Why this append function in python didn't work out?

Question

def random_row(board, length):
    return randint(0, len(board) - length)


def random_col(board, length):
    return randint(0, len(board[0]) - length)

ship_all = []

for length in range(1,6):
    ship = []
    ship_row = random_row(board, length)
    ship.append(ship_row)
    ship_col = random_col(board, length)
    ship.append(ship_col)
    i = 0
    if randint(0,1) == 0:
        while i<length:
            ship_all.append(ship)
            ship[0] += 1
            print ship
            i+=1
    else:
        while i<length:
            ship_all.append(ship)
            ship[1] += 1
            print ship
            i+=1

print ship_all

The ship_all.append(ship) just give the final result of the ship, but print ship works out correctly, how to solve this problem?

Because you're filling `ship_all` with *multiple references to the same list*. Move `ship = []` *inside* the `while` loops. — jonrsharpe, Feb 27 '15 at 12:07

score 2 · Accepted Answer · edited May 23 '17 at 11:56

2

You are not adding copies of ship to ship_all, you are adding the same list object over and over again, altering that object each iteration.

Create a copy of the list each time you append:

ship_all.append(ship[:])

or create the ship list from scratch inside the while loops.

edited May 23 '17 at 11:56

Community

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1

answered Feb 27 '15 at 12:11

Martijn Pieters

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Thanks! It worked out, but the why, what does `ship[:]` stands for? – Liye Feb 27 '15 at 12:16
@Liye: It creates a copy by slicing the list from start to end, copying across all elements. You can also use `list(ship)`. – Martijn Pieters Feb 27 '15 at 12:17

Why this append function in python didn't work out?

1 Answers1