In python : I have 2 pieces of code behaving different when I modify values:

X = Y = Number

x = y = 2
x = 3print(x)
print(y)Output:
3
2

X = Y = [List]

x = y = ['a','b']
x[0] = 'd'print(x)
print(y)Output:
['d', 'b']
['d', 'b']

Why don't LISTS respect the original value of variable Y if X is changed?

These two cases are more different than they seem, and it isn't because of the types involved.

x = 3

This changes what value x is looking at. Reassigning x using = does not change the 3. It wouldn't matter if the right hand side were a int, or a list. Whatever is on the right is put into the variable on the left.

x[0] = 'd'

This looks the same, but it's actually quite different. Instead of a simple reassignment, this is actually internally translated to

x.__setitem__(0, 'd')

Instead of being just a reassignment, this is actually a mutative operation that changes the object x is holding.

So the reason the latter acts differently is that instead of changing what object x is holding, you're altering the object itself that x is holding.

You can tell the two cases apart based on what's on the left of =. If the left is a simple variable name (x =), it's a plain reassignment. If the left side uses [] (x[0] =) or anything other kind of "accessor" syntax like a . (x.attr =), you're altering the object that x is holding.