def test_shift_left():
grid = Grid()
assert(grid.shift_left([2, 0, 0, 0]) == [2, 0, 0, 0])
assert(grid.shift_left([0, 0, 0, 2]) == [2, 0, 0, 0])
assert(grid.shift_left([2, 2, 0, 0]) == [4, 0, 0, 0])
assert(grid.shift_left([2, 0, 2, 0]) == [4, 0, 0, 0])
assert(grid.shift_left([2, 0, 0, 2]) == [4, 0, 0, 0])
assert(grid.shift_left([0, 0, 2, 2]) == [4, 0, 0, 0])
assert(grid.shift_left([2, 2, 2, 0]) == [4, 2, 0, 0])
assert(grid.shift_left([2, 4, 0, 0]) == [2, 4, 0, 0])
assert(grid.shift_left([0, 0, 2, 4]) == [2, 4, 0, 0])
assert(grid.shift_left([2, 2, 2, 2]) == [4, 4, 0, 0])
assert(grid.shift_left([4, 4, 4, 4]) == [8, 8, 0, 0])
assert(grid.shift_left([2, 0, 4, 4]) == [2, 8, 0, 0])
assert(grid.shift_left([2, 2, 0, 2]) == [4, 2, 0, 0])
assert(grid.shift_left([2, 2, 4, 4]) == [4, 8, 0, 0])
assert(grid.shift_left([8, 8, 4, 2]) == [16, 4, 2, 0])
print "Left tests passed."
