Why is arr[0] * 7 changing all element of arr to [7]

Question

arr =[[]]*3 #[[],[],[]]
arr[0].append(7)
print(arr[0]) #[7]
print(arr) #[[7],[7],[7]]
try:
print(arr[1][0]) #7
except IndexError:
print("0")

Answer 1

[[]] * 3 creates a list with 3 identical references of an empty list []. You can change either of the 3 and changes will reflect in all.