On 29/09/12 11:30, Alan Gauld wrote:
On 28/09/12 13:15, Mark Lawrence wrote:
from Calvin Spealman is typical "Also, I'd be completely in support of
dropping round() and agree it gets misused
and leads to too much confusion. We should promote the right ways, and
sometimes to show the right path you need to lock another door and throw
away the key.".
As a matter of interest what is the "right path" that is being proposed?
If it takes much more than 7 keypresses then I suspect it will be opposed!
(I'm too lazy to look up the thread myself! :-)
It is already opposed because it breaks existing, working code unnecessarily.
The replacements suggested are:
- use Decimal values, and round them instead;
- use string formatting
Neither suggestion has really thought things through clearly. The first has
identified the problem correctly -- it is *binary floats*, not round, which
causes the problem, but to round a Decimal you need the round built-in (or
at least a replacement):
py> from decimal import Decimal as D
py> x = D('2.123456')
py> x.round
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Decimal' object has no attribute 'round'
py> round(x, 3)
Decimal('2.123')
The second is, well, poorly thought out. Here is an example of the sort
of thing where round can give funny results. You expect that on half-way
cases, it should round to the nearest EVEN number:
# as expected
py> round(1.125, 2)
1.12
py> round(1.135, 2)
1.14
# but unexpected
py> round(2.675, 2)
2.67
Why? Because 2.675 is not actually a half-way case, it is actually a binary
float a tiny bit under the decimal 2.675:
py> print("%.17f" % 2.675)
2.67499999999999982
Fair enough. So let's try the recommended solution:
py> "%.2f" % 2.675
'2.67'
Wait, that gives the same result as rounding. So how is this better?
--
Steven
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