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Currently, Python switches from %f to %g formatting at 1e50. This 
applies both to the old-style percent formatting, and to the newer PEP 
3101-style formatting:
>>> '%f' % 2.**166
'93536104789177786765035829293842113257979682750464.000000'
>>> '%f' % 2.**167
'1.87072e+50'
>>> format(2.**166, 'f')
'93536104789177786765035829293842113257979682750464.000000'
>>> format(2.**167, 'f')
'1.87072e+50'
The main reason for the switch seems to have been implementation 
convenience: it makes it possible to use a fixed-size buffer in
the float formatting routines.
I propose removing this feature for Python 3.1, but leaving it in place 
for 2.7.
See
http://mail.python.org/pipermail/python-dev/2009-April/089030.html
for additional discussion.