Clarify sys.float_info documentation.
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Mark Dickinson
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@ -296,43 +296,65 @@ always available.
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.. data:: float_info
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A structseq holding information about the float type. It contains low level
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information about the precision and internal representation. Please study
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your system's :file:`float.h` for more information.
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information about the precision and internal representation. The values
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correspond to the various floating-point constants defined in the standard
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header file :file:`float.h` for the 'C' programming language; see section
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5.2.4.2.2 of the 1999 ISO/IEC C standard [C99]_, 'Characteristics of
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floating types', for details.
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+---------------------+--------------------------------------------------+
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| attribute | explanation |
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+=====================+==================================================+
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| :const:`epsilon` | Difference between 1 and the next representable |
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| | floating point number |
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+---------------------+--------------------------------------------------+
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| :const:`dig` | digits (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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| :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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| :const:`max` | maximum representable finite float |
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+---------------------+--------------------------------------------------+
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| :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
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| | range of finite representable floats |
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+---------------------+--------------------------------------------------+
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| :const:`max_10_exp` | maximum int e such that 10**e is in the |
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| | range of finite representable floats |
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+---------------------+--------------------------------------------------+
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| :const:`min` | Minimum positive normalizer float |
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+---------------------+--------------------------------------------------+
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| :const:`min_exp` | minimum int e such that radix**(e-1) is a |
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| | normalized float |
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+---------------------+--------------------------------------------------+
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| :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
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| | float |
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+---------------------+--------------------------------------------------+
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| :const:`radix` | radix of exponent |
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+---------------------+--------------------------------------------------+
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| :const:`rounds` | addition rounds (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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+---------------------+----------------+--------------------------------------------------+
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| attribute | float.h macro | explanation |
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+=====================+================+==================================================+
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| :const:`epsilon` | DBL_MAX | difference between 1 and the least value greater |
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| | | than 1 that is representable as a float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`dig` | DBL_DIG | maximum number of decimal digits that can be |
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| | | faithfully represented in a float; see below |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`mant_dig` | DBL_MANT_DIG | float precision: the number of base-``radix`` |
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| | | digits in the significand of a float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`max` | DBL_MAX | maximum representable finite float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`max_exp` | DBL_MAX_EXP | maximum integer e such that ``radix**(e-1)`` is |
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| | | a representable finite float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`max_10_exp` | DBL_MAX_10_EXP | maximum integer e such that ``10**e`` is in the |
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| | | range of representable finite floats |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`min` | DBL_MIN | minimum positive normalized float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`min_exp` | DBL_MIN_EXP | minimum integer e such that ``radix**(e-1)`` is |
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| | | a normalized float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that ``10**e`` is a |
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| | | normalized float |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`radix` | FLT_RADIX | radix of exponent representation |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`rounds` | FLT_ROUNDS | constant representing rounding mode |
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| | | used for arithmetic operations |
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+---------------------+----------------+--------------------------------------------------+
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.. note::
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The attribute :attr:`sys.float_info.dig` needs further explanation. If
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``s`` is any string representing a decimal number with at most
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:attr:`sys.float_info.dig` significant digits, then converting ``s`` to a
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float and back again will recover a string representing the same decimal
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value::
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The information in the table is simplified.
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>>> import sys
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>>> sys.float_info.dig
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15
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>>> s = '3.14159265358979' # decimal string with 15 significant digits
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>>> format(float(s), '.15g') # convert to float and back -> same value
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'3.14159265358979'
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But for strings with more than :attr:`sys.float_info.dig` significant digits,
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this isn't always true::
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>>> s = '9876543211234567' # 16 significant digits is too many!
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>>> format(float(s), '.16g') # conversion changes value
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'9876543211234568'
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.. versionadded:: 2.6
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@ -992,3 +1014,8 @@ always available.
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first three characters of :const:`version`. It is provided in the :mod:`sys`
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module for informational purposes; modifying this value has no effect on the
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registry keys used by Python. Availability: Windows.
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.. rubric:: Citations
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.. [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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