[Python-Dev] PEP 3101: floats format 'f' and 'F'

Eric Smith eric+python-dev at trueblade.com
Wed Jul 16 17:15:40 CEST 2008

Mark Dickinson wrote:
> On Wed, Jul 16, 2008 at 3:35 PM, Eric Smith
> <eric+python-dev at trueblade.com> wrote:
>> Does anyone know why 'F' is the same as 'f'?  Wouldn't it make more sense to
>> either drop it, or make it convert the exponent to upper case
> What exponent?  Isn't the point of 'f' formatting that there is no exponent?

There's no exponent until the number gets large.  I haven't looked up 
how big the number has to get.  On my Mac, it's somewhere between 1e50 
and 1e60.

$ ./python.exe
Python 3.0b1+ (py3k:64984:64985, Jul 15 2008, 20:17:06)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
 >>> '%f' % 1e100
 >>> '%F' % 1e100
 >>> '%f' % 1.2

str.format() works the same.

> In C, the only difference seems to be that a NaN or infinity formatted with '%F'
> is turned into "NAN" or "INF" instead of "nan" or "inf".

Not so in Python.
 >>> '%f' % float('nan')
 >>> '%F' % float('nan')

But it is the case with 'e':
 >>> '%e' % float('nan')
 >>> '%E' % float('nan')

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