TKInter按钮的全局变量

Question

I use a multiple frame window from Steven M. Vascellaro example Switch between two frames in tkinter . And I don't understand is it possible to use global variable, because each frame is destroyed every time it is changed. 1st (of 2 buttons) button at StartPage is deactivated, if I click on 2nd button and go to PageOne and do something there and return to StartPage 1st button must be activated.

If I declare global in StartPage class

_tkinter.TclError: invalid command name ".!startpage.!button2"

If I declare it in PageOne class

NameError: name 'install_svc' is not defined

Not good idea, but I think about 3rd frame where I define buttons again.

EDIT1:

class MyGUI(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)

        self._frame = None
        self.switch_frame(StartPage)
        self.geometry("400x300")

    def switch_frame(self, frame_class):
        """Destroys current frame and replaces it with a new one."""
        new_frame = frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame = new_frame
        self._frame.pack()

    def quit_app(self):
        self._frame.destroy()
        self.destroy()

class StartPage(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        global install_svc

class PageOne(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
    def entry_check(self,parent):
        global install_svc

Answer 1

Declare and initialize install_svc at the root level (global scope). The statement global install_svc wont auto-magically create, by itself, the variable in the global scope: it will just do a lookup for a global variable with such name but, if you try to use that variable without first assigning something to it, a NameError will be thrown.

Code

install_svc = None # Or some other initial value

class MyGUI(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)

        self._frame = None
        self.switch_frame(StartPage)
        self.geometry("400x300")

    def switch_frame(self, frame_class):
        """Destroys current frame and replaces it with a new one."""
        new_frame = frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame = new_frame
        self._frame.pack()

    def quit_app(self):
        self._frame.destroy()
        self.destroy()

class StartPage(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        global install_svc

        # You can use install_svc safely
        print(install_svc)

class PageOne(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
    def entry_check(self,parent):
        global install_svc

        # You can use install_svc safely
        print(install_svc)

You could also check if the variable exists, doing a try..except , but that will be really ugly (at least for me) and you will need to do such check everywhere:

class MyGUI(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)

        self._frame = None
        self.switch_frame(StartPage)
        self.geometry("400x300")

    def switch_frame(self, frame_class):
        """Destroys current frame and replaces it with a new one."""
        new_frame = frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame = new_frame
        self._frame.pack()

    def quit_app(self):
        self._frame.destroy()
        self.destroy()

class StartPage(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        global install_svc

        try:
            install_svc
        except NameError:
            install_svc = None # Or some initial value

class PageOne(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
    def entry_check(self,parent):
        global install_svc

        try:
            install_svc
        except NameError:
            install_svc = None # Or some initial value