Pass data frame through Tkinter classes

I am using Python 2.7 and Tkinter. I am almost new to Object Oriented programs. I have a long program with many Tkinter windows and at some point I ask the user to load an Excel file that I read with Pandas, and want to permanently use and update that value (of a data variable). The way that I am doing it now is with global variables but I know that it is dangerous, inefficient and not elegant at all.

Even though I could do controller.show_frame(framename) given the way my gui class is built, I ended up building some of the frames myself just so the data variable would update itself.

I read and tried some answers in Stack Overflow but may have implemented them wrong:

• Tried creating a dictionary inside the gui class, something like self.app_data = {data=[],filename=""} and updating it from other windows, the thing here is that I think that the class gui is instanced only once and it kind of creates all of the other window classes so this did not work. Maybe I did something wrong there. (not shown on the code).
• Tried to do something as what was suggested here but I could just not make it work.

Main frame is some sort of intermediate step that I need for other purposes; the following code is a simplification of my program.

I know this is an awful nightmare code! Thank you :)

import Tkinter as tk
import pandas as pd 
import tkFileDialog
import tkMessageBox
global data, strat_columns, filename
data = pd.DataFrame([])
strat_columns = []
filename = ""

class gui(tk.Tk):

    data = pd.DataFrame([])
    filename = ""
    def __init__(self):
        tk.Tk.__init__(self)
        container = tk.Frame(self)
        container.pack(side="top",fill="both",expand=True)
        self.frames = {}

        for F in (main_frame, first_frame):
            frame = F(container, self)
            self.frames[F] = frame
            frame.grid(row=0, column=0, sticky="nsew")
        self.show_frame(main_frame)

    def show_frame(self,sel_frame):
        frame = self.frames[sel_frame]
        frame.tkraise()

    def get_page(self, page_class):
        return self.frames[page_class]


class main_frame(tk.Frame):

    def __init__(self,parent,controller):

        tk.Frame.__init__(self,parent)
        self.parent = parent 
        self.controller = controller
        button_new = tk.Button(self,
                           text="New window",
                           command=lambda: self.button_new_callback())
        button_new.pack()

    def button_new_callback(self,*args,**kwargs):
        self.controller.show_frame(first_frame)


class first_frame(tk.Frame):

    def __init__(self,parent,controller):
        tk.Frame.__init__(self,parent)
        self.controller = controller
        self.parent = parent
        self.show_frame = controller.show_frame
        statusText.set("Press Browse button and browse for file, then press the Go button")
        label = tk.Label(self, text="Please load a file: ")
        label.pack()
        entry = tk.Entry(self, width=50)
        entry.pack()
        button_go = tk.Button(self,
                           text="Go",
                           command=lambda: self.button_go_callback(entry,statusText,message))
        button_browse = tk.Button(self,
                               text="Browse",
                               command=lambda: self.button_browse_callback(entry))
        button_go.pack()
        button_browse.pack()
        message = tk.Label(self, textvariable=statusText)
        message.pack()

    def button_browse_callback(self,entry):
        global filename
        filename = tkFileDialog.askopenfilename()
        entry.delete(0, tk.END)
        entry.insert(0, filename)

    def button_go_callback(self,entry,statusText,message):
        global data
        input_file = entry.get()
        data = pd.read_excel(filename)
        sf = second_frame(self.parent, self)
        sf.grid(row=0, column=0, sticky="nsew")
        sf.tkraise()

class second_frame(tk.Frame):
     pass

if __name__ == "__main__":

    my_gui = gui()
    my_gui.mainloop()
    my_gui.title("TEST")