Python - How come zzz[0] += [[]] is not the same as zzz[0]= zzz[0] + [[]] ?

Question

I'm comparing two chunks of code:

Case 1:

zzz = [[]] *3
print(zzz) # outputs [[], [], []]
zzz[0]= zzz[0] + [[]]
print(zzz) # outputs [[[]], [], []]

Case 2:

zzz = [[]] *3
print(zzz) # outputs [[], [], []]
zzz[0] += [[]]
print(zzz) # outputs [[[]], [[]], [[]]]

In the second case, why does zzz[0] += [[]] add a pair of square brackets in EACH item instead of just the first one?

Answer 1

The following link explains well about how list+=item and list=list+item works

https://stackoverflow.com/questions/2347265/why-does-behave-unexpectedly-on-lists

when you do [[]]*3 it creates 3 list object pointing to same location ,so [][][] are all pointing to same location ,and change to anyone one of them will effect other as well

In zzz[0]=zzz[0]+[[]], we are adding new item to the first list of zzz ,doing this way calls add method which creates a new list object so zzz[0] now is assigned a different list object ,and since this Operation created a new list object other two lists aren't affected at all

But doing this zzz[0]+=[[]] calls iadd method which changes the original list data instead of creating a new object ,so you must know by now what will happen to other two list object ,since they all were pointing to same location you get output as [[[]],[[]],[[]]]

Answer 2

The + operator tries to call the __add__ dunder method. <-- Case 1
The += operator tries to call the __iadd__ dunder method. <-- Case 2

If __iadd__ is unavailable, __add__ is used instead.
--> For mutable types like lists, the += changes the original object's value
--> For immutable types like tuples/strings/integers, a NEW object is returned
(ie. a += b becomes equivalent to a = a + b)

In the case of zzz above, it is a mutable type with three objects pointing to same location.