为什么空格会影响相等字符串的身份比较？

Question

我注意到为相同的字符串添加空格使得它们比较不相等is,而非空间版本比较相等.

a = 'abc'
b = 'abc'
a is b
#outputs: True

a = 'abc abc'
b = 'abc abc'
a is b
#outputs: False

我已经阅读了关于将字符串与==和进行比较的问题is.我认为这是一个不同的问题,因为空格字符正在改变行为,而不是字符串的长度.看到:

a = 'abc'
b = 'abc'
a is b # True

a = 'gfhfghssrtjyhgjdagtaerjkdhhgffdhfdah'
b = 'gfhfghssrtjyhgjdagtaerjkdhhgffdhfdah'
a is b # True

为什么在字符串中添加空格会改变此比较的结果？

Answer 1

python解释器根据某些条件缓存一些字符串,第一个abc字符串被缓存并用于两者,但第二个字符串不是.它是由小整数同-5来256.

由于字符串被扣留/高速缓存分配a和b对"abc"品牌a和b因此使用指向相同的对象在内存中is,检查如果两个对象实际上是相同的对象,返回True.

第二个字符串abc abc未缓存,因此它们是内存中两个完全不同的对象,因此使用is返回进行身份检查False.这一次a是不是 b.它们都指向内存中的不同对象.

In [43]: a = "abc" # python caches abc
In [44]: b = "abc" # it reuses the object when assigning to b
In [45]: id(a)
Out[45]: 139806825858808    # same id's, same object in memory
In [46]: id(b)
Out[46]: 139806825858808    
In [47]: a = 'abc abc'   # not cached  
In [48]: id(a)
Out[48]: 139806688800984    
In [49]: b = 'abc abc'    
In [50]: id(b)         # different id's different objects
Out[50]: 139806688801208

缓存字符串的条件是字符串在字符串中只有字母,下划线和数字,因此在您的情况下,空格不符合条件.

使用解释器有一种情况,即使字符串不符合上述条件,多个赋值,您也可以最终指向同一个对象.

In [51]: a,b  = 'abc abc','abc abc'

In [52]: id(a)
Out[52]: 139806688801768

In [53]: id(b)
Out[53]: 139806688801768

In [54]: a is b
Out[54]: True

查看 codeobject.c来决定我们看到的标准来NAME_CHARS决定可以实现的内容:

#define NAME_CHARS \
    "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz"

/* all_name_chars(s): true iff all chars in s are valid NAME_CHARS */

static int
all_name_chars(unsigned char *s)
{
    static char ok_name_char[256];
    static unsigned char *name_chars = (unsigned char *)NAME_CHARS;

    if (ok_name_char[*name_chars] == 0) {
        unsigned char *p;
        for (p = name_chars; *p; p++)
            ok_name_char[*p] = 1;
    }
    while (*s) {
        if (ok_name_char[*s++] == 0)
            return 0;
    }
    return 1;
}

我们可以PyString_FromStringAndSize在stringobject.c源代码中看到长度为0或1的字符串.

/* share short strings */
    if (size == 0) {
        PyObject *t = (PyObject *)op;
        PyString_InternInPlace(&t);
        op = (PyStringObject *)t;
        nullstring = op;
        Py_INCREF(op);
    } else if (size == 1 && str != NULL) {
        PyObject *t = (PyObject *)op;
        PyString_InternInPlace(&t);
        op = (PyStringObject *)t;
        characters[*str & UCHAR_MAX] = op;
        Py_INCREF(op);
    }
    return (PyObject *) op;
}

不直接相关的问题,但对于那些有兴趣PyCode_New从还codeobject.c源展示了如何更弦建设codeobject时一旦弦满足标准被拘留all_name_chars.

PyCodeObject *
PyCode_New(int argcount, int nlocals, int stacksize, int flags,
       PyObject *code, PyObject *consts, PyObject *names,
       PyObject *varnames, PyObject *freevars, PyObject *cellvars,
       PyObject *filename, PyObject *name, int firstlineno,
       PyObject *lnotab)
{
    PyCodeObject *co;
    Py_ssize_t i;
    /* Check argument types */
    if (argcount < 0 || nlocals < 0 ||
        code == NULL ||
        consts == NULL || !PyTuple_Check(consts) ||
        names == NULL || !PyTuple_Check(names) ||
        varnames == NULL || !PyTuple_Check(varnames) ||
        freevars == NULL || !PyTuple_Check(freevars) ||
        cellvars == NULL || !PyTuple_Check(cellvars) ||
        name == NULL || !PyString_Check(name) ||
        filename == NULL || !PyString_Check(filename) ||
        lnotab == NULL || !PyString_Check(lnotab) ||
        !PyObject_CheckReadBuffer(code)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    intern_strings(names);
    intern_strings(varnames);
    intern_strings(freevars);
    intern_strings(cellvars);
    /* Intern selected string constants */
    for (i = PyTuple_Size(consts); --i >= 0; ) {
        PyObject *v = PyTuple_GetItem(consts, i);
        if (!PyString_Check(v))
            continue;
        if (!all_name_chars((unsigned char *)PyString_AS_STRING(v)))
            continue;
        PyString_InternInPlace(&PyTuple_GET_ITEM(consts, i));
    }

这个答案基于使用cpython解释器的简单赋值,就函数或简单赋值之外的任何其他功能而言是实际的,没有被提及也没有被回答.

如果对c代码有更深入理解的人有任何添加的内容,请随时编辑.

还有一个更详尽的解释这里的整串实习的.