因?yàn)閜ython的對(duì)象占用的內(nèi)存空間比較大，創(chuàng)建開(kāi)銷也大，對(duì)于int類型有個(gè)內(nèi)部的緩存機(jī)制，所以a=1和b=1實(shí)際上是指向了同一個(gè)對(duì)象，詳情可以參見(jiàn)python源碼（后附）。

另外要注意，題主這種寫(xiě)法python解釋器是分別解析執(zhí)行每一句的。如果寫(xiě)成"a = 999; b = 999; a is b"或者分成多行放在一個(gè)源碼文件執(zhí)行，就可以看到輸出還是True。使用compiler.parse("a = 999\nb = 999\a is b")可以看出ast里有兩個(gè)Const(999), 應(yīng)該是在解釋執(zhí)行的時(shí)候做了相應(yīng)的優(yōu)化。

Python2.6.8/Objects/intobject.c

#define NSMALLPOSINTS           257
#define NSMALLNEGINTS           5
...
static PyIntObject *small_ints[NSMALLNEGINTS + NSMALLPOSINTS];
...
PyObject *PyInt_FromLong(long ival)
{
    register PyIntObject *v;
#if NSMALLNEGINTS + NSMALLPOSINTS > 0
    //解釋：如果 -5 <= ival && ival < 257
    if (-NSMALLNEGINTS <= ival && ival < NSMALLPOSINTS) {
        v = small_ints[ival + NSMALLNEGINTS];
        Py_INCREF(v);
#ifdef COUNT_ALLOCS
        if (ival >= 0)
            quick_int_allocs++;
        else 
            quick_neg_int_allocs++;
#endif
        return (PyObject *) v;
    }    
#endif
    if (free_list == NULL) {
        if ((free_list = fill_free_list()) == NULL)
            return NULL;
    }    
    /* Inline PyObject_New */
    v = free_list;
    free_list = (PyIntObject *)Py_TYPE(v);
    PyObject_INIT(v, &PyInt_Type);
    v->ob_ival = ival;
    return (PyObject *) v;
}