import tkinter as tk
root = tk.Tk()
text = tk.Text(root)
text.insert('1.0', 'line 1\nline 2\n')
text.insert('insert', 'line 3\n', 'sel')
text.pack()
text.selection_own()  # Needed, else next two lines give None, TclError.
print('owner:', text.selection_own_get())  # .!text
print('text: ', text.selection_get())  # 'line 3'
text.selection_clear()
print('text: ', text.selection_get())  # TclError, no selection