Why the List is [[7],[7],[7]],not[[7],[0],[0]]?

Question

this is a code questions which I originally meet when I challenge other in sololearn(I add some code wanting to figure out the list)I wonder that why the List is [[7],[7],[7]],not[[7],[0],[0]]?thanks~

arr=[[]]*3
print(arr)#output is [[],[],[]]
arr[0].append(7)
print (arr)#output is [[7],[7],[7]] #why not [[7]],[],[]]
try:
print(arr[1][0])
except IndexError:
print("0")

Answer 1

When you create a two dimensional array in python like the following
arr=[[]]*3

All the three rows refer to single row in memory

Row 0-->[ X, X, X]
Row 1 --|
Row 2--|

So if we assign a value to any row, it will be affected in all other rows.

To avoid this , you can create a matrix like the following.
columns = 3
rows = 3
mat = [ [0 for i in range(columns)] for j in range(rows)]
mat[0][0] = 1 # Will not affect other rows
print(mat([1][0]) # Prints Zero

Answer 2

thank you very much!!!