why "a+=b" and "a = a+b" has different output in below codes?

Question

def test1(a,b):
#a = a + b
a += b
A = [11,22]
B = [33]

test1(A,B)
print(A)

It will be [11,22,33] with a +=b
But will be [11,22] with a = a+b

Answer 1

Because when you use
a = a + b
The compiler creates a new variable to store the value of a + b
And because test1 is not returned, the variable a in the global scope is not affected

But when you run
a += b
the value of a + b is stored in the same variable a, because the argument has been modified, the variable in global scope of a is affected.

You can use id() to check if they are the same variable, like in this demo:
https://code.sololearn.com/c8uqMqsiZxv9/?ref=app