为什么 Tkinter 条目的 get function 没有返回任何东西？

Question

I'm trying to use two dialogs to get manual input, and then work with that data.
All source I've found claim I should use the get() function, but I wrote a simple mini program yet, and I can't make the second dialog work.
I hope someone can tell me what I'm doing wrong. Here's a file:

from tkinter import *
from tkinter.filedialog import askdirectory
from tkinter import messagebox

def getpath():
    def selectPath():
        path_ = askdirectory()
        path.set(path_)
    root = Tk()
    root.title('select path')
    path = StringVar()
    def close():
        if(path.get()==""):
            messagebox.showinfo("","nothing")
        else:
            root.withdraw()
            root.quit()
    Label(root,text="path：").grid(row=0,column=0)
    Entry(root,textvariable = path).grid(row=0,column=1)
    Button(root,text="select",command=selectPath).grid(row=0,column=2)
    Button(root,text="enter",command=close).grid(row=0,column=3)
    root.mainloop()
    return path.get()

def getname():
    def get_need_name():
        name = need_name.get()
        print('hereherehere'+name) #does not work
    root = Tk()
    root.title('select name')
    need_name = StringVar()
    Label(root,text="name：").grid(row=0,column=0)
    entry = Entry(root,bd=10,textvariable=need_name)
    entry.grid(row=0,column=1)
    Button(root,text="enter", font=16, bg="silver", relief='groove', command=get_need_name).grid(row=0,column=2)
    root.mainloop()
    return name.get()

def main():
    path = getpath()
    print("mypath："+path)
    print('******************')
    print('done!')
    name = getname()
    print("myname:"+name)

if __name__ == '__main__':
    main()

This give me two dialogs I can type in, but only the first dialog works.

Answer 1

The reason is that you are creating multiple instances of Tk, and you don't destroy the instances when you are done with them. This causes two problems. First is a memory leak. Each time you call one of these functions you create a new window and a new tcl interpreter.

The second problem is that the first root window becomes the default window when a root isn't specified. When you create a StringVar in the second function, because you didn't specify which root window it belongs to it will be assigned to the first root window. When you use it as the target of textvariable in a second instance of Tk , tkinter thinks the variable doesn't exist so it creates a new one for the second window. However, your reference is still to the one created in the first root window and is never updated by user input in the second window.

Confusing? Yes, which is why you typically shouldn't be creating more than one instance of Tk .

To make your code work with as few changes as possible and to remove the memory leak caused by not destroying the windows, you can change the last couple of lines in your method to look like the following. This destroys the root window when you are done with it, removing the memory leak and the side effect of having more than one root window.

root = Tk()
...
root.mainloop()
value = path.get()
root.destroy()
return value

The second dialog should look similar:

root = Tk()
...
root.mainloop()
value = name.get()
root.destroy()
return value

This retrieves the value after mainloop exits but before the underlying tcl interpreter is deleted, and then destroys the window and its tcl interpreter.

The next time you create an instance of Tk , that instance will become the new default root, and any new instance of StringVar will go to that root.

Another solution would be to specify the master for the StringVar instance, but that leaves the memory leak in place so it's only half of a solution.

Arguably a better solution is to create a single root window, and either reuse it or create instances of Toplevel rather than Tk . Effbot has some decent documentation on how to create a modal window with wait_window .

Answer 2

After some testing, and googling the root.quit() is the problem here is a working example for you to look at and mess with.

from tkinter import *
from tkinter.filedialog import askdirectory
from tkinter import messagebox

root = Tk()
path = StringVar()
def select_path():
    #uses the return value to set no need to create an additional variable
    path.set(askdirectory())
def close():
    if path.get() == "":
        messagebox.showinfo("","Please select path")
    else:
        get_name_frame.tkraise()
def get_name():
    print("hereherehere", name.get())


get_path_frame = Frame(root)
get_path_frame.grid(row=0, column=0, sticky="nsew")
Label(get_path_frame,text="path：").grid(row=0,column=0)
Entry(get_path_frame,textvariable = path).grid(row=0,column=1)
Button(get_path_frame,text="select",command=select_path).grid(row=0,column=2)
Button(get_path_frame,text="enter",command=close).grid(row=0,column=3)

get_name_frame = Frame(root)
get_name_frame.grid(row=0, column=0,sticky="nsew")
Label(get_name_frame, text="name: ").grid(row=0, column=0)
name = StringVar()
entry = Entry(get_name_frame, bd=10, textvariable = name)
entry.grid(row=0, column=1)
Button(get_name_frame,text="enter", font=16, bg="silver", relief='groove', command=get_name).grid(row=0,column=2)

get_path_frame.tkraise()
root.mainloop()