用户单击后如何更改选项菜单的值？

Question

I'm playing around with option menu. I have a list of countries called options. The option menu is set to the first index of options. How can I update the value if a user clicks on a different country? Basically the function doesn't work even though I click on the 2nd (options[1]) country in the option menu.

def first_country():
    from_country = start_clicked.get()
    if from_country == options[1]:
        my_pic = Image.open("usa_flag.png")
        resized = my_pic.resize((200, 100), Image.ANTIALIAS)
        new_pic = ImageTk.PhotoImage(resized)
        flag_label = Label(root, image=new_pic)
        flag_label = Label(root, text="function works")
        flag_label.grid(row=3, column=0)

start_clicked = StringVar()
start_clicked.set(options[0])
dropdown = OptionMenu(root, start_clicked, *options, command=first_country())  

Answer 1

In Python, whenever you add () to the end of a function, it invokes it. (aside from declaration)

This being the case, it is usually so that when you are to pass a function to something, you need only pass a reference to the function

Effectively, just remove the ()

dropdown = OptionMenu(root, start_clicked, *options, command=first_country)    

Comment from acw1668 explained it well:

command=first_country() should be command=first_country instead. The former one will execute the function immediately and assign None to command .

Answer 2

command=first_country() should be command=first_country instead. The former one will execute the function immediately and assign None (result of the function) to command option.

Also the callback for command option of OptionMenu expects an argument which is the selected item:

def first_country(from_country):
    if from_country == options[1]:
        my_pic = Image.open("usa_flag.png")
        resized = my_pic.resize((200, 100), Image.ANTIALIAS)
        new_pic = ImageTk.PhotoImage(resized)
        # better create the label once and update its image here
        flag_label.config(image=new_pic)
        flag_label.photo = new_pic # save a reference of the image

...
dropdown = OptionMenu(root, start_clicked, *options, command=first_country)
...
# create the label for the flag image
flag_label = Label(root)
flag_label.grid(row=3, column=0)
...

Note that I have created the label for the flag image once and update its image inside the function instead. Also you need to save the reference of the image if it is created inside a function, otherwise it will be garbage collected.