my_list = [42, 42, 18, 73, 11, 28, 29, 0, 10, 16, 22, 53, 41]

print("The list is :")
print(my_list)

i, j = 4, 8
my_key = 9

my_list[i:j] = [my_key] * (j - i)

print("The result is:")
print(my_list)

The list is :
[42, 42, 18, 73, 11, 28, 29, 0, 10, 16, 22, 53, 41]
The result is:
[42, 42, 18, 73, 9, 9, 9, 9, 10, 16, 22, 53, 41]