Списки - изменяемые объекты. Поэтом, при присваивани другой переменной, списки нужно обязательно копировать. Либо с помощью модуля copy, либо через slice
first = []
second = []
first.append(1)
print(f'1) First: {first} Second: {second} ')
second = first
print(f'2) First: {first} Second: {second} ')<code></code>
first.append(2)
print(f'3) First: {first} Second: {second} ')
1) First: [1] Second: []
2) First: [1] Second: [1]
3) First: [1, 2] Second: [1, 2]
Process finished with exit code 0
А теперь скопируем
first = []
second = []
first.append(1)
print(f'1) First: {first} Second: {second} ')
second = first[:]
print(f'2) First: {first} Second: {second} ')
first.append(2)
print(f'3) First: {first} Second: {second} ')
1) First: [1] Second: []
2) First: [1] Second: [1]
3) First: [1, 2] Second: [1]
Process finished with exit code 0