Python Tkinter报错：name 'buttonOnOff'未定义，按钮切换功能失效

问题描述 #

我尝试实现点击按钮切换开关状态的功能，但一直触发NameError。试过调整函数和按钮的创建顺序：把按钮放函数前面的话，函数又调用不了。

原代码 #

#import all of tkinter and also updated widgets
from tkinter import *
from tkinter import ttk
import main

#sets up main application window
root = Tk()
root.title("Winstreak Counter GUI")

#creating a content frame (frame widget, hold content of user interface)
#we do this because it'll allow us to control the background color and allow for more themed widgets
mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=10, row=10, sticky=(N, W, E, S))
root.columnconfigure(10, weight=1)
root.rowconfigure(10, weight=1)

f = open('/Users/ripkoye/Desktop/WinstreakProject/winstreak.txt', 'r')
#store the information into a variable
winstreak = StringVar(mainframe)
winstreak.set(f.read())
#creates a winstreaklabel
winstreaklabel = ttk.Label(mainframe,width=20, padding='5 5 5 5')
winstreaklabel.grid()
winstreaklabel['textvariable'] = winstreak
#assigns the text to the variable named winstreak
f.close()

is_on = True

def switch():
    global is_on

    if is_on:
        buttonOnOff.config(image = off)
        is_on = False
    else:
        buttonOnOff.config(image = on)
        is_on = True

on = PhotoImage(file='on.png')
off = PhotoImage(file='off.png')
buttonOnOff = ttk.Button(mainframe, image=on, width=0.5, padding='5 5 5 5', command=switch())
buttonOnOff.grid()


root.mainloop()

报错信息 #

Traceback (most recent call last):
  File "/Users/ripkoye/Desktop/WinstreakProject/gui.py", line 42, in <module>
    buttonOnOff = ttk.Button(mainframe, image=on, width=0.5, padding='5 5 5 5', command=switch())
  File "/Users/ripkoye/Desktop/WinstreakProject/gui.py", line 34, in switch
    buttonOnOff.config(image = off)
NameError: name 'buttonOnOff' is not defined

解决步骤 #

1. 修正按钮command参数：
   创建按钮时command=switch()会立即执行switch函数，而非点击按钮时触发。需改成command=switch（去掉括号），这样才会把函数对象绑定到按钮点击事件上。

2. 让函数能访问buttonOnOff变量：
   在switch函数里，除声明global is_on外，还要加上global buttonOnOff，确保函数能识别这个全局变量。

修改后的代码 #

from tkinter import *
from tkinter import ttk
import main

root = Tk()
root.title("Winstreak Counter GUI")

mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=10, row=10, sticky=(N, W, E, S))
root.columnconfigure(10, weight=1)
root.rowconfigure(10, weight=1)

f = open('/Users/ripkoye/Desktop/WinstreakProject/winstreak.txt', 'r')
winstreak = StringVar(mainframe)
winstreak.set(f.read())
winstreaklabel = ttk.Label(mainframe,width=20, padding='5 5 5 5')
winstreaklabel.grid()
winstreaklabel['textvariable'] = winstreak
f.close()

is_on = True

def switch():
    global is_on, buttonOnOff  # 新增buttonOnOff的全局声明

    if is_on:
        buttonOnOff.config(image = off)
        is_on = False
    else:
        buttonOnOff.config(image = on)
        is_on = True

on = PhotoImage(file='on.png')
off = PhotoImage(file='off.png')
# 去掉switch后面的括号
buttonOnOff = ttk.Button(mainframe, image=on, width=0.5, padding='5 5 5 5', command=switch)
buttonOnOff.grid()

root.mainloop()

内容的提问来源于stack exchange，提问作者Ryan Dang