Tkinter:Call another GUI window but widgets don't respond

Question

I want to call GUI class in B.py by button Measure in A.py. It works,but I find something strange in class B

1.entry doesn't show text "test" that defined by self.var_PSA .

2.Presses button PSA , but the content of entry doesn't change too.

If I run B.py, all funcions work well. How could I solve the problem?

A.py

import tkinter as tk 
import B

class main_win(tk.Tk):

    def __init__(self):
        tk.Tk.__init__(self)
        self._frame = None
        self.show_frame(Page3)

    def show_frame(self, frame_class):
        new_frame = frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame = new_frame
        self._frame.pack()


class Page3(tk.Frame):
 
    def __init__(self, master):

        tk.Frame.__init__(self,master, width=900, height=620, bg = "#d8d8d8")
        self.master = master

        self.button_meas = tk.Button(self, command = lambda:self.CallB())
        self.button_meas.place(relx=0.278, rely=0.5, height=31, width=94)
        self.button_meas.configure(activebackground="#ececec")
        self.button_meas.configure(activeforeground="#000000")
        self.button_meas.configure(background="#d9d9d9")
        self.button_meas.configure(disabledforeground="#a3a3a3")
        self.button_meas.configure(foreground="#000000")
        self.button_meas.configure(highlightbackground="#d9d9d9")
        self.button_meas.configure(highlightcolor="black")
        self.button_meas.configure(pady="0")
        self.button_meas.configure(text='''Measure''')

    def CallB(self):

        B.GUI()

app = main_win()

app.mainloop()

B.py

import tkinter as tk

class GUI(tk.Tk):


    def __init__(self):

        tk.Tk.__init__(self)

        tk.Tk.geometry(self, "1200x820")

        self.Frame1 = tk.Frame(self)
        self.Frame1.place(relx=0.057, rely=0.147, relheight=0.263
                , relwidth=0.895)
        self.Frame1.configure(relief='groove')
        self.Frame1.configure(borderwidth="2")
        self.Frame1.configure(relief="groove")
        self.Frame1.configure(background="#d9d9d9")
        self.Frame1.configure(highlightbackground="#d9d9d9")
        self.Frame1.configure(highlightcolor="black")

        self.var_PSA = tk.StringVar()
        self.var_PSA.set("test")
        self.PSA_Entry = tk.Entry(self.Frame1,textvariable = self.var_PSA)
        self.PSA_Entry.place(relx=0.676, rely=0.465, height=31
                , relwidth=0.296)
        self.PSA_Entry.configure(background="white")
        self.PSA_Entry.configure(disabledforeground="#a3a3a3")
        self.PSA_Entry.configure(font="TkFixedFont")
        self.PSA_Entry.configure(foreground="#000000")
        self.PSA_Entry.configure(highlightbackground="#d9d9d9")
        self.PSA_Entry.configure(highlightcolor="black")
        self.PSA_Entry.configure(insertbackground="black")
        self.PSA_Entry.configure(selectbackground="#c4c4c4")
        self.PSA_Entry.configure(selectforeground="black")

        self.button_PSA = tk.Button(self.Frame1, command = lambda: self.PSA_event())
        self.button_PSA.place(relx=0.594, rely=0.465, height=31, width=70)
        self.button_PSA.configure(activebackground="#ececec")
        self.button_PSA.configure(activeforeground="#000000")
        self.button_PSA.configure(background="#d9d9d9")
        self.button_PSA.configure(disabledforeground="#a3a3a3")
        self.button_PSA.configure(foreground="#000000")
        self.button_PSA.configure(highlightbackground="#d9d9d9")
        self.button_PSA.configure(highlightcolor="black")
        self.button_PSA.configure(pady="0")
        self.button_PSA.configure(text='''PSA''')
    
    def PSA_event(self):
        self.var_PSA.set("hello")


if __name__ == '__main__':
    app = GUI()
    app.mainloop()

Answer 1

You should change the self.var_PSA = tk.StringVar() to self.var_PSA = tk.StringVar(master=self) in B.py file. You have a Tk() instance in A.py and another one in B.py . You should tell to widgets that they are where connected in case of two Tk() .

I recommend to use Toplevel() instead of second Tk() instance in case of multi window application. Read more: http://effbot.org/tkinterbook/toplevel.htm