用预留容量和缩容阈值避免列表反复搬迁

51    /* Bypass realloc() when a previous overallocation is large enough52       to accommodate the newsize.  If the newsize falls lower than half53       the allocated size, then proceed with the realloc() to shrink the list.54    */55    if (allocated >= newsize && newsize >= (allocated >> 1)) {56        assert(self->ob_item != NULL || newsize == 0);57        Py_SET_SIZE(self, newsize);58        return 0;59    }

71    new_allocated = ((size_t)newsize + (newsize >> 3) + 6) & ~(size_t)3;72    /* Do not overallocate if the new size is closer to overallocated size73     * than to the old size.74     */75    if (newsize - Py_SIZE(self) > (Py_ssize_t)(new_allocated - newsize))76        new_allocated = ((size_t)newsize + 3) & ~(size_t)3;7778    if (newsize == 0)79        new_allocated = 0;80    if (new_allocated <= (size_t)PY_SSIZE_T_MAX / sizeof(PyObject *)) {81        num_allocated_bytes = new_allocated * sizeof(PyObject *);82        items = (PyObject **)PyMem_Realloc(self->ob_item, num_allocated_bytes);