(a+=b) != (a=a+b)

You might be familiar with the compound operator in Python.

For those, who didn't.They are used to shorten the expressions.

Example

a = 2

If we want to increment the value of a by 1, we need to use the below notation.

a = a + 1

But, with the help of a compound operator, you can do it as below.

a += 1

Which is essentially equal to a=a+1 (umm.. not always)

There are some cases that don't follow this above rule.

Example

Consider the below example,

a = [1,2,3]
b = a
a += [4,5] # Compund operator
print(a)
print(b)

This will output,

[1,2,3,4,5]
[1,2,3,4,5]

That's what we expected right? Now here comes the twist, Now let's try without a compound operator, and let's see the output.

a = [1,2,3]
b = a
a = a + [4,5] # Without compound operator
print(a)
print(b)

This will output,

[1,2,3,4,5]
[1,2,3]

As you can see, the output is the same for the list a but the list b alters.

What's the reason?

a += [4,5]

In the above case, the list will be extended. So, as we assigned this list to another list, both will be changed.

But, when we use the below method. it creates a new list and appends the values. so it won't affect the list b

a = a + [4,5]