Issue 7630: Strange behaviour of decimal.Decimal (original) (raw)

Issue7630

Created on 2010-01-04 05:41 by parmax, last changed 2022-04-11 14:56 by admin. This issue is now closed.

Messages (3)
msg97192 - (view)	Author: parmax (parmax)	Date: 2010-01-04 05:41
>>> from decimal import Decimal >>> dec = Decimal(2 ** 1024) >>> dec Decimal('179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216') >>> dec += Decimal('0.1') >>> dec Decimal('1.797693134862315907729305191E+308') >>> dec == Decimal(2 ** 1024) + Decimal('0.1') True >>> dec -= Decimal(2 ** 1024) >>> dec Decimal('2.109752663820230210576934273E+280') >>> dec == Decimal('0.1') False >>>
msg97196 - (view)	Author: Mark Dickinson (mark.dickinson) *	Date: 2010-01-04 08:58
You don't say what behaviour you were expecting! :) By design, almost all Decimal operations (but not creation of a Decimal from an integer or string) round to the precision given by the current context. By default that precision is 28 significant digits. See the documentation, particularly the quick-start tutorial, http://docs.python.org/library/decimal.html#quick-start-tutorial for more information, including how to change the default precision.
msg97199 - (view)	Author: Mark Dickinson (mark.dickinson) *	Date: 2010-01-04 09:09
It might also help to note that, with decimal, what you see (from repr() of a Decimal instance) is *exactly* what you get. So when you see >>> dec Decimal('1.797693134862315907729305191E+308') that number really is exactly what's stored in dec: there are no 'hidden' digits waiting to appear.