[Python-3000] str.format() -- poss. code or doc bug?

Eric Smith eric+python-dev at trueblade.com
Fri Nov 30 16:44:20 CET 2007

Mark Summerfield wrote:
> Hi,
> I'm using Python 30a.
> The docs for str.format()'s 'g' format say
>     "General format. This prints the number as a fixed-point number,
>     unless the number is too large, in which case it switches to 'e'
>     exponent notation."
> The fixed-point format uses the 'f' character.
> But this does not seem to happen in practice:
>>>> "[{0:12.4e}] [{0:12.4f}] [{0:12.4g}]".format(10**4 * math.pi)
> '[  3.1416e+04] [  31415.9265] [   3.142e+04]'
>>>> "[{0:12.4e}] [{0:12.4f}] [{0:12.4g}]".format(10**3 * math.pi)
> '[  3.1416e+03] [   3141.5927] [        3142]'
> I thought this was a bug in Python 3, but Python 2 does the same thing:
>>>> n = 10**4 * math.pi
>>>> m = 10**3 * math.pi
>>>> "[%12.4e] [%12.4f] [%12.4g]" % (n, n, n)
> '[  3.1416e+04] [  31415.9265] [   3.142e+04]'
>>>> "[%12.4e] [%12.4f] [%12.4g]" % (m, m, m)
> '[  3.1416e+03] [   3141.5927] [        3142]'

They're the same because I copied the '%' code when I created the 
__format__ code.  I copied, instead of refactoring and using the same 
code, because at the time I did it I thought the direction was going to 
be to remove '%' formatting.  Plus, the string/unicode work was making 
it more complex at the time.  If I had it to do over, I might spend some 
more time refactoring, or maybe even modifying '%' to call str.format() 

As to whether the documentation or the code is correct, I can't say 
which is correct or more desirable.  Changing how this works would no 
doubt break all sorts of code.

> Python 2's docs are different from Python 3's regarding 'g' format:
>     "Floating point format. Uses exponential format if exponent is
>     greater than -4 or less than precision, decimal format otherwise."
> There is no "decimal format", but there is "Signed integer decimal"
> format which is what seems to being used.
> So is this a doc bug?
> BTW I notice that decimal.Decimal() numbers can't be used with the 'e',
> 'f', or 'g' formats. I know that these numbers aren't floating-point
> under the hood, but this still seems a bit counter-intuitive to me.

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