Python常见面试题010. Python的整形占多大内存?

010. Python的整形占多大内存?

示例代码

import sys
a = 0
print(a.__sizeof__())  # 24
print(sys.getsizeof(a)) # 24

• 所以答案是24？并不是！看下面

import sys
b = 1
print(b.__sizeof__()) # 28
print(sys.getsizeof(b)) # 28

• 变了
• 难道都会变？

import sys
c = 3
print(c.__sizeof__()) # 28
print(sys.getsizeof(c))   # 28

• 不是你想的那样的！
• 但的确是动态的

import sys
d = 1073741823
e = 1073741824
print(d.__sizeof__()) # 28
print(sys.getsizeof(e)) # 32

• 有点规律了，1073741824是什么鬼？它是2**30
• 实际上，python的int类型就是动态的，每230增加4个字节**
• 那下一个边界是？2**60！

print(sys.getsizeof(2**60))  # 36

• 那其他类型呢？可以参考附录

sizeof()

• 返回内存中的大小，单位字节

 |  __sizeof__(self, /)
 |      Returns size in memory, in bytes.

getsizeof()

• 这是sys模块的一个方法
• 在pycharm中你只能看到如下内容，不过大致也是返回对象的大小，单位是字节

def getsizeof(p_object, default): # real signature unknown; restored from __doc__
    """
    getsizeof(object, default) -> int
    
    Return the size of object in bytes.
    """
    return 0

• 2个貌似一样

• 其实不然

  b = []
  print(b.__sizeof__())  # 40
  print(sys.getsizeof(b))   # 56 不一样(我在jupyter中执行的结果) # 如果你在pycharm中执行可能此处是 64
  

import sys
l = []
w =[1, 2]
x =[4, 5, 7, 9]
y =[2, 8, 6, 56, 45, 89, 88]

print('sizeof:%d,getsize:%d' %(l.__sizeof__(),sys.getsizeof(l))) 
print('sizeof:%d,getsize:%d' %(w.__sizeof__(),sys.getsizeof(w)))
print('sizeof:%d,getsize:%d' %(x.__sizeof__(),sys.getsizeof(x)))

# sizeof:40,getsize:64 # 此处就是在pycharm中执行的
# sizeof:56,getsize:80
# sizeof:72,getsize:96

• getsizeof() 方法调用__sizeof__方法，但同时会附带一些额外的GC操作(arbage collector overhead). 因此前者的大小比后者要大一些

• 列表初始化的时候是40，每加一个元素是8个字节

• 代码大了之后，内存管理就显得非常重要了，现在仅作了解

附录

https://stackoverflow.com/questions/449560/how-do-i-determine-the-size-of-an-object-in-python

• 对于其他类型，有人做了一些测试

• 基于 64-bit Python 3.6，使用Anaconda

  Empty
  Bytes  type        scaling notes
  28     int         +4 bytes about every 30 powers of 2
  37     bytes       +1 byte per additional byte
  49     str         +1-4 per additional character (depending on max width)
  48     tuple       +8 per additional item
  64     list        +8 for each additional
  224    set         5th increases to 736; 21nd, 2272; 85th, 8416; 341, 32992
  240    dict        6th increases to 368; 22nd, 1184; 43rd, 2280; 86th, 4704; 171st, 9320
  136    func def    does not include default args and other attrs
  1056   class def   no slots 
  56     class inst  has a __dict__ attr, same scaling as dict above
  888    class def   with slots
  16     __slots__   seems to store in mutable tuple-like structure
                     first slot grows to 48, and so on.