学步园

创建一个PyStringObject对象，通常Python会申请内存，创建一个PyStringObject对象。而若是申请很多个相同的对象，则会大量浪费内存。因此，Python引入了intern机制，在创建一个String对象的时候，先查找是否已经创建过内容相同的对象，如果有，则直接将已有的对象地址返回，否则就新创建。

void
PyString_InternInPlace(PyObject **p)
{
    register PyStringObject *s = (PyStringObject *)(*p);
    PyObject *t;
    if (s == NULL || !PyString_Check(s))
        Py_FatalError("PyString_InternInPlace: strings only please!");
    /* If it's a string subclass, we don't really know what putting
       it in the interned dict might do. */
    if (!PyString_CheckExact(s))
        return;
    if (PyString_CHECK_INTERNED(s))
        return;
    if (interned == NULL) {
        interned = PyDict_New();
        if (interned == NULL) {
            PyErr_Clear(); /* Don't leave an exception */
            return;
        }
    }
    t = PyDict_GetItem(interned, (PyObject *)s);
    if (t) {
        Py_INCREF(t);
        Py_DECREF(*p);
        *p = t;
        return;
    }

    if (PyDict_SetItem(interned, (PyObject *)s, (PyObject *)s) < 0) {
        PyErr_Clear();
        return;
    }
    /* The two references in interned are not counted by refcnt.
       The string deallocator will take care of this */
    Py_REFCNT(s) -= 2;
    PyString_CHECK_INTERNED(s) = SSTATE_INTERNED_MORTAL;
}

实际上，Python还是会创建这一个String对象，然而在创建后，若是存在相同的，则会将String的ob_refcn置为0，对象被析构。