List Reference

a = [
    1, 2, 3, 4
]

b = a

b

[1, 2, 3, 4]

b.append(5)

a

[1, 2, 3, 4, 5]

As b is referring to a, whatever is added to b, will be reflected to a as well

If you don’t want such behavior, just use .copy() or a[:]

c = a.copy()

c.append(6)

a

[1, 2, 3, 4, 5]