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，我开始学习 Tkinter，但我不知道如何在 GUI 上单击鼠标时返回事件。

点击canvas我想画点。我无法在 paint 函数中引用 canvas 变量。

这段代码是基于中的问题

每当我尝试这个例子时，我都会在绘画函数中收到一个错误，上面写着

NameError: name 'canvas' is not defined;

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。您在函数中创建的变量无法在该函数之外使用。

>>> 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 canvas 传递给绘画函数。要通过 bind 做到这一点，我们需要创建一个 "closure"，我们可以使用 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()

您真的应该为这些类型的程序考虑一个 class 结构，这样您就可以使变量可用于其他方法。

如何使用 Tkinter 在 Python Canvas 上引用绘制事件？

How reference back a paint event on Python Canvas using Tkinter?

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tk

canvas

tkinter