else rel_tolerance
        super().__init__(expected, rel=rel, abs=tolerance, nan_ok=False)

    def __repr__(self) -> str:
        return f"{self.expected} ± {self.abs}"

View on GitHub (pinned to 98b357f69e)

Solutions

  1. Pass a finite rel: approx(td, rel=0.01).
  2. Guard the producing computation against division by zero.
  3. Validate the config value at load time.

Example fix

// before
assert got == approx(td, rel=float('inf'))
// after
assert got == approx(td, rel=0.01)
Defensive patterns

Strategy: validation

Validate before calling

import math

def approx_td_rel(value, rel):
    if isinstance(rel, float) and math.isinf(rel):
        raise ValueError('rel is infinite')
    return pytest.approx(value, rel=rel)

Type guard

import math

def is_finite_number(v) -> bool:
    return isinstance(v, (int, float)) and not math.isnan(v) and not math.isinf(v)

Prevention

When it happens

Trigger: Call pytest.approx(some_timedelta, rel=float('inf')) or rel=math.inf.

Common situations: rel computed from a division by zero; tolerance loaded from a config that allows inf; copy-paste error.

Related errors

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/40e1a7ee4082622f.json. Report an issue: GitHub.