Undocumented alternate form for %#f ?

[Dino Viehland]

Ahh, cool...  Thanks for the explanation!

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-----Original Message-----
From: python-list-bounces+dinov=microsoft.com at python.org [mailto:python-list-bounces+dinov=microsoft.com at python.org] On Behalf Of Dave Hughes
Sent: Friday, April 28, 2006 1:00 PM
To: python-list at python.org
Subject: Re: Undocumented alternate form for %#f ?

Dino Viehland wrote:

> I'm assuming this is by-design, but it doesn't appear to be
> documented:
>
> >>> '%8.f' % (-1)
> '      -1'
> >>> '%#8.f' % (-1)
> '     -1.'
>
>
> The docs list the alternate forms, but there isn't one listed for
> f/F.   It would seem the alternate form for floating points is
> truncate & round the floating point value, but always display the .
> at the end.  Is that correct?

The Python % operator follows the C sprintf function pretty darn
closely in behaviour (hardly surprising really, though I've never
peeked at the implementation). Hence "man sprintf" can provide some
clues here. From man sprintf on my Linux box:

#
The  value  should be converted to an ``alternate form''.  For o
conversions, the first character of the output  string  is  made
zero (by prefixing a 0 if it was not zero already).  For x and X
conversions, a non-zero result has the string `0x' (or `0X'  for
X  conversions) prepended to it.  For a, A, e, E, f, F, g, and G
conversions, the result will always  contain  a  decimal  point,
even  if  no digits follow it (normally, a decimal point appears
in the results of those conversions only if  a  digit  follows).
For g and G conversions, trailing zeros are not removed from the
result as they would otherwise be.  For other  conversions,  the
result is undefined.

Hence, I don't think it's the # doing the truncating here, but it
certainly is producing the mandatory decimal point. If you get rid of
the "." in the specification, it uses the default decimal precision (6):

>>> "%8f" % (-1)
'-1.000000'
>>> "%#8f" % (-1)
'-1.000000'

No difference with the alternate specification here as the precision is
non-zero. Again, from man sprintf:

The precision
[snip]
If the precision is given as
just `.', or the precision is negative, the precision is  taken  to  be
zero.   This  gives the minimum number of digits to appear for d, i, o,
u, x, and X conversions, the number of digits to appear after the radix
character  for  a, A, e, E, f, and F conversions, the maximum number of
significant digits for g and G conversions, or the  maximum  number  of
characters to be printed from a string for s and S conversions.


HTH,

Dave.
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