Difference between __sizeof__() and getsizeof() method – Python

Memory management is of utmost priority when we write large chunks of code. So in addition to good coding knowledge, it is important to be able to write programs, so as to handle memory efficiently. 

So let us look at the two ways of getting the size of a particular object in Python. These are getsizeof() method and __sizeof() method. 
The getsizeof() is a system-specific method and hence we have to import the sys module to use it. A sample code is as shown below for calculating the size of a list.

Python3

import sys
a =[1, 2]
b =[1, 2, 3, 4]
c =[1, 2, 3, 4]
d =[2, 3, 1, 4, 66, 54, 45, 89]
print(sys.getsizeof(a))
print(sys.getsizeof(b))
print(sys.getsizeof(c))
print(sys.getsizeof(d))

Output: 

80
96
96
128

getsizeof() method calls the __sizeof__() method o the object with an additional garbage collector overhead. Hence the size returned by the getsize() method will be more than that returned by the __sizeof()__ method. 
For example, the getsizeof() method returns 64 bytes for an empty list and then 8 bytes for every additional element.
Now let’s look at the __sizeof__() method. It returns the size of the object without any overhead.

Python3

w =[1, 2]
x =[4, 5, 7, 9]
y =[2, 8, 6, 56, 45, 89, 88]
z =[54, 45, 12, 23, 24, 90, 20, 40]
print(w.__sizeof__())
print(x.__sizeof__())
print(y.__sizeof__())
print(z.__sizeof__())

Output:

56
72
96
104

For example, the __sizeof__() method returns 40 bytes for an empty list and then 8 bytes for every additional element.