list object
- 可变对象头
- ob_item: 指向分配的空间，元素是指针
- allocated: 真实容量.
Objects/listobjects.c
/* Ensure ob_item has room for at least newsize elements, and set
* ob_size to newsize. If newsize > ob_size on entry, the content
* of the new slots at exit is undefined heap trash; it's the caller's
* responsibility to overwrite them with sane values.
* The number of allocated elements may grow, shrink, or stay the same.
* Failure is impossible if newsize <= self.allocated on entry, although
* that partly relies on an assumption that the system realloc() never
* fails when passed a number of bytes <= the number of bytes last
* allocated (the C standard doesn't guarantee this, but it's hard to
* imagine a realloc implementation where it wouldn't be true).
* Note that self->ob_item may change, and even if newsize is less
* than ob_size on entry.
*/
static int
list_resize(PyListObject *self, Py_ssize_t newsize)
{
PyObject **items;
size_t new_allocated, num_allocated_bytes;
Py_ssize_t allocated = self->allocated;
/* Bypass realloc() when a previous overallocation is large enough
to accommodate the newsize. If the newsize falls lower than half
the allocated size, then proceed with the realloc() to shrink the list.
*/
if (allocated >= newsize && newsize >= (allocated >> 1)) {
assert(self->ob_item != NULL || newsize == 0);
Py_SET_SIZE(self, newsize);
return 0;
}
/* This over-allocates proportional to the list size, making room
* for additional growth. The over-allocation is mild, but is
* enough to give linear-time amortized behavior over a long
* sequence of appends() in the presence of a poorly-performing
* system realloc().
* Add padding to make the allocated size multiple of 4.
* The growth pattern is: 0, 4, 8, 16, 24, 32, 40, 52, 64, 76, ...
* Note: new_allocated won't overflow because the largest possible value
* is PY_SSIZE_T_MAX * (9 / 8) + 6 which always fits in a size_t.
*/
new_allocated = ((size_t)newsize + (newsize >> 3) + 6) & ~(size_t)3;
/* Do not overallocate if the new size is closer to overallocated size
* than to the old size.
*/
if (newsize - Py_SIZE(self) > (Py_ssize_t)(new_allocated - newsize))
new_allocated = ((size_t)newsize + 3) & ~(size_t)3;
if (newsize == 0)
new_allocated = 0;
num_allocated_bytes = new_allocated * sizeof(PyObject *);
items = (PyObject **)PyMem_Realloc(self->ob_item, num_allocated_bytes);
if (items == NULL) {
PyErr_NoMemory();
return -1;
}
self->ob_item = items;
Py_SET_SIZE(self, newsize);
self->allocated = new_allocated;
return 0;
}
list resize
import sys
def print_list_size(nums=None):
nums_size = sys.getsizeof(nums)
if nums:
print(f"id({id(nums)}) v:{nums[-1]} len:{len(nums)} size:{nums_size}")
else:
print(f"id({id(nums)}) v:{nums} len:{len(nums)} size:{nums_size}")
def test_0_4():
data = []
print_list_size(nums=data)
for i in range(1, 35):
data.append(i)
print_list_size(nums=data)
test_0_4()
def resize_list(x):
return (x + (x >> 3) + 6) & (~3)
"""
## 3.9
Add padding to make the allocated size multiple of 4
The growth pattern is: 0, 4, 8, 16, 24, 32, 40, 52, 64, 76
https://github.com/python/cpython/blob/3.9/Objects/listobject.c
