[Python-Dev] [Python-checkins] r64424 - in python/trunk:Include/object.h Lib/test/test_sys.py Misc/NEWSObjects/intobject.c Objects/longobject.c Objects/typeobject.cPython/bltinmodule.c

Thu Jun 26 23:47:31 CEST 2008

Guido van Rossum wrote:
> On Thu, Jun 26, 2008 at 1:46 PM, Mark Dickinson <dickinsm at gmail.com> wrote:
>> I'd be delighted with '%a' support.
>> Remind me what %a does?

It's a C99 feature. From the spec (7.19.6.1p8)
 a,A A double argument representing a floating-point
 number is converted in the style [-]0xh.hhhhp±d,
 where there is one hexadecimal digit (which is
 nonzero if the argument is a normalized floating-
 point number and is otherwise unspecified) before
 the decimal-point character235) and the number of
 hexadecimal digits after it is equal to the
 precision; if the precision is missing and FLT_RADIX
 is a power of 2, then the precision is sufficient
 for an exact representation of the value; if the
 precision is missing and FLT_RADIX is not a power of
 2, then the precision is sufficient to
 distinguish236) values of type double, except that
 trailing zeros may be omitted; if the precision is
 zero and the # flag is not specified, no decimal-
 point character appears. The letters abcdef are
 used for a conversion and the letters ABCDEF for A
 conversion. The A conversion specifier produces a
 number with X and P instead of x and p. The
 exponent always contains at least one digit, and
 only as many more digits as necessary to represent
 the decimal exponent of 2. If the value is zero,
 the exponent is zero.
 A double argument representing an infinity or NaN is
 converted in the style of an f or F conversion
 specifier.
Footnotes
 235) Binary implementations can choose the hexadecimal digit
 to the left of the decimal-point character so that
 subsequent digits align to nibble (4-bit) boundaries.
 236) The precision p is sufficient to distinguish values of
 the source type if 16p-1>bn where b is FLT_RADIX and n is
 the number of base-b digits in the significand of the
 source type. A smaller p might suffice depending on the
 implementation's scheme for determining the digit to the
 left of the decimal-point character.
This is symmetric with C99's hexadecimal floating point literals:
 hexadecimal-floating-constant:
 hexadecimal-prefix hexadecimal-fractional-constant
 binary-exponent-part floating-suffix-opt
 hexadecimal-prefix hexadecimal-digit-sequence
 binary-exponent-part floating-suffix-opt
 hexadecimal-fractional-constant:
 hexadecimal-digit-sequence-opt . hexadecimal-digit-sequence
 hexadecimal-digit-sequence .
 binary-exponent-part:
 p sign-opt digit-sequence
 P sign-opt digit-sequence
 hexadecimal-digit-sequence:
 hexadecimal-digit
 hexadecimal-digit-sequence hexadecimal-digit
scanf and strtod support the same format.
Regards,
Martin