[Python-checkins] peps: PEP 410: use :sup: and explain the loss of precision with other tuple formats

[victor.stinner]

http://hg.python.org/peps/rev/f5f31e154b20
changeset:   4034:f5f31e154b20
user:        Victor Stinner <victor.stinner at haypocalc.com>
date:        Sat Feb 04 00:06:26 2012 +0100
summary:
  PEP 410: use :sup: and explain the loss of precision with other tuple formats

files:
  pep-0410.txt |  48 +++++++++++++++++++++++++---------------
  1 files changed, 30 insertions(+), 18 deletions(-)


diff --git a/pep-0410.txt b/pep-0410.txt
--- a/pep-0410.txt
+++ b/pep-0410.txt
@@ -30,12 +30,12 @@
  * time module: clock_gettime(), clock_getres(), wallclock()
 
 The Python float type uses binary64 format of the IEEE 754 standard. With a
-resolution of 1 nanosecond (10^-9), float timestamps lose precision for values
-bigger than 2^24 seconds (194 days: 1970-07-14 for an Epoch timestamp).
+resolution of 1 nanosecond (10\ :sup:`-9`), float timestamps lose precision for values
+bigger than 2\ :sup:`24` seconds (194 days: 1970-07-14 for an Epoch timestamp).
 
 .. note::
-   With a resolution of 1 microsecond (10^-6), float timestamps lose precision
-   for values bigger than 2^33 seconds (272 years: 2242-03-16 for an Epoch
+   With a resolution of 1 microsecond (10\ :sup:`-6`), float timestamps lose precision
+   for values bigger than 2\ :sup:`33` seconds (272 years: 2242-03-16 for an Epoch
    timestamp).
 
 
@@ -107,17 +107,18 @@
 Add a new IEEE 754-2008 quad-precision float type. The IEEE 754-2008 quad
 precision float has 1 sign bit, 15 bits of exponent and 112 bits of mantissa.
 
-128 bits float is supported by GCC (4.3), Clang and ICC. The problem is that
-Visual C++ 2008 doesn't support it. Python must be portable and so cannot rely
-on a type only available on some platforms. Another example: GCC 4.3 does not
-support __float128 in 32-bit mode on x86 (but gcc 4.4 does).
+128 bits float is supported by GCC (4.3), Clang and ICC compilers. Python must
+be portable and so cannot rely on a type only available on some platforms. For
+example, Visual C++ 2008 doesn't support it 128 bits float, whereas it is used
+to build the official Windows executables. Another example: GCC 4.3 does not
+support __float128 in 32-bit mode on x86 (but GCC 4.4 does).
 
-Intel CPUs have FPU supporting 80-bit floats, but not using SSE intructions.
-Other CPU vendors don't support this float size.
+Intel CPUs have FPU (x87) supporting 80-bit floats, but not using SSE
+intructions.  Other CPU vendors don't support this float size.
 
-There is also a license issue: GCC uses the MPFR library which is distributed
-under the GNU LGPL license. This license is incompatible with the Python
-license.
+There is also a license issue: GCC uses the MPFR library for 128 bits float,
+library distributed under the GNU LGPL license. This license is not compatible
+with the Python license.
 
 datetime.datetime
 -----------------
@@ -166,13 +167,24 @@
 
 Other formats have been proposed:
 
- * A: (sec, nsec): value = sec + nsec * 10 ** -9
- * B: (intpart, floatpart, exponent): value = intpart + floatpart * 10 ** exponent
- * C: (intpart, floatpart, base, exponent): value = intpart + floatpart * base ** exponent
+ * A: (sec, nsec): value = sec + nsec * 10\ :sup:`-9`
+ * B: (intpart, floatpart, exponent): value = intpart + floatpart * 10\ :sup:`exponent`
+ * C: (intpart, floatpart, base, exponent): value = intpart + floatpart * base\ :sup:`exponent`
 
 The format A only supports nanosecond resolution. Formats A and B lose
-precision if the clock frequency is not a power of 10. The format C has a
-similar issue.
+precision if the clock frequency cannot be written as a power of 10: if the
+clock frequency is not coprime with 2 and 5.
+
+For some clocks, like ``QueryPerformanceCounter()`` on Windows, the frequency
+is only known as runtime. The base and exponent has to be computed. If
+computing the base and the exponent is too expensive (or not possible, e.g. if
+the frequency is a prime number), exponent=1 can be used. The format (C) is
+just a fractionn if exponent=1.
+
+The only advantage of these formats is a small optimization if the base is 2
+for float or if the base 10 for Decimal. In other cases, frequency = base\
+:sup:`exponent` must be computed again to convert a timestamp as float or
+Decimal. Storing directly the frequency in the denominator is simpler.
 
 
 Alternatives: API design

-- 
Repository URL: http://hg.python.org/peps