Frank Millman wrote:
I have been trying to explain why they should use the decimal module. They have had a counter-argument from someone else who says they should just use the rounding technique in my third example above.
It's possible to get away with this by judicious use of rounding. There's a business software package I work with that does this -- every number is rounded to a defined number of decimal places before being stored in its database, so the small inaccuracies resulting from inexact representations don't get a chance to accumulate. If you're going to do this, I would NOT recommend using the rounding technique in your example -- it seems to itself be relying on accidents of the arithmetic. Use the round() function: >>> 1.1 + 2.2 3.3000000000000003 >>> round(1.1 + 2.2, 1) 3.3 Also, if you go this route, always keep in mind *why* it works -- it relies on the actual value always being close enough to a multiple of a power of 10 that it rounds to the correct value when converted to decimal.
Are there edge cases where that rounding method could fail?
Yes, it's easy to come up with examples that make it fail, e.g. multiplying by a large number, or adding up a few billion monetary amounts before rounding. These things are very unlikely to happen in an accounting application, but they are theoretically possible. So, you can probably make it work if you're careful, but for peace of mind I would still recommend using Decimal, because Python has it, and it eliminates all of these potential problems. -- Greg -- https://mail.python.org/mailman/listinfo/python-list
