Tkinter checkbox in a for loop

I'm trying to learn python and tkinter since few days. I would like to create dynamic checkboxes (values from a file) in a frame.

This is my code:

import tkinter as tk
lst="lst.txt"

class DisplayApp:
    def __init__(self):
        self.root = tk.Tk()
        self.root.title("My Menu")
        self.build_gui()

    def build_gui(self):
        frame_1 = tk.LabelFrame(self.root, text="Frame 1")
        frame_1.grid(row=2, columnspan=3, sticky='WE',
                            padx=5, pady=5, ipadx=5, ipady=5)
        linestring = open(lst, 'r').read()
        for checkBoxName in linestring:
                c = tk.Checkbutton(frame_1, text=checkBoxName,)
                c.pack(side=tk.LEFT)

    def clicked(self):
        pass

    def main(self):
        self.root.mainloop()

if __name__ == "__main__":
        app = DisplayApp()
        app.main()

This is the file lst.txt:

laptopname_1
laptopname_2
laptopname_3
laptopname_n
...

This the error:

[root@hdid_master ~]# ./test_menu_2.py
  File "./test_menu_2.py", line 35
    for checkBoxName in linestring:
                                  ^
TabError: inconsistent use of tabs and spaces in indentation

Many thanks for your help. Best Regards.