float.md

float

contents

• related file
• memory layout
• example
  • 0
  • 1
  • 0.1
  • 1.1
  • -0.1
• free_list

related file

• cpython/Objects/floatobject.c
• cpython/Include/floatobject.h
• cpython/Objects/clinic/floatobject.c.h

memory layout

PyFloatObject is no more than a wrapper of c type double, which takes 8 bytes to represent a floating point number

you can refer to IEEE 754/IEEE-754标准与浮点数运算 for more detail

example

0

the binary representation of 0.0 in IEEE 754 format is 64 zero bits

f = 0.0

1.0

f = 1.0

0.1

f = 0.1

1.1

the difference between 1.1 and 0.1 is the last few exponent bits

-0.1

the difference between -0.1 and 0.1 is the first sign bit

free_list

#ifndef PyFloat_MAXFREELIST
#define PyFloat_MAXFREELIST    100
#endif
static int numfree = 0;
static PyFloatObject *free_list = NULL;

free_list is a single linked list, store at most PyFloat_MAXFREELIST PyFloatObject

The linked list is linked via the field ob_type

>>> f = 0.0
>>> id(f)
4551393664
>>> f2 = 1.0
>>> id(f2)
4551393616
del f
del f2

f3 comes from the head of the free_list

>>> f3 = 3.0
>>> id(f3)
4551393616