Method __round__

Lib/_pydecimal.py:1781–1841

Round self to the nearest integer, or to a given precision. If only one argument is supplied, round a finite Decimal instance self to the nearest integer. If self is infinite or a NaN then a Python exception is raised. If self is finite and lies exactly halfway bet

(self, n=None)

    )

    def __round__(self, n=None):
        """Round self to the nearest integer, or to a given precision.

        If only one argument is supplied, round a finite Decimal
        instance self to the nearest integer.  If self is infinite or
        a NaN then a Python exception is raised.  If self is finite
        and lies exactly halfway between two integers then it is
        rounded to the integer with even last digit.

        >>> round(Decimal('123.456'))
        123
        >>> round(Decimal('-456.789'))
        -457
        >>> round(Decimal('-3.0'))
        -3
        >>> round(Decimal('2.5'))
        2
        >>> round(Decimal('3.5'))
        4
        >>> round(Decimal('Inf'))
        Traceback (most recent call last):
          ...
        OverflowError: cannot round an infinity
        >>> round(Decimal('NaN'))
        Traceback (most recent call last):
          ...
        ValueError: cannot round a NaN

        If a second argument n is supplied, self is rounded to n
        decimal places using the rounding mode for the current
        context.

        For an integer n, round(self, -n) is exactly equivalent to
        self.quantize(Decimal('1En')).

        >>> round(Decimal('123.456'), 0)
        Decimal('123')
        >>> round(Decimal('123.456'), 2)
        Decimal('123.46')
        >>> round(Decimal('123.456'), -2)
        Decimal('1E+2')
        >>> round(Decimal('-Infinity'), 37)
        Decimal('NaN')
        >>> round(Decimal('sNaN123'), 0)
        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")