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Date: Tue, 30 Oct 2012 11:13:53 -0400
From: Dave Angel <d@davea.name>
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To: andrew.mackeith@3ds.com
Subject: Re: Float to String "%.7e" - diff between Python-2.6 and Python-2.7
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On 10/30/2012 10:47 AM, andrew.mackeith@3ds.com wrote:
> When formatting a float using the exponential format, the rounding is different in Python-2.6 and Python-2.7. See example below.
> Is this intentional?
>
> Is there any way of forcing the Python-2.6 behavior (for compatibility reasons when testing)?
>
>> c:\python26\python
> r:\asiData\abaObjects\lib>c:\python26\python
> Python 2.6.5 (r265:79096, Mar 19 2010, 18:02:59) [MSC v.1500 64 bit (AMD64)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> x = [2.096732130e+02,2.096732140e+02,2.096732150e+02,2.096732151e+02,2.096732160+02]
>>>> for a in x:
> ...     print ' %.9e    %.7e'%(a,a)
> ...
>  2.096732130e+02    2.0967321e+02
>  2.096732140e+02    2.0967321e+02
>  2.096732150e+02    2.0967322e+02 <<<<<<<<
>  2.096732151e+02    2.0967322e+02
>  4.096732160e+00    4.0967322e+00
>

Looks to me that the indicated value was rounded wrong in 2.6, so
apparently that was fixed in 2.7

The actual binary fp value stored for 2.096732150 e+02 is slightly
smaller than the decimal string specified, so it'll round towards
1.0967321 when you specify 7 digits.

I don't know of any way to re-introduce the earlier version.  But you
could fix them both by using Decimal, where there's no quantization
error.  Or if this is a fake example, adapt by just validating that the
results are 'close'

-- 

DaveA