Python 标准类型比较

请看下面的例子：

>>> a=1
>>> b=1
>>> a is b
True
>>> c=1.0
>>> d=1.0
>>> c is d
False

是不是非常奇怪，下面解释原因：

上面的例子a,b指向了同一个整形对象，但是c,d却不是只想同一对象，原因在于

整形和字符串对象是不可变对象，Python高效的缓存这些整形和字符串，Python应用程序中经常会用到这些小型整数，因此Python会缓存这些整数

源码如下：

#include "Python.h"
#include "longintrepr.h"

#include <float.h>
#include <ctype.h>
#include <stddef.h>

#include "clinic/longobject.c.h"
/*[clinic input]
class int "PyObject *" "&PyLong_Type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=ec0275e3422a36e3]*/

#ifndef NSMALLPOSINTS
#define NSMALLPOSINTS           257
#endif
#ifndef NSMALLNEGINTS
#define NSMALLNEGINTS           5
#endif

_Py_IDENTIFIER(little);
_Py_IDENTIFIER(big);

/* convert a PyLong of size 1, 0 or -1 to an sdigit */
#define MEDIUM_VALUE(x) (assert(-1 <= Py_SIZE(x) && Py_SIZE(x) <= 1),   
         Py_SIZE(x) < 0 ? -(sdigit)(x)->ob_digit[0] :   
             (Py_SIZE(x) == 0 ? (sdigit)0 :                             
              (sdigit)(x)->ob_digit[0]))

PyObject *_PyLong_Zero = NULL;
PyObject *_PyLong_One = NULL;





/* Create a new int object from a C long int */

PyObject *
PyLong_FromLong(long ival)
{
    PyLongObject *v;
    unsigned long abs_ival;
    unsigned long t;  /* unsigned so >> doesn't propagate sign bit */
    int ndigits = 0;
    int sign;

    CHECK_SMALL_INT(ival);

    if (ival < 0) {
        /* negate: can't write this as abs_ival = -ival since that
           invokes undefined behaviour when ival is LONG_MIN */
        abs_ival = 0U-(unsigned long)ival;
        sign = -1;
    }
    else {
        abs_ival = (unsigned long)ival;
        sign = ival == 0 ? 0 : 1;
    }

    /* Fast path for single-digit ints */
    if (!(abs_ival >> PyLong_SHIFT)) {
        v = _PyLong_New(1);
        if (v) {
            Py_SIZE(v) = sign;
            v->ob_digit[0] = Py_SAFE_DOWNCAST(
                abs_ival, unsigned long, digit);
        }
        return (PyObject*)v;
    }

#if PyLong_SHIFT==15
    /* 2 digits */
    if (!(abs_ival >> 2*PyLong_SHIFT)) {
        v = _PyLong_New(2);
        if (v) {
            Py_SIZE(v) = 2*sign;
            v->ob_digit[0] = Py_SAFE_DOWNCAST(
                abs_ival & PyLong_MASK, unsigned long, digit);
            v->ob_digit[1] = Py_SAFE_DOWNCAST(
                  abs_ival >> PyLong_SHIFT, unsigned long, digit);
        }
        return (PyObject*)v;
    }
#endif

    /* Larger numbers: loop to determine number of digits */
    t = abs_ival;
    while (t) {
        ++ndigits;
        t >>= PyLong_SHIFT;
    }
    v = _PyLong_New(ndigits);
    if (v != NULL) {
        digit *p = v->ob_digit;
        Py_SIZE(v) = ndigits*sign;
        t = abs_ival;
        while (t) {
            *p++ = Py_SAFE_DOWNCAST(
                t & PyLong_MASK, unsigned long, digit);
            t >>= PyLong_SHIFT;
        }
    }
    return (PyObject *)v;
}

（范围在（-5,257）），再有新的引用时，并不创建新的对象。