My python function won't return or update the value

Question

def getMove(win,playerX,playerY):

    #Define variables.
    movePos = 75
    moveNeg = -75
    running = 1

    #Run while loop constantly to update mouse's coordinates.
    while(running):

        mouseCoord = win.getMouse()
        mouseX = mouseCoord.getX()
        mouseY = mouseCoord.getY()
        print "Mouse X = ", mouseX
        print "Mouse Y = ", mouseY

        if mouseX >= playerX:
            playerX = movePos + playerX
            running = 0
        elif mouseX <= playerX:
            playerX = moveNeg + playerX           
            running = 0
        elif mouseY >= playerY:
            playerY = movePos + playerY            
            running = 0
        elif mouseY <= playerY:
            playerY = moveNeg + playerY            
            running = 0
    return playerX,playerY

def main():

    #Create game window.
    win = GraphWin("Python Game", 500, 500)
    drawBoard(win)

    #Define variables.
    playerX = 75
    playerY = 125
    keyX = 325
    keyY = 375
    running = 1


    #Create Key and Player objects, draw the key, but don't draw the player yet.
    key = Text(Point(keyX,keyY),"KEY")
    key.draw(win)

    while(running):
        print "player X = ", playerX
        print "Player Y = ", playerY
        drawBoard(win)
        getMove(win,playerX,playerY)
        player = Circle(Point(playerX,playerY),22)
        player.setFill('yellow')
        player.draw(win)
main()

I am using a graphics library to create a game. My player and key are drawn in the correct places. However, when calling the getMove function, my playerX and playerY do not update. I have added debug print statements to find their values while running the game and it is always 75 and 125. Help!

Answer 1

In python, integers are immutable - when you assign a new integer value to a variable, you are just making the variable point to a new integer, not changing what the old integer it pointed to's value was.

(An example of a mutable object in python is the list, which you can modify and all variables pointing to that list will notice the change - since the LIST has changed.)

Similarly, when you pass a variable into a method in python and then alter what the variable points to in that method, you do not alter what the variable points to outside of that method because it is a new variable.

To fix this, assigned the returned playerX,playerY to your variables outside the method:

playerX, playerY = getMove(win,playerX,playerY)