关于 python 的整数在内存的疑问

我装的是32位的python2.7.9的python，然后发现1~256这个范围的数字的内存是共享，比如我：
a = 1 # id(a) = 31341232
b = 1 # id(b) = 31341232
c = 1 # id(c) = 31341232
他们的id是一样的，

但是一旦数字超过了256,
e = 257 #id(e) = 40104988
f = 257 #id(f) = 40104940
他们的id是不同的。按理说，id不同才比较复合预期，所以我想问，为什么会出现上述id相同的情况，是不是有什么机制啊？！求解~

Python

整数

按理说

11 replies • 2015-07-18 23:01:07 +08:00

echo1937

Jul 17, 2015

学python的时候没有接触"可变对象""不可变对象""引用计数"这样的概念吗?

你看的都什么出版社的书啊.

aragakiiyui

Jul 17, 2015

@echo1937 数字是不可变对象吧，而且引用计数不是针对可变对象么？！

Jul 17, 2015

@echo1937 呵呵。

Jul 17, 2015

@aragakiiyui 回答你的问题：
https://docs.python.org/2/c-api/int.html#c.PyInt_FromLong

The current implementation keeps an array of integer objects for all integers between -5 and 256, when you create an int in that range you actually just get back a reference to the existing object. So it should be possible to change the value of 1. I suspect the behaviour of Python in this case is undefined. :-)

bearzk

Jul 17, 2015

https://docs.python.org/2/c-api/int.html
```
PyObject* PyInt_FromLong(long ival)
Return value: New reference.
Create a new integer object with a value of ival.

The current implementation keeps an array of integer objects for all integers between -5 and 256, when you create an int in that range you actually just get back a reference to the existing object. So it should be possible to change the value of 1. I suspect the behaviour of Python in this case is undefined. :-)
```

文档说是-5 ~ 256都是存好的, 可能是为了快吧.

lzjun

Jul 17, 2015

[-5, 256]这些小对象由于使用频率高，python把他们缓存在内存中

aragakiiyui

Jul 17, 2015

@11 @bearzk @lzjun 理解了~~谢谢

luobuda

Jul 17, 2015

Cpython代码

#ifndef NSMALLPOSINTS
#define NSMALLPOSINTS 257
#endif
#ifndef NSMALLNEGINTS
#define NSMALLNEGINTS 5
#endif
#if NSMALLNEGINTS + NSMALLPOSINTS > 0
/* References to small integers are saved in this array so that they
can be shared.
The integers that are saved are those in the range
-NSMALLNEGINTS (inclusive) to NSMALLPOSINTS (not inclusive).
*/
static PyIntObject *small_ints[NSMALLNEGINTS + NSMALLPOSINTS];
#endif

small_ints就是小整数缓存 05~256
可以修改重新编译

BUPTGuo

Jul 17, 2015

Java也有类似的策略，恩

mingyun

Jul 18, 2015

@lzjun 原来

aheadlead

Jul 18, 2015

@echo1937 希望还是尽量说一些能解决问题的话…