如何在類的__init__函數中使用canvas參數？

Question

我製作了這個模塊叫做gamesmodule，我正在研究一個讓畫面中的球產生並反彈的程序。我用了一個

class Ball: 
    def __init__(self, canvas): 
     self.canvas = canvas 
     self.id = canvas.create_oval(10, 10, 25, 25, fill='red') 
     self.canvas.move(self.id, 25, 25) 
     self.x = 0 
     self.y = -1 
     self.canvas_height = self.canvas.winfo_height 
     self.canvas_width = self.canvas.winfo_width 

和初始化函數包含一個畫布參數，輸入一個名稱的畫布。我已經預先定義在畫布上，並創建在類球：

ball = Ball(canvas) 

但Python的殼輸出這個錯誤：

Traceback (most recent call last): 
File "<pyshell#59>", line 1, in <module> 
ball = Ball(canvas) 
NameError: name 'canvas' is not defined 

Answer 1

你必須定義canvas

canvas = tkinter.Canvas(...) 

ball = Ball(canvas) 

也許你在錯誤的地方定義canvas - 即。在某些功能中 - 執行Ball(canvas)

Answer 2

Python使用空白（縮進）來定義一個代碼塊的界限。您需要對齊__init__中的所有行：

class Ball: 
    def __init__(self, canvas): 
     self.canvas = canvas 
     # Lines below this one were unindented in your example: 
     self.id = self.canvas.create_oval(10, 10, 25, 25, fill='red') 
     self.canvas.move(self.id, 25, 25) 
     self.x = 0 
     self.y = -1 
     self.canvas_height = self.canvas.winfo_height 
     self.canvas_width = self.canvas.winfo_width 

但是，這並不會產生您的錯誤。看說明：

ball = Ball(canvas) 
NameError: name 'canvas' is not defined 

變量canvas不是在你打電話Ball(canvas)塊定義。在這種情況下，Python不知道什麼是canvas。