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Commit 76908929 authored by Mark Dickinson's avatar Mark Dickinson
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Merged revisions 82452 via svnmerge from 

svn+ssh://pythondev@svn.python.org/python/branches/py3k

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  r82452 | mark.dickinson | 2010-07-02 21:26:07 +0100 (Fri, 02 Jul 2010) | 9 lines

  Merged revisions 82446 via svnmerge from
  svn+ssh://pythondev@svn.python.org/python/trunk

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    r82446 | mark.dickinson | 2010-07-02 19:06:52 +0100 (Fri, 02 Jul 2010) | 1 line

    Clarify sys.float_info documentation.
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Doc/library/sys.rst
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...@@ -224,44 +224,65 @@ always available. ...@@ -224,44 +224,65 @@ always available.
.. data:: float_info .. data:: float_info
A structseq holding information about the float type. It contains low level A structseq holding information about the float type. It contains low level
information about the precision and internal representation. Please study information about the precision and internal representation. The values
your system's :file:`float.h` for more information. correspond to the various floating-point constants defined in the standard
header file :file:`float.h` for the 'C' programming language; see section
5.2.4.2.2 of the 1999 ISO/IEC C standard [C99]_, 'Characteristics of
floating types', for details.
+---------------------+----------------+--------------------------------------------------+
| attribute | float.h macro | explanation |
+=====================+================+==================================================+
| :const:`epsilon` | DBL_MAX | difference between 1 and the least value greater |
| | | than 1 that is representable as a float |
+---------------------+----------------+--------------------------------------------------+
| :const:`dig` | DBL_DIG | maximum number of decimal digits that can be |
| | | faithfully represented in a float; see below |
+---------------------+----------------+--------------------------------------------------+
| :const:`mant_dig` | DBL_MANT_DIG | float precision: the number of base-``radix`` |
| | | digits in the significand of a float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max` | DBL_MAX | maximum representable finite float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max_exp` | DBL_MAX_EXP | maximum integer e such that ``radix**(e-1)`` is |
| | | a representable finite float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max_10_exp` | DBL_MAX_10_EXP | maximum integer e such that ``10**e`` is in the |
| | | range of representable finite floats |
+---------------------+----------------+--------------------------------------------------+
| :const:`min` | DBL_MIN | minimum positive normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`min_exp` | DBL_MIN_EXP | minimum integer e such that ``radix**(e-1)`` is |
| | | a normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that ``10**e`` is a |
| | | normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`radix` | FLT_RADIX | radix of exponent representation |
+---------------------+----------------+--------------------------------------------------+
| :const:`rounds` | FLT_ROUNDS | constant representing rounding mode |
| | | used for arithmetic operations |
+---------------------+----------------+--------------------------------------------------+
The attribute :attr:`sys.float_info.dig` needs further explanation. If
``s`` is any string representing a decimal number with at most
:attr:`sys.float_info.dig` significant digits, then converting ``s`` to a
float and back again will recover a string representing the same decimal
value::
+---------------------+--------------------------------------------------+ >>> import sys
| attribute | explanation | >>> sys.float_info.dig
+=====================+==================================================+ 15
| :const:`epsilon` | Difference between 1 and the next representable | >>> s = '3.14159265358979' # decimal string with 15 significant digits
| | floating point number | >>> format(float(s), '.15g') # convert to float and back -> same value
+---------------------+--------------------------------------------------+ '3.14159265358979'
| :const:`dig` | digits (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
| :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
| :const:`max` | maximum representable finite float |
+---------------------+--------------------------------------------------+
| :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
| | range of finite representable floats |
+---------------------+--------------------------------------------------+
| :const:`max_10_exp` | maximum int e such that 10**e is in the |
| | range of finite representable floats |
+---------------------+--------------------------------------------------+
| :const:`min` | Minimum positive normalizer float |
+---------------------+--------------------------------------------------+
| :const:`min_exp` | minimum int e such that radix**(e-1) is a |
| | normalized float |
+---------------------+--------------------------------------------------+
| :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
| | float |
+---------------------+--------------------------------------------------+
| :const:`radix` | radix of exponent |
+---------------------+--------------------------------------------------+
| :const:`rounds` | addition rounds (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
.. note::
The information in the table is simplified. But for strings with more than :attr:`sys.float_info.dig` significant digits,
this isn't always true::
>>> s = '9876543211234567' # 16 significant digits is too many!
>>> format(float(s), '.16g') # conversion changes value
'9876543211234568'
.. data:: float_repr_style .. data:: float_repr_style
...@@ -875,3 +896,8 @@ always available. ...@@ -875,3 +896,8 @@ always available.
first three characters of :const:`version`. It is provided in the :mod:`sys` first three characters of :const:`version`. It is provided in the :mod:`sys`
module for informational purposes; modifying this value has no effect on the module for informational purposes; modifying this value has no effect on the
registry keys used by Python. Availability: Windows. registry keys used by Python. Availability: Windows.
.. rubric:: Citations
.. [C99] ISO/IEC 9899:1999. "Programming languages -- C." A public draft of this standard is available at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf .
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