这些列表有什么区别？

Question

In my intro computer science class we just finished writing a program to create a tic tac toe board and the way I made my game board was like this;

game_board = [[', ', '], [', ', '],[', ', ']]

I viewed similar problems on the internet and saw another way it was written like this

different_board = [[' '] * 3 for row in range(3)]

I was wondering how the second one would look compared to the first one if it were written out, would they be the same or would it look different?

Answer 1

You can just ask Python:

a = [[' ', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]
b = [[' '] * 3 for row in range(3)]
print(a == b)

Result:

True

Also, beware:

c = c = [[' '] * 3] * 3
d = [[' ' for _ in range(3)] for _ in range(3)]
print(a == c)
print(a == d)

They appear the same, but they are only similar - try modifying c and you'll see why:

c[0][1] = 'x'
print(c)

Result:

[[' ', 'x', ' '], [' ', 'x', ' '], [' ', 'x', ' ']]

It's three references to the same list! The definition of d is correct, but it's not immediately clear to most programmers why * 3 has that problem and for _ in range(3) would not, so I'd stay away from these "efficient" definitions.

And you could see what it looks like yourself:

print(b)

Result:

[[' ', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]