如何使用 Tkinter 在 Python Canvas 上引用绘制事件

Question

我知道这可能是有史以来最简单的问题。但是，我正在开始学习 Tkinter，我不知道如何在 GUI 上单击鼠标时引用事件。

我想在单击画布时画一些点。我无法在油漆函数中引用画布变量。

这段代码基于 SO 中的问题

每当我尝试该示例时，我都会在 paint 函数中出现错误，该错误显示

名称

错误：未定义名称"画布";

def main():
    master = Tk()
    # Right side of the screen / img holder
    right_frame = Frame(master, width=500, height=500, cursor="dot")
    right_frame.pack(side=LEFT)
    # Retrieve image
    image = Image.open("./image/demo.JPG")
    image = image.resize((800, 700), Image.ANTIALIAS)
    photo = ImageTk.PhotoImage(image)
    # Create canvas
    canvas = Canvas(right_frame, width=800, height=700)
    canvas.create_image(0, 0, image=photo, anchor="nw")
    canvas.pack()
    canvas.bind("<B1-Motion>", paint)
    message = Label(right_frame, text="Press the mouse to draw")
    message.pack(side=BOTTOM)

    mainloop()

def paint(event):
    python_green = "#476042"
    x1, y1 = (event.x - 1), (event.y - 1)
    x2, y2 = (event.x + 1), (event.y + 1)
    canvas.create_oval(x1, y1, x2, y2, fill=python_green)

if __name__ == "__main__":
    main()

Answer 1

这是python范围规则的一个简单错误。在函数中创建的变量不能在该函数外部使用。

>>> def func():
...   a = 42
... 
>>> func()
>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

您必须将画布传递给绘画函数。要通过bind做到这一点，我们需要做一个"闭包"，我们可以用functools.partial函数来完成。

from tkinter import *
from functools import partial
def main():
    master = Tk()
    # Right side of the screen / img holder
    right_frame = Frame(master, width=500, height=500, cursor="dot")
    right_frame.pack(side=LEFT)
    # Create canvas
    canvas = Canvas(right_frame, width=800, height=700)
    canvas.create_image(0, 0, image=photo, anchor="nw")
    canvas.pack()
    canvas.bind("<B1-Motion>", partial(paint, canvas))
    message = Label(right_frame, text="Press the mouse to draw")
    message.pack(side=BOTTOM)
    mainloop()
def paint(canvas, event):
    python_green = "#476042"
    x1, y1 = (event.x - 1), (event.y - 1)
    x2, y2 = (event.x + 1), (event.y + 1)
    canvas.create_oval(x1, y1, x2, y2, fill=python_green)
if __name__ == "__main__":
    main()

您确实应该考虑这些类型的程序的类结构，以便可以使变量可用于其他方法。