Cloze splitter

import random
import tkinter
import tkinter.filedialog
import io

def gen_notes(book):
    start = 0
    while start < len(book):
        passage = book[start:start + 25]
        out = ""
        start += 10
        cloze_done = False
        for line,s in enumerate(passage):
            if line >= 15 and not cloze_done:
                words = s.split()
                candidates = [i for (i,w) in enumerate(words) if len(w) > 4]
                try:
                    idx = random.choice(candidates)
                    s = ' '.join(words[:idx]) + ' {{c1::' + words[idx] + '}} ' + ' '.join(words[idx+1:])
                    cloze_done = True
                except IndexError:
                    pass
                pass
            out += (s + '\n')
        yield out

tk = tkinter.Tk()

bookname = tkinter.filedialog.askopenfilename()
notes = gen_notes(io.open(bookname, encoding="utf8").read().splitlines())

def copy_note():
    global tk, notes
    tk.clipboard_clear()
    tk.clipboard_append(next(notes))

tkinter.Button(tk, text = "copy next", command = copy_note)\
       .pack(fill=tkinter.BOTH, expand=True)
tk.mainloop()