python/cpython · error · OverflowError

cannot round an infinity

Error message

cannot round an infinity

What it means

round(Decimal('Infinity')) — one-argument form — raises OverflowError('cannot round an infinity'); there is no nearest integer to an unbounded value. Mirrors __int__/__trunc__ behavior on infinities.

Source

Thrown at Lib/_pydecimal.py:1846

        ...     round(Decimal('-Infinity'), 37)
        ...     round(Decimal('sNaN123'), 0)
        Decimal('NaN')
        Decimal('NaN123')

        """
        if n is not None:
            # two-argument form: use the equivalent quantize call
            if not isinstance(n, int):
                raise TypeError('Second argument to round should be integral')
            exp = _dec_from_triple(0, '1', -n)
            return self.quantize(exp)

        # one-argument form
        if self._is_special:
            if self.is_nan():
                raise ValueError("cannot round a NaN")
            else:
                raise OverflowError("cannot round an infinity")
        return int(self._rescale(0, ROUND_HALF_EVEN))

    def __floor__(self):
        """Return the floor of self, as an integer.

        For a finite Decimal instance self, return the greatest
        integer n such that n <= self.  If self is infinite or a NaN
        then a Python exception is raised.

        """
        if self._is_special:
            if self.is_nan():
                raise ValueError("cannot round a NaN")
            else:
                raise OverflowError("cannot round an infinity")
        return int(self._rescale(0, ROUND_FLOOR))

    def __ceil__(self):

View on GitHub (pinned to bc6749cc3b)

Solutions

1. Guard with is_infinite() and render a sentinel like '∞'/'inf' or reject
2. Fix the source: enable the Overflow trap or raise Context.prec so infinity never appears
3. Clamp to a domain maximum before rounding when a bounded display is acceptable

Example fix

// before
shown = round(v)  # OverflowError when v is Infinity

// after
shown = 'inf' if v.is_infinite() else round(v)

def safe_round1(d):
    if d.is_infinite():
        raise OverflowError(f'cannot round {d}')
    return round(d)

def is_roundable(d) -> bool:
    return not d._is_special

try:
    n = round(d)
except OverflowError:
    n = display_cap  # clamp for display only
except ValueError:
    n = 0  # NaN policy

When it happens

Trigger: round(Decimal('Infinity')); round(Decimal('-inf')); values that overflowed to infinity earlier (Overflow trap disabled) then rounded for display.

Common situations: Formatting computed results for report output where an earlier operation overflowed silently; parsing 'Infinity' from JSON (json.loads accepts it) then rounding.

Related errors

• Cannot convert infinity to integer
• Second argument to round should be integral
• cannot round a NaN
• cannot convert Infinity to integer ratio
• timedelta # of days is too large: %d

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/4469e8241f7f2d61. Report an issue: GitHub.