I have a Snake class defined below along with a method Move which is supposed to calculate the new position of the head according to an action , append this element to the blocks array to the snake's block array and then popleft the first element of this list.
class Snake:
actions = [np.array([-1, 0]), np.array([1, 0]), np.array([0, -1]), np.array([0, 1])]
def __init__(self, start_position, start_direction_idx):
"""
:type start_position: 2x1 numpy Array
"""
self.startPosition = None
self.alive = True
self.direction = start_direction_idx
self.length = 1
self.currentPosition = start_position
self.blocks = deque([start_position])
self.blockAtEndofTail = None
def move(self, action):
if self.isMovingBackwards(action):
action = self.direction
else:
self.direction = action
print('Blocks before anything', self.blocks)
print('Current position before action',self.currentPosition)
self.currentPosition += self.actions[self.direction]
print('Current position after action', self.currentPosition)
print('Blocks before pop',self.blocks)
self.blockAtEndofTail = self.blocks.popleft()
print('Blocks after pop', self.blocks)
self.blocks.append(self.currentPosition)
print('Blocks after append', self.blocks)
print(self.blocks)
Here is some sample output below that I get when I run the program .
Blocks before anything deque([array([10, 10])])
Current position before action [10 10]
Current position after action [ 9 10]
Blocks before pop deque([array([ 9, 10])])
Blocks after pop deque([])
Blocks after append deque([array([ 9, 10])])
deque([array([ 9, 10])])
I got the above but I expected this below:
Blocks before anything deque([array([10, 10])])
Current position before action [10 10]
Current position after action [ 9 10]
Blocks before pop deque([array([ 10, 10])])
Blocks after pop deque([])
Blocks after append deque([array([ 9, 10])])
deque([array([ 9, 10])])
How is the value in the deque being changed by my method?
In python, objects are references. the value in the blocks
deque is actually a reference to the same numpy array that currentPosition
is referencing, since they both were initialized from start_position
. If you want them to be independent, try copying the value itself, and not the reference, using the built-in copy
function in Snake.__init__
:
self.blocks = deque([start_position.copy()])