local/global variables in functions

[abccba]

x = 1


def f1(y):
    y += 1
    return y


x1 = f1(x)
print('x = ', x, '  x1 = ', x1)

answer:
x = 1 x1 = 2

x_list = ['1','2']


def f2(y_list):
    y_list[0] = '0'
    return y_list


x1_list = f2(x_list)
print('x_list = ', x_list, '  x1_list = ', x1_list)

answer:
x_list = ['0', '2'] x1_list = ['0', '2']

why in the first case the global variable is preserved and not in the second case?