[Bernhard@localhost ~]$ python
Python 2.5.1 (r251:54863, Oct 30 2007, 13:54:11)
[GCC 4.1.2 20070925 (Red Hat 4.1.2-33)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> f1=0.55
>>> f2=2.25
>>> print '%.1f, %.1f' % (f1, f2)
0.6, 2.2
>>>
 
f1 is rounded to the next higher value, but f2 is not.


C:\>python
Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit 
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f1=0.55
>>> f2=2.25
>>> print '%.1f, %.1f' % (f1, f2)
0.6, 2.3
>>>

Under windows it works properly.


If the floating point numbers are encapsulated behind "round(f, 1)" 
both OS show the same output.

Is there anything in the Python documentation that implies that '%.1f' % 2.25 should be the string '2.3'?  I know 
that the documentation for the builtin round function implies that round(2.25, 1) should be (the closest 
representable float to) 2.3, but that's a separate issue.

As far as I can tell, the float formatting of Python derives its behaviour fairly directly from that of the C 
library.  And the C language specification seems to be pretty much silent on the exact rounding behaviour of the 
%e, %f and %g conversion specifiers, so it's not clear that either the Linux result or the Windows result is wrong.  
I'm actually more surprised by the Windows result:  I'd expect %f to do round-to-nearest, with halfway results like 
this one rounded to the string with even last digit.

Incidentally, on OS X 10.4.11/Intel, I get the same as on Linux:

Python 2.5.1 (r251:54863, Dec 18 2007, 11:48:56) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print '%.1f' % 2.25
2.2

Right, Unix-derived C libraries generally do IEEE-754 "round to
nearest/even" rounding, while Microsoft's do "add a half and chop"
rounding.  The Python reference manual says nothing about this, and
neither does the C standard (well, C89 doesn't; unsure about C99).

Python's round() function is independent of the platform C
string<->float conversions, and intentionally does (as documented) "add
a half and chop" rounding.

It's worth noting that Python's round function doesn't quite work as advertised, either:

Python 2.6a0 (trunk:59634M, Dec 31 2007, 17:27:56) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> round(562949953421312.5, 1)
562949953421312.62

I'm closing this as invalid.