class GUI:
    def __init__(self):
        self.root = Tk()
        ...
        self.FolderBut = Button(self.root, width=31, height=23, text='...')
        ...
        self.FolderBut.bind('<Button-1>', self.openfile)
        self.ShiftBut1.bind('<Button-1>', self.setleft)
        self.ShiftBut2.bind('<Button-1>', self.setright)

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

    def openfile(ev):
        op = askopenfilename()
        self.FolderText.insert(END, op)

def openfile(self, ev):

        self.f = open(self.op)
        for i in self.f.readline():
            self.res += chr(ord(i) + self.n)

self.res = ""
for i in self.f.readline():
            self.res += chr(ord(i) + self.n)

# или
tmp = [chr(ord(i) + self.n) for  i in self.f ]
self.res = ''.join(tmp)

self.n = Entry(self.root, width=20)

self.entry = Entry(self.root, width=20)
self.entry.insert(0,'0') 
......
self.n  = int(self.entry.get())