new_allocated = ((size_t)newsize + (newsize >> 3) + 6) & ~(size_t)3;
id(4554271168) v:[] len:0 size:56
id(4554271168) v:1 len:1 size:88
id(4554271168) v:2 len:2 size:88
id(4554271168) v:3 len:3 size:88
id(4554271168) v:4 len:4 size:88
id(4554271168) v:5 len:5 size:120
id(4554271168) v:6 len:6 size:120
id(4554271168) v:7 len:7 size:120
id(4554271168) v:8 len:8 size:120
id(4554271168) v:9 len:9 size:184
id(4554271168) v:10 len:10 size:184
id(4554271168) v:11 len:11 size:184
id(4554271168) v:12 len:12 size:184
id(4554271168) v:13 len:13 size:184
id(4554271168) v:14 len:14 size:184
id(4554271168) v:15 len:15 size:184
id(4554271168) v:16 len:16 size:184
id(4554271168) v:17 len:17 size:248
id(4554271168) v:18 len:18 size:248
id(4554271168) v:19 len:19 size:248
id(4539054528) v:20 len:20 size:248
id(4539054528) v:21 len:21 size:248
id(4539054528) v:22 len:22 size:248
id(4539054528) v:23 len:23 size:248
id(4539054528) v:24 len:24 size:248
id(4506298816) v:25 len:25 size:312
id(4506298816) v:26 len:26 size:312
id(4506298816) v:27 len:27 size:312
id(4506298816) v:28 len:28 size:312
id(4506298816) v:29 len:29 size:312
id(4506298816) v:30 len:30 size:312
id(4506298816) v:31 len:31 size:312
id(4506298816) v:32 len:32 size:312
id(4506298816) v:33 len:33 size:376
id(4506298816) v:34 len:34 size:376
## 3.7/3.8
The growth pattern is: 0, 4, 8, 16, 25, 35, 46, 58, 72, 88
https://github.com/python/cpython/blob/3.7/Objects/listobject.c
https://github.com/python/cpython/blob/3.8/Objects/listobject.c
new_allocated = (size_t)newsize + (newsize >> 3) + (newsize < 9 ? 3 : 6);
(py3) ➜ stu git:(hyh_debug_01_main) ✗ python --version
Python 3.7.4
(py3) ➜ stu git:(hyh_debug_01_main) ✗ python test_list_resize.py
id(4538606048) v:[] len:0 size:72
id(4538606048) v:1 len:1 size:104
id(4538606048) v:2 len:2 size:104
id(4538606048) v:3 len:3 size:104
id(4538606048) v:4 len:4 size:104 4个空间使用完. 增加4
id(4538606048) v:5 len:5 size:136
id(4538606048) v:6 len:6 size:136
id(4538606048) v:7 len:7 size:136
id(4538606048) v:8 len:8 size:136 8个空间使用完. 增加4
id(4538606048) v:9 len:9 size:200
id(4538606048) v:10 len:10 size:200
id(4538606048) v:11 len:11 size:200
id(4538606048) v:12 len:12 size:200
id(4538606048) v:13 len:13 size:200
id(4538606048) v:14 len:14 size:200
id(4538606048) v:15 len:15 size:200
id(4538606048) v:16 len:16 size:200 16个空间使用完. 增加8
id(4538606048) v:17 len:17 size:272
id(4538606048) v:18 len:18 size:272
id(4538606048) v:19 len:19 size:272
id(4538606048) v:20 len:20 size:272
id(4538606048) v:21 len:21 size:272
id(4538606048) v:22 len:22 size:272
id(4538606048) v:23 len:23 size:272
id(4538606048) v:24 len:24 size:272
id(4538606048) v:25 len:25 size:272 25个空间使用完. 增加9
id(4538606048) v:26 len:26 size:352
id(4538606048) v:27 len:27 size:352
id(4538606048) v:28 len:28 size:352
id(4538606048) v:29 len:29 size:352
id(4538606048) v:30 len:30 size:352
id(4538606048) v:31 len:31 size:352
id(4538606048) v:32 len:32 size:352
id(4538606048) v:33 len:33 size:352
id(4538606048) v:34 len:34 size:352
# 关于size不一致问题，_typeobject 类型对象结构体在py3.9中删除了部分元素比3.8更小
"""
来源：oschina
链接：https://my.oschina.net/tplinuxhyh/blog/4495519