About cumulative assignment of lists and numpy arrays

Whether the object can be modified

Python list, in numpy array a = a + b ʻA + = b` (called cumulative assignment) It should be noted that the behavior of is different.

In Python cumulative assignment, perform in-place operations if possible. That is, if the contents of the object can be changed, a new object is not created.

Therefore, it is important whether the object is mutable or immutable. Numeric types are immutable objects, and lists are mutable objects.

Experiment with id function

There is an id function as a built-in function of Python. You can get the identification value of the object by using the id function. The identification value is guaranteed to be unique and constant for the life of the object. So, I will explain the difference between the two using this id function.

Numeric type

First of all, from ʻa + = 1`.

test1.py

a = 1
print (id(a))
a += 1
print (id(a))

test1.py execution result

1663828464
1663828496

It can be seen that the identification value of a has changed. In Python, numeric types are immutable objects, This is because the reference destination of a changes depending on the processing of ʻa + = 1. Next, let's look at ʻa = a + 1.

test2.py

a = 1
print (id(a))
a = a + 1
print (id(a))

test2.py execution result

1656226288
1656226320

After all, it can be seen that the identification value of a has changed. There should be no problem so far.

list

This is the main subject.

test3.py

a = [0,1,2]
b = [2,3,4]
print (id(a))
a  =  a + b
print (id(a))

test3.py execution result

53560776
53560200

The identification value of a has changed. next,

test4.py

a = [0,1,2]
b = [2,3,4]
print (id(a))
a  += b
print (id(a))

test4.py execution result

53560008
53560008

This time, the identification value of a has not changed.

numpy array

I will omit the experimental results for the numpy array because it will be tedious. The result is similar to a regular list. The official numpy tutorial also states:

Arithmetic operators on arrays apply elementwise. A new array is created and filled with the result.

Some operations, such as += and *=, act in place to modify an existing array rather than create a new one.

Summary

a = a + b a += b It should be noted that the description such as is not always equivalent.