如何解决Tkinter嵌套函数中combo box下拉选择框取值不更新问题

你代码的核心问题是变量作用域匹配错误：

• dropDownBoxTempValue是tempConverter函数的内部局部变量，不属于全局作用域
• storedTemperature函数中使用global关键字声明的变量会去全局作用域查找，根本找不到你定义的dropDownBoxTempValue，所以下拉选择触发的赋值操作完全没有生效，convert函数读取到的永远是初始赋值的Celsius

你完全不需要额外维护dropDownBoxTempValue变量和storedTemperature回调函数，OptionMenu绑定的variable本身就是StringVar类型，直接在转换函数中读取它的当前值即可，修改后的完整代码如下：

from tkinter import *
from tkinter import ttk

def tempConverter():
    temperatureConverter = Tk()
    temperatureConverter.geometry('650x650')

    def convert():
        # 直接读取OptionMenu绑定的StringVar当前值即可
        dropDownBoxTempValue = variable.get()
        temperature = enterTemperature.get()
        if dropDownBoxTempValue == 'Celsius':
            f = (float(temperature) * 9 / 5) + 32
            temperatureLabel['text'] = f'{round(f, 2)}° Fahrenheit'
            k = (float(temperature)) + 273.15
            temperatureLabel2['text'] = f'{round(k, 2)}° Kelvin'

        elif dropDownBoxTempValue == 'Fahrenheit':
            c = (float(temperature) - 32) * 5 / 9
            temperatureLabel['text'] = f'{round(c, 2)}° Celsius'
            k = (float(temperature) - 32) * 5 / 9 + 273.15
            temperatureLabel2['text'] = f'{round(k, 2)}° Kelvin'

        elif dropDownBoxTempValue == 'Kelvin':
            c = (float(temperature) - 273.15)
            temperatureLabel['text'] = f'{round(c, 2)}° Celsius'
            f = (float(temperature) - 273.15) * 9 / 5 + 32
            temperatureLabel2['text'] = f'{round(f, 2)}° Fahrenheit'

    inputNumber = StringVar()
    variable = StringVar()

    enterTemperature = Entry(temperatureConverter, textvariable=inputNumber, font='Helvetica 14')
    enterTemperature.pack()
    temperatureLabel = Label(temperatureConverter, text="")
    temperatureLabel.pack()
    temperatureLabel2 = Label(temperatureConverter, text="")
    temperatureLabel2.pack()

    dropDownList = ["Celsius", "Fahrenheit", "Kelvin"]
    # 去掉多余的command回调
    temperaturedropDown = OptionMenu(temperatureConverter, variable, *dropDownList)
    temperaturedropDown.pack()
    variable.set(dropDownList[0])

    convertButton = Button(temperatureConverter, text="CONVERT", command=convert)
    convertButton.pack()

    temperatureConverter.mainloop()

tempConverter()

如果不想删除你原来写的storedTemperature回调和单独的存储变量，只需要把storedTemperature里的global关键字换成nonlocal即可，nonlocal的作用是声明当前变量引用外层嵌套函数的局部变量：

def storedTemperature(setTemp):
    nonlocal dropDownBoxTempValue
    dropDownBoxTempValue = setTemp

内容的提问来源于stack exchange，提问作者Ryeux