更新Tkinter画布上的文本字段

第一次，我在Python中使用tkinter创建了一个GUI。我使用过wxPython，但没有遇到同样的问题，但是我从未使用过画布。画布背后的思考过程是最终添加动画。我创建了一个网格布局，其中一个网格是画布。我正在尝试使用itemconfig更新分数字段，但没有运气，无法在画布上更新两个分数字段。我的代码如下。关于如何更新画布上的两个分数的任何建议？我希望在新分数写入屏幕之前删除旧分数。谢谢参观。请原谅全局变量，并在此过程中将其删除。

class Gui(Frame):
def __init__(self, root):
    self.root=root
    self.initUI()

def initUI(self):
    global score1, score2   # these are ints
    score1str = str(score1)
    score2str = str(score2)

    root.title("Table Tennis Scoreboard")

    self.canvas = Canvas(root, width=640, height=480, background='green')
    self.canvas.grid(row=0,column=1)

    self.canvas.create_text(185, 90, anchor=W, font=('Calibri', 20),
                                             text="Let's play Table Tennis!")

    self.canvas.create_rectangle(75, 120, 275, 320, outline="red", fill="red")
    self.canvas.create_rectangle(370, 120, 565, 320, outline="blue", fill="blue")

    self.canvas.create_text(125, 130, anchor=W, font=('Calibri', 20),
                                             text="Player 1")
    self.canvas.create_text(425, 130, anchor=W, font=('Calibri', 20),
                                             text="Player 2")

    self.score1_id = self.canvas.create_text(135, 250, anchor=W, font=('Arial', 100),
                                             text=score1str)
    self.score2_id = self.canvas.create_text(435, 250, anchor=W, font=('Arial', 100),
                                             text=score2str)

    frame = Frame(self.root, relief=RAISED, borderwidth=2, background='white')
    frame.grid(row=0,column=0)

    p1up = Button(frame,text="Player1 +1", height=2, width=10, background='red', command=self.add('one'))
    p1up.grid(row = 0,column = 0, padx=10, pady=(20,5))
    p1down = Button(frame,text="Player1 -1", height=2, width=10, background='red', command=self.subtract('one'))
    p1down.grid(row = 1,column = 0, padx=10, pady=(5,100))
    p2up = Button(frame,text="Player2 +1", height=2, width=10, background='blue', command=self.add('two'))
    p2up.grid(row = 3,column = 0, padx=10, pady=5)
    p2down = Button(frame,text="Player2 -1", height=2, width=10, background='blue', command=self.subtract('two'))
    p2down.grid(row = 4,column = 0, padx=10, pady=(5,100))
    reset = Button(frame,text="Reset", height=2, width=10, command=self.new_game())
    reset.grid(row = 6,column = 0, padx=10, pady=(5,20))

def new_game(self):
    global score1, score2   # these are ints
    score1 = 0
    score2 = 0
    self.update_score()

def add(self, player):
    global score1, score2  # these are ints
    self.player = player
    if self.player == 'one':
        #if score1 < 11:
        score1 += 1
    elif self.player == 'two':
        #if score2 < 11:
        score2 += 1
    self.update_score()
    return

def subtract(self, player):
    global score1, score2  # these are ints
    self.player = player
    if self.player == 'one':
        if score1 > 0:
            score1 = score1 - 1
    elif self.player == 'two':
        if score2 > 0:
            score2 -= 1
    self.update_score()
    return

def update_score(self):
    global score1, score2   # these are ints

    score1str = str(score1)
    score2str = str(score2)

    self.canvas.itemconfig(self.score1_id, text=score1str)
    self.canvas.itemconfig(self.score2_id, text=score2str)

    return

if __name__== '__main__':
    root=Tk()
    gui=Gui(root)
    root.mainloop()