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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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Mark Dickinson 2010-07-02 20:26:07 +00:00
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Doc/library/sys.rst
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@ -223,44 +223,65 @@ always available.
.. data:: float_info
A structseq holding information about the float type. It contains low level
information about the precision and internal representation. Please study
your system's :file:`float.h` for more information.
information about the precision and internal representation. The values
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 | explanation |
+=====================+==================================================+
| :const:`epsilon` | Difference between 1 and the next representable |
| | floating point number |
+---------------------+--------------------------------------------------+
| :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`) |
+---------------------+--------------------------------------------------+
+---------------------+----------------+--------------------------------------------------+
| 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 |
+---------------------+----------------+--------------------------------------------------+
.. note::
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::
The information in the table is simplified.
>>> import sys
>>> sys.float_info.dig
15
>>> s = '3.14159265358979' # decimal string with 15 significant digits
>>> format(float(s), '.15g') # convert to float and back -> same value
'3.14159265358979'
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
@ -957,3 +978,8 @@ always available.
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
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 .