如何测试带有 Tkinter 主循环的 Python 代码？

Question

How can I test the following code with nosetests?

# GUI
try:
    # for Python2
    import Tkinter as tk
except ImportError:
    # for Python3
    import tkinter as tk


def main():
    root = tk.Tk()
    canvas = tk.Canvas(root, width=250, height=250)
    canvas.pack()
    root.bind('<B1-Motion>', lambda x: True)
    root.bind('<Button-1>', lambda x: True)
    root.mainloop()

When I call main() the mainloop gets executed. How can I prevent that?

Answer 1

Instead of calling root.mainloop() the idea is to call root.update(). This will run the tk loop only once (any pending event will hence be dispatched and handled by your application).

Testing the application behavior becomes then a succession of steps like:

root.event_generate('<B1-Motion>') # will generate a "fake" event <B1-Motion>
root.update()
# test that '<B1-Motion>' callback was correctly executed (by eg checking an expected state of you app
root.event_generate('<Button-1>')
root.update()
# test ....

Note that you can also directly invoke button (as if they were clicked) by calling button.invoke()

Answer 2

You can redesign this into a Class:

import tkinter as tk

class GUI(tk.Tk):
    def __init__(self):
        super().__init__()
        canvas = tk.Canvas(self, width=250, height=250)
        canvas.pack()
        self.bind('<B1-Motion>', lambda x: True)
        self.bind('<Button-1>', lambda x: True)
    


if __name__ == "__main__":
    
    root = GUI()
    root.mainloop()

Your test should workaround the object created from GUI class easily, and to guarantee that the mainloop wouldn't run once you import this file elsewhere, I just added the if __name__ == "__main__": block at the end.

Answer 3

To stop the mainloop() running, you could comment out the line, this would run main() without Tkinter creating the canvas.

try:
    # for Python2
     import Tkinter as tk
except ImportError:
     # for Python3
    import tkinter as tk


def main():
    root = tk.Tk()
    canvas = tk.Canvas(root, width=250, height=250)
    canvas.pack()
    root.bind('<B1-Motion>', lambda x: True)
    root.bind('<Button-1>', lambda x: True)
    #root.mainloop() """#stops the line being run"""
main()

or have the root.mainloop() outside the main() function and comment out mainloop()

try:
    # for Python2
     import Tkinter as tk
except ImportError:
     # for Python3
    import tkinter as tk
def main():
    root = tk.Tk()
    canvas = tk.Canvas(root, width=250, height=250)
    canvas.pack()
    root.bind('<B1-Motion>', lambda x: True)
    root.bind('<Button-1>', lambda x: True)
main()
#root.mainloop()