2007-06-13
操作符is的1个诡异问题
请看这段程序
猜猜结果，呵呵结果是
13013836 257
13013812 257
原因在这里：
大家可以看下intobject.c的有关这部分的源码：
呵呵，一目了然吧.
这边给我们的教训就是判断数字相等不要用 is操作符，哈哈.
a = 0
b = 0
while a is b:
a += 1
b += 1
print id(a), a
print id(b), b
猜猜结果，呵呵结果是
13013836 257
13013812 257
原因在这里：
引用
This is really hardcoded limit in the current CPython implementation :)
The interpreter preallocates numbers from 0 to 256.
The interpreter preallocates numbers from 0 to 256.
大家可以看下intobject.c的有关这部分的源码：
#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 #ifdef COUNT_ALLOCS int quick_int_allocs, quick_neg_int_allocs; #endif
呵呵，一目了然吧.
这边给我们的教训就是判断数字相等不要用 is操作符，哈哈.
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