PySimpleGUI。 运行时错误：主线程不在主循环中

Question

As a Python learner I decided to try out PySimpleGUI, and wrote a script in which the relevant snippet is:

`    
import PySimpleGUI as sg 
....
window = sg.Window('Output Filename Creator').Layout(layout)
while True:
    event, values = window.Read()
    if event is None  or event == "Cancel":
        window.Close()
        sys.exit()
    else:
        outfile = values['file']
        window.Close()
        return outfile    `  

I use Windows 10, Python 3.7, Idle 3.7, and PySimpleGUI-3.24.0. After running the script that contains the snippet above (no execution errors) I go to the Idle shell and try to type in len('1'). At entering the open bracket the following error is generated:

Traceback (most recent call last): File "C:\\Users\\Paul\\AppData\\Local\\Programs\\Python\\Python37-32\\lib\\tkinter\\__init__.py", line 332, in __del__ if self._tk.getboolean(self._tk.call("info", "exists", self._name)): RuntimeError: main thread is not in main loop

I know that PySimpleGUI is based on tkinter, but that is as far as my knowledge goes. I don't know how threading works in Python or how PySimpleGUI is interfaced with Tk. Yet I would like to know where the error comes from and what I can do to avoid it.

Update: the code reduced to bare essentials still gives the same error when window is closed by clicking on cross in upper right corner:

def OutputFileName(default):
    import PySimpleGUI as sg      

    layout = [
              [sg.In(default, key='file', size=(70,1)),
               sg.SaveAs('Browse')],
              [sg.Save(), sg.Text(' '*35), sg.Cancel()]
            ]

    window = sg.Window(' ').Layout(layout)
    event, values = window.Read()
    if event is None or event == "Cancel":
        return
    elif event == 'Save':
        return values['file']
outf = OutputFileName('foo.txt')

Answer 1

This problem is due to IDLE being written using tkinter.

tkinter is very picky about resources and threads. There is a warning in the PySimpleGUI documentation about utilizing PySimpleGUI in a threaded environment because you can get into situations like this one where resources are freed in the incorrect thread or tkinter gets confused about who is running the mainloop.

Here is an older post that talks about problems running multiple mainloops when using IDLE.

https://groups.google.com/forum/#!topic/comp.lang.python/kr7lKj4qMl4