Python中的内存管理之python list内存使用详解

第Python中的内存管理之pythonlist内存使用详解使用Python的时候，我们知道list是一个长度可变对的数组，可以通过insert，append和extend轻易的拓展其中的元素个数。也可以使用运算符如：[1]+[2]生成新的数组[1,2]

extend()、"+"、"+="的区别

"+"将两个list相加，会返回到一个新的list对象

append在原list上进行修改，没有返回值

从以下代码可以看到，调用b=b+[3,4]之后，通过id(b)查看b变成了一个新对象。

In[5]:b=[1,2]

In[6]:id(b)

Out[6]:1628740249224

In[7]:b=b+[3,4]

In[8]:id(b)

Out[8]:1628740456520

使用extend()完成相同的步骤，可以看到对象c的id保持和原来的一致

In[9]:c=[1,2]

In[10]:id(c)

Out[10]:1628740392584

In[11]:c.extend([3,4])

In[12]:id(c)

Out[12]:1628740392584

使用"+="连接列表，看到效果和extend()是相同的。

In[1]:a=[1,2]

In[2]:id(a)

Out[2]:1628740021448

In[3]:a+=[3,4]

In[4]:id(a)

Out[4]:1628740021448

结论：减少内存的拷贝，修改一个列表的数据时，应避免使用list1=list1+list2这样的语法。

List的内存使用

一个示例：

In[1]:importsys

In[2]:lst1=[1]

In[3]:lst2=[]

In[4]:lst2.append(1)

In[5]:lst1==lst2

Out[5]:True

In[6]:sys.getsizeof(lst1)

Out[6]:72

In[7]:sys.getsizeof(lst2)

Out[7]:96

可以看到，lst1==lst2，但是当使用sys.getsizeof获取对象的内存大小时，两者却是不同的。

如下图所示，list_a长度为4，当执行append(4)时，底层的数据长度其实申请了4个元素的空间，当再次执行append(5)的时候，不需要再次申请内存。

因为执行append()操作时，Python将一次拓展N个元素的内存，因为一个append操作很可能是很多append操作的开始，通过额外分配内存来减少可能的内存分配和内存copy的次数。

In[1]:importsys

In[2]:l=[]

...:print(f'listinitialsize{sys.getsizeof(l)}')

...:foriinrange(80):

...:cur_size=sys.getsizeof(l)

...:l.append(i)

...:new_size=sys.getsizeof(l)

...:print(f'listlen{i+1}:\tcurrent_size{new_size}\tnew_allocated8*{(new_size-cur_size)/8}')

...:

listinitialsize64

listlen1:current_size96new_allocated8*4.0

listlen2:current_size96new_allocated8*0.0

listlen3:current_size96new_allocated8*0.0

listlen4:current_size96new_allocated8*0.0

listlen5:current_size128new_allocated8*4.0

listlen6:current_size128new_allocated8*0.0

listlen7:current_size128new_allocated8*0.0

listlen8:current_size128new_allocated8*0.0

listlen9:current_size192new_allocated8*8.0

listlen10:current_size192new_allocated8*0.0

listlen11:current_size192new_allocated8*0.0

listlen12:current_size192new_allocated8*0.0

listlen13:current_size192new_allocated8*0.0

listlen14:current_size192new_allocated8*0.0

listlen15:current_size192new_allocated8*0.0

listlen16:current_size192new_allocated8*0.0

listlen17:current_size264new_allocated8*9.0

listlen18:current_size264new_allocated8*0.0

listlen19:current_size264new_allocated8*0.0

listlen20:current_size264new_allocated8*0.0

listlen21:current_size264new_allocated8*0.0

listlen22:current_size264new_allocated8*0.0

listlen23:current_size264new_allocated8*0.0

listlen24:current_size264new_allocated8*0.0

listlen25:current_size264new_allocated8*0.0

listlen26:current_size344new_allocated8*10.0

listlen27:current_size344new_allocated8*0.0

listlen28:current_size344new_allocated8*0.0

listlen29:current_size344new_allocated8*0.0

listlen30:current_size344new_allocated8*0.0

listlen31:current_size344new_allocated8*0.0

listlen32:current_size344new_allocated8*0.0

listlen33:current_size344new_allocated8*0.0

listlen34:current_size344new_allocated8*0.0

listlen35:current_size344new_allocated8*0.0

listlen36:current_size432new_allocated8*11.0

listlen37:current_size432new_allocated8*0.0

listlen38:current_size432new_allocated8*0.0

listlen39:current_size432new_allocated8*0.0

listlen40:current_size432new_allocated8*0.0

listlen41:current_size432new_allocated8*0.0

listlen42:current_size432new_allocated8*0.0

listlen43:current_size432new_allocated8*0.0

listlen44:current_size432new_allocated8*0.0

listlen45:current_size432new_allocated8*0.0

listlen46:current_size432new_allocated8*0.0

listlen47:current_size528new_allocated8*12.0

listlen48:current_size528new_allocated8*0.0

listlen49:current_size528new_allocated8*0.0

listlen50:current_size528new_allocated8*0.0

listlen51:current_size528new_allocated8*0.0

listlen52:current_size528new_allocated8*0.0

listlen53:current_size528new_allocated8*0.0

listlen54:current_size528new_allocated8*0.0

listlen55:current_size528new_allocated8*0.0

listlen56:current_size528new_allocated8*0.0

listlen57:current_size528new_allocated8*0.0

listlen58:current_size528new_allocated8*0.0

listlen59:current_size640new_allocated8*14.0

listlen60:current_size640new_allocated8*0.0

listlen61:current_size640new_allocated8*0.0

listlen62:current_size640new_allocated8*0.0

listlen63:current_size640new_allocated8*0.0

listlen64:current_size640new_allocated8*0.0

listlen65:current_size640new_allocated8*0.0

listlen66:current_size640new_allocated8*0.0

listlen67:current_size640new_allocated8*0.0

listlen68:current_size640new_allocated8*0.0

listlen69:current_size640new_allocated8*0.0

listlen70:current_size640new_allocated8*0.0

listlen71:current_size640new_allocated8*0.0

listlen72:current_size640new_allocated8*0.0

listlen73:current_size768new_allocated8*16.0

listlen74:current_size768new_allocated8*0.0

listlen75:current_size768new