浮点数对象
**********

type PyFloatObject

   这个C类型 "PyObject" 的子类型代表一个Python浮点数对象。

PyTypeObject PyFloat_Type
    * 属于 稳定 ABI.*

   这是个属于C类型 "PyTypeObject" 的代表Python浮点类型的实例。在Python
   层面的类型 "float" 是同一个对象。

int PyFloat_Check(PyObject *p)

   如果它的参数是一个 "PyFloatObject" 或者 "PyFloatObject" 的子类型则
   返回真值。 此函数总是会成功执行。

int PyFloat_CheckExact(PyObject *p)

   如果它的参数是一个 "PyFloatObject" 但不是 "PyFloatObject" 的子类型
   则返回真值。 此函数总是会成功执行。

PyObject *PyFloat_FromString(PyObject *str)
    *返回值：新的引用。** 属于 稳定 ABI.*

   根据字符串 *str* 的值创建一个 "PyFloatObject"，失败时返回 "NULL"。

PyObject *PyFloat_FromDouble(double v)
    *返回值：新的引用。** 属于 稳定 ABI.*

   根据 *v* 创建一个 "PyFloatObject" 对象，失败时返回 "NULL"。

double PyFloat_AsDouble(PyObject *pyfloat)
    * 属于 稳定 ABI.*

   Return a C double representation of the contents of *pyfloat*.  If
   *pyfloat* is not a Python floating point object but has a
   "__float__()" method, this method will first be called to convert
   *pyfloat* into a float. If "__float__()" is not defined then it
   falls back to "__index__()". This method returns "-1.0" upon
   failure, so one should call "PyErr_Occurred()" to check for errors.

   在 3.8 版本发生变更: Use "__index__()" if available.

double PyFloat_AS_DOUBLE(PyObject *pyfloat)

   Return a C double representation of the contents of *pyfloat*, but
   without error checking.

PyObject *PyFloat_GetInfo(void)
    *返回值：新的引用。** 属于 稳定 ABI.*

   返回一个 structseq 实例，其中包含有关 float 的精度、最小值和最大值
   的信息。 它是头文件 "float.h" 的一个简单包装。

double PyFloat_GetMax()
    * 属于 稳定 ABI.*

   Return the maximum representable finite float *DBL_MAX* as C
   double.

double PyFloat_GetMin()
    * 属于 稳定 ABI.*

   Return the minimum normalized positive float *DBL_MIN* as C double.


Pack and Unpack functions
=========================

The pack and unpack functions provide an efficient platform-
independent way to store floating-point values as byte strings. The
Pack routines produce a bytes string from a C double, and the Unpack
routines produce a C double from such a bytes string. The suffix (2, 4
or 8) specifies the number of bytes in the bytes string.

On platforms that appear to use IEEE 754 formats these functions work
by copying bits. On other platforms, the 2-byte format is identical to
the IEEE 754 binary16 half-precision format, the 4-byte format
(32-bit) is identical to the IEEE 754 binary32 single precision
format, and the 8-byte format to the IEEE 754 binary64 double
precision format, although the packing of INFs and NaNs (if such
things exist on the platform) isn't handled correctly, and attempting
to unpack a bytes string containing an IEEE INF or NaN will raise an
exception.

On non-IEEE platforms with more precision, or larger dynamic range,
than IEEE 754 supports, not all values can be packed; on non-IEEE
platforms with less precision, or smaller dynamic range, not all
values can be unpacked. What happens in such cases is partly
accidental (alas).

在 3.11 版本加入.


Pack functions
--------------

The pack routines write 2, 4 or 8 bytes, starting at *p*. *le* is an
int argument, non-zero if you want the bytes string in little-endian
format (exponent last, at "p+1", "p+3", or "p+6" "p+7"), zero if you
want big-endian format (exponent first, at *p*). The "PY_BIG_ENDIAN"
constant can be used to use the native endian: it is equal to "1" on
big endian processor, or "0" on little endian processor.

Return value: "0" if all is OK, "-1" if error (and an exception is
set, most likely "OverflowError").

There are two problems on non-IEEE platforms:

* What this does is undefined if *x* is a NaN or infinity.

* "-0.0" and "+0.0" produce the same bytes string.

int PyFloat_Pack2(double x, unsigned char *p, int le)

   Pack a C double as the IEEE 754 binary16 half-precision format.

int PyFloat_Pack4(double x, unsigned char *p, int le)

   Pack a C double as the IEEE 754 binary32 single precision format.

int PyFloat_Pack8(double x, unsigned char *p, int le)

   Pack a C double as the IEEE 754 binary64 double precision format.


Unpack functions
----------------

The unpack routines read 2, 4 or 8 bytes, starting at *p*.  *le* is an
int argument, non-zero if the bytes string is in little-endian format
(exponent last, at "p+1", "p+3" or "p+6" and "p+7"), zero if big-
endian (exponent first, at *p*). The "PY_BIG_ENDIAN" constant can be
used to use the native endian: it is equal to "1" on big endian
processor, or "0" on little endian processor.

Return value: The unpacked double.  On error, this is "-1.0" and
"PyErr_Occurred()" is true (and an exception is set, most likely
"OverflowError").

Note that on a non-IEEE platform this will refuse to unpack a bytes
string that represents a NaN or infinity.

double PyFloat_Unpack2(const unsigned char *p, int le)

   Unpack the IEEE 754 binary16 half-precision format as a C double.

double PyFloat_Unpack4(const unsigned char *p, int le)

   Unpack the IEEE 754 binary32 single precision format as a C double.

double PyFloat_Unpack8(const unsigned char *p, int le)

   Unpack the IEEE 754 binary64 double precision format as a C double.
