pytest-dev/pytest · error · TypeError

absolute tolerance for a scalar value must be an int, float

Error message

absolute tolerance for a scalar value must be an int, float or Decimal, got {type(abs).__name__}

What it means

When constructing an ApproxScalar, the abs (absolute tolerance) parameter must be None or an int, float, or Decimal. If abs is any other type (string, complex, etc.), pytest raises TypeError in __init__. This validates the tolerance type before any comparison occurs.

Source

Thrown at src/_pytest/approx.py:460

        expected: ExpectedT,
        rel: float | Decimal | timedelta | None,
        abs: float | Decimal | timedelta | None,
        nan_ok: bool,
    ) -> None:
        __tracebackhide__ = True
        if rel is not None:
            if not isinstance(rel, (int, float, Decimal)):
                raise TypeError(
                    f"relative tolerance for a scalar value must be an int, float or Decimal, "
                    f"got {type(rel).__name__}"
                )
            if not isinstance(expected, SupportsAbs):
                raise TypeError(
                    f"expected value must support abs(...) when relative tolerance is used, "
                    f"got {type(expected).__name__}"
                )
        if abs is not None and not isinstance(abs, (int, float, Decimal)):
            raise TypeError(
                f"absolute tolerance for a scalar value must be an int, float or Decimal, "
                f"got {type(abs).__name__}"
            )
        super().__init__(expected, rel=rel, abs=abs, nan_ok=nan_ok)

    def __repr__(self) -> str:
        """Return a string communicating both the expected value and the
        tolerance for the comparison being made.

        For example, ``1.0 ± 1e-6``, ``(3+4j) ± 5e-6 ∠ ±180°``.
        """
        # Don't show a tolerance for values that aren't compared using
        # tolerances, i.e. non-numerics and infinities. Need to call abs to
        # handle complex numbers, e.g. (inf + 1j).
        if (
            _is_bool(self.expected)
            or (not isinstance(self.expected, Complex | Decimal))
            or math.isinf(abs(self.expected))

View on GitHub (pinned to 98b357f69e)

Solutions

Convert abs to a Python float: approx(1.0, abs=float(value)).
Pass abs as a plain numeric literal: approx(1.0, abs=0.001).
Use Decimal explicitly if needed: approx(Decimal('1.0'), abs=Decimal('0.001')).

Example fix

# before
approx(1.0, abs='1e-3')  # string

# after
approx(1.0, abs=1e-3)  # float

Defensive patterns

Strategy: type-guard

Validate before calling

from decimal import Decimal

def validate_abs_tolerance(abs_tol):
    if abs_tol is not None and not isinstance(abs_tol, (int, float, Decimal)):
        raise TypeError(f'abs must be int, float, or Decimal, got {type(abs_tol).__name__}')
    return abs_tol

Type guard

from decimal import Decimal

def is_valid_abs_tolerance(value) -> bool:
    return value is None or isinstance(value, (int, float, Decimal))

Prevention

Always pass abs as a plain float literal or variable.
Convert config/string-sourced tolerance values with float() before passing to approx().
Use Decimal type consistently if working with Decimal expected values.

When it happens

Trigger: Calling approx(1.0, abs='0.001') or approx(1.0, abs=1e-3j). The isinstance(abs, (int, float, Decimal)) check fails.

Common situations: Passing a tolerance value from a parsed config string, env var, or JSON without converting to float, or accidentally using a complex number.

Related errors

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/a3f15e461a1d991a.json. Report an issue: GitHub.