Effect of Update v.s. Reassign in Python

This post demonstrates behaviors of reassgin(e.g. x = x + 1) v.s. update (e.g. x += 1) in Python.

For primative types (e.g., int), both reassign and update get a new id.

x = 1
old_id = id(x)
x = x + 1
new_id = id(x)
old_id == new_id
False
x = 1
old_id = id(x)
x += 1
new_id = id(x)
old_id == new_id
False

But for a list, only update gets a new id, but not reassign.

x = [1]
old_id = id(x)
x += [2]
new_id = id(x)
x, old_id == new_id
([1, 2], True)
x = [1]
old_id = id(x)
x = x + [2]
new_id = id(x)
x, old_id == new_id
([1, 2], False)
Need to be aware of this difference to make list behave as expected. Compare below:
​
Reassign `x`, where `y` is not updated:
x = [1]
y = [x]
print(y)
x = x + [2]
print(y)
[[1]]
[[1]]
Update `x`, where `y` is also updated:
x = [1]
y = [x]
print(y)
x += [2]
print(y)
[[1]]
[[1, 2]]
Same comparison for set in stead of list:

Reassign:

x = set([1])
y = [x]
print(y)
x = x | set([1])
print(y)
[{1}]
[{1}]
Update:
x = set([1])
y = [x]
print(y)
x |= set([2])
print(y)
[{1}]
[{1, 2}]

Code is here: https://github.com/yang-zhang/yang-zhang.github.io/blob/master/coding/reassign_vs_update.ipynb