Why is this different?

Question

Scenario 1

y = [1, 2, 3]

for x in y:
y = y + [x] #note this line
print(y)

Output: [1, 2, 3, 1][1, 2, 3, 1, 2][1, 2, 3, 1, 2, 3]

Scenario 2

y = [1, 2, 3]

for x in y:
y += [x] #note this line
print(y)

Outputs: [1, 2, 3, 1][1, 2, 3, 1, 2][1, 2, 3, 1, 2, 3][1, 2, 3, 1, 2, 3, 1] .... so on, to infinity

Answer 1

y = [ 1, 2, 3 ]
print( id( y ), '<-- Original list' )
for x in y:
y = y + [ x ]
print( id( y ) )

# Here a new `list` object named <y> is created in each loop iteration. Thus the original `list` <y> remains intact.

print()

y = [ 1, 2, 3 ]
print( id( y ), '<-- Original list' )
for x in y:
y += [ x ]
print( id( y ) )

# Here original `list` <y> is updated by adding new element in each loop iteration.

Because we're adding new items, each update makes the `for...in` loop extends the loop range (due to <y> growing longer), and this in turn, turn it a close-to-infinite loop which will *probably* stop only when the system runs out of memory to allocate for <y> growth.

Answer 2

I think the difference is between __add__ and __iadd__ magic methods of list.

__iadd__ (means in-place add) mutates current list, and __add__ returns new list.

In scenario 2, the expression of y += [x] calls __iadd__ and therefore y list updates every time.

Answer 3

Both will give the same output their is no difference
+= Is same as
= +
(a+=b ) == (a+b) //

Answer 4

+ is an arithmetic operator

+= is an assignment operator