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这是一个创建于 3418 天前的主题，其中的信息可能已经有所发展或是发生改变。
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~
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~
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1
echo1937 2015-07-17 17:59:54 +08:00
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. |
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2
aragakiiyui OP @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？！
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4
11 2015-07-17 18:07:58 +08:00  1
@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. :-) |
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5
bearzk 2015-07-17 18:09:23 +08:00
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. |
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6
lzjun 2015-07-17 18:09:41 +08:00
[-5, 256]这些小对象由于使用频率高，python把他们缓存在内存中
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7
aragakiiyui OP |
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8
luobuda 2015-07-17 21:03:53 +08:00
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 |
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9
BUPTGuo 2015-07-17 21:09:21 +08:00
Java也有类似的策略，恩
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