当将简短的Python脚本作为函数隔离时，为什么运行方式不同？

Question

As part of a program that hits some points on a particular web interface, this component enables the user to click three spots & writes a config file that instructs the program precisely where to put the cursor each time it runs. These lines work as intended (Python 3.6) when they're in the main body of the program; the problem is, I want to isolate them as a function so I can enable the user to choose whether to reconfigure or not. (If user hasn't switched browsers nor resized, existing config file is fine.)

I hope a knowledgeable Pythonista can help me understand why simply indenting these lines beginning with window=Tk() four/eight spaces as appropriate and putting them in a function causes them to not run properly, instead hanging up in the middle of the first call to on_click. (Edit: The error message from terminal is below the script)

import threading, pynput, os, configparser
from pynput.mouse import Listener
from tkinter import *
configfile_name = "config.ini"
cfg = configparser.ConfigParser()

def write_config(x_coord, y_coord):
   global configfile_name
   c = open(configfile_name, "a", encoding="utf-8")
   c.write('x=' + x_coord + '\r')
   c.write('y=' + y_coord)
   c.write('\r\n')
   c.close()

def write_first_section(section_name):
   global configfile_name
   c = open(configfile_name, "w", encoding="utf-8")
   c.write('[' + section_name + '] \r')
   c.close()

def write_section(section_name):
   global configfile_name
   c = open(configfile_name, "a", encoding="utf-8")
   c.write('[' + section_name + '] \r')
   c.close()

def on_click(x, y, button, pressed) :
   global count, window, listener
   write_config(str(x), str(y))
   window.after(2000, window.destroy)    
   listener.stop()  

window = Tk()
window.title("Map your screen")
window.geometry('350x80+300+225')
window.lift()
write_first_section('tab')
lbl = Label(window, text="With the QP chat interface screen up,\nclick the 'New' tab at upper left above the blue bar")
lbl.grid(column=0, row=0)
with Listener(on_click=on_click) as listener:
   window.mainloop()
   listener.join()
listener.stop()
write_section('pickup')
window = Tk()
window.title("Step two!")
window.geometry('350x80+300+225')
window.lift()
lbl = Label(window, text="Now, click just below the blue bar")
lbl.grid(column=0, row=0)
with Listener(on_click=on_click) as listener:
   window.mainloop()
   listener.join()
listener.stop()
write_section('paste')
window = Tk()
window.title("Step three!")
window.geometry('350x80+300+225')
window.lift()
lbl = Label(window, text="Lastly, click in the text box")
lbl.grid(column=0, row=0)
with Listener(on_click=on_click) as listener:
   window.mainloop()

When placed in a function, script throws these errors:

Unhandled exception in listener callback
Traceback (most recent call last):
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/__init__.py", line 157, in inner
    return f(self, *args, **kwargs)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/xorg.py", line 458, in _handler
    self._handle(self._display_stop, event)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/mouse/_xorg.py", line 141, in _handle
    self.on_click(px, py, self._button(event.detail), True)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/__init__.py", line 78, in inner
    if f(*args) is False:
  File "does_it_all.py", line 37, in on_click
    window.after(2000, window.destroy)
NameError: name 'window' is not defined
Traceback (most recent call last):
  File "does_it_all.py", line 231, in <module>
    map()
  File "does_it_all.py", line 192, in map
    listener.join()
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/__init__.py", line 205, in join
    six.reraise(exc_type, exc_value, exc_traceback)
  File "/home/bruce/.local/lib/python3.6/site-packages/six.py", line 692, in reraise
    raise value.with_traceback(tb)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/__init__.py", line 157, in inner
    return f(self, *args, **kwargs)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/xorg.py", line 458, in _handler
    self._handle(self._display_stop, event)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/mouse/_xorg.py", line 141, in _handle
    self.on_click(px, py, self._button(event.detail), True)
  File "/home/bruce/.local/lib/python3.6/site-packages/pynput/_util/__init__.py", line 78, in inner
    if f(*args) is False:
  File "does_it_all.py", line 37, in on_click
    window.after(2000, window.destroy)    
NameError: name 'window' is not defined

When I try putting everything after the on_click(....) function definition into a function called map(), and then try to run map(), it hangs up after writing [tab] into the config.ini file, leaving the first window stuck on the screen. But when I run it as written above, it properly progresses through all three windows and writes all three parts of the config file. I'm sure there are terrible blunders in the script, but nevertheless: Why is it those 32 lines would run fine when out in the open, but break when they comprise a function? Thanks for any help and constructive scolding!

Answer 1

When you moved those lines into a function, the window (and listener ) variables you defined stopped being global variables, and became local to that function. Your on_click functions was relying on window (and listener ) to be defined globally, and since they no longer existed as globals at all, your code broke.

The minimal fix would be to add:

global window, listener

to the top of your new function, so it treats window and listener as global variables, rather than the default behavior of treating them as locals.

A more thorough solution would be to write a class where window and listener (and possibly other things) were instance attributes, accessed via self , with your new function and on_click being methods of that class. It's not critical in this case, but any time you find yourself reliant on global for mutable global state, it's usually a bad design, and you really wanted a class in the first place.