diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst index 3e33581f96f..db323802a6f 100644 --- a/Doc/library/decimal.rst +++ b/Doc/library/decimal.rst @@ -897,6 +897,48 @@ Decimal objects :const:`Rounded`. If given, applies *rounding*; otherwise, uses the rounding method in either the supplied *context* or the current context. + Decimal numbers can be rounded using the :func:`.round` function: + + .. describe:: round(number) + .. describe:: round(number, ndigits) + + If *ndigits* is not given or ``None``, + returns the nearest :class:`int` to *number*, + rounding ties to even, and ignoring the rounding mode of the + :class:`Decimal` context. Raises :exc:`OverflowError` if *number* is an + infinity or :exc:`ValueError` if it is a (quiet or signaling) NaN. + + If *ndigits* is an :class:`int`, the context's rounding mode is respected + and a :class:`Decimal` representing *number* rounded to the nearest + multiple of ``Decimal('1E-ndigits')`` is returned; in this case, + ``round(number, ndigits)`` is equivalent to + ``self.quantize(Decimal('1E-ndigits'))``. Returns ``Decimal('NaN')`` if + *number* is a quiet NaN. Raises :class:`InvalidOperation` if *number* + is an infinity, a signaling NaN, or if the length of the coefficient after + the quantize operation would be greater than the current context's + precision. In other words, for the non-corner cases: + + * if *ndigits* is positive, return *number* rounded to *ndigits* decimal + places; + * if *ndigits* is zero, return *number* rounded to the nearest integer; + * if *ndigits* is negative, return *number* rounded to the nearest + multiple of ``10**abs(ndigits)``. + + For example:: + + >>> from decimal import Decimal, getcontext, ROUND_DOWN + >>> getcontext().rounding = ROUND_DOWN + >>> round(Decimal('3.75')) # context rounding ignored + 4 + >>> round(Decimal('3.5')) # round-ties-to-even + 4 + >>> round(Decimal('3.75'), 0) # uses the context rounding + Decimal('3') + >>> round(Decimal('3.75'), 1) + Decimal('3.7') + >>> round(Decimal('3.75'), -1) + Decimal('0E+1') + .. _logical_operands_label: