[ python-Bugs-1778207 ] rounding inconsisntency using string formatting

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Bugs item #1778207, was opened at 2007-08-20 16:20
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Category: Python Interpreter Core
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jim Hurlburt (jim_hurlburt)
Assigned to: Nobody/Anonymous (nobody)
Summary: rounding inconsisntency using string formatting

Initial Comment:
Sirs:

Using python to generate a text file for transfer of data between systems, I seem to be getting inconsistent rounding results using a text formatting string

Python 2.4.3 (#69, Mar 29 2006, 17:35:34) [MSC v.1310 32 bit (Intel)] on win32

print "%9.4f" % (229.0 * .325,)            ->     74.4250   correct
print "%9.2f" % (round(74.425, 2),)        ->     74.43     correct
print "%9.2f" % (74.425,)                  ->     74.42     wrong
print "%9.2f" % (167.255,)                 ->    167.26     correct
print "%9.2f" % (167.2551,)                ->    167.26     correct
print "%9.2f" % (167.2549,)                ->    167.25     correct

It appears as if the string formatting code is a bit flakey.  Perhaps working in binary with too small a precision?

For a bit I thought it might be "Round even up, odd down at the .005 break, but that doesn't appear to be the case.

Now that I know it's there, I can do a workaround.
Seems as if it deserves a fix or at least a document notation of the problem.

Thanks in advance,

Jim Hurlburt
Atrium Windows and Doors
Yakima, WA

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