From b1affc517f67037fcd9027cf92fda3424b80c1ea Mon Sep 17 00:00:00 2001 From: Mark Dickinson Date: Sun, 4 Apr 2010 22:09:21 +0000 Subject: [PATCH] Add versionchanged entry for Decimal(float) construction. Also add an example of constructing a Decimal directly from a float, update the quickstart tutorial, and tweak another couple of sentences. --- Doc/library/decimal.rst | 22 +++++++++++++++------- 1 file changed, 15 insertions(+), 7 deletions(-) diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst index 6addd483211..2697026f058 100644 --- a/Doc/library/decimal.rst +++ b/Doc/library/decimal.rst @@ -133,10 +133,9 @@ precision, rounding, or enabled traps:: >>> getcontext().prec = 7 # Set a new precision -Decimal instances can be constructed from integers, strings, or tuples. To -create a Decimal from a :class:`float`, first convert it to a string. This -serves as an explicit reminder of the details of the conversion (including -representation error). Decimal numbers include special values such as +Decimal instances can be constructed from integers, strings, floats, or tuples. +Construction from an integer or a float performs an exact conversion of the +value of that integer or float. Decimal numbers include special values such as :const:`NaN` which stands for "Not a number", positive and negative :const:`Infinity`, and :const:`-0`. @@ -145,6 +144,8 @@ representation error). Decimal numbers include special values such as Decimal('10') >>> Decimal('3.14') Decimal('3.14') + >>> Decimal(3.14) + Decimal('3.140000000000000124344978758017532527446746826171875') >>> Decimal((0, (3, 1, 4), -2)) Decimal('3.14') >>> Decimal(str(2.0 ** 0.5)) @@ -343,8 +344,9 @@ Decimal objects If *value* is a :class:`float`, the binary floating point value is losslessly converted to its exact decimal equivalent. This conversion can often require - upto 53 digits of precision. For example, ``Decimal(float('1.1'))`` converts - to ``Decimal('1.100000000000000088817841970012523233890533447265625')``. + 53 or more digits of precision. For example, ``Decimal(float('1.1'))`` + converts to + ``Decimal('1.100000000000000088817841970012523233890533447265625')``. The *context* precision does not affect how many digits are stored. That is determined exclusively by the number of digits in *value*. For example, @@ -362,6 +364,9 @@ Decimal objects leading and trailing whitespace characters are permitted when creating a Decimal instance from a string. + .. versionchanged:: 2.7 + The argument to the constructor is now permitted to be a :float:`instance`. + Decimal floating point objects share many properties with the other built-in numeric types such as :class:`float` and :class:`int`. All of the usual math operations and special methods apply. Likewise, decimal objects can be @@ -523,6 +528,9 @@ Decimal objects `0x1.999999999999ap-4`. That equivalent value in decimal is `0.1000000000000000055511151231257827021181583404541015625`. + .. note:: From Python 2.7 onwards, a :class:`Decimal` instance + can also be constructed directly from a :class:`float`. + .. doctest:: >>> Decimal.from_float(0.1) @@ -1955,7 +1963,7 @@ value unchanged:: Q. Is there a way to convert a regular float to a :class:`Decimal`? -A. Yes, all binary floating point numbers can be exactly expressed as a +A. Yes, any binary floating point number can be exactly expressed as a Decimal though an exact conversion may take more precision than intuition would suggest: