# Tk_list_search.py

# Tk_list_search.py

import Tkinter as Tk

master = Tk.Tk()

listbox = Tk.Listbox(master)
listbox.pack()

# Insert few elements in listbox:
for item in ["zero", "one", "two", "three", "four", "five", "six", "seven"]:
    listbox.insert(Tk.END, item)
# Return index of desired element to seek for
def check_index(element):
   try:
       index = listbox.get(0, "end").index(element)
       return index
   except ValueError:
       print'Item can not be found in the list!'
       index = -1 # Or whatever value you want to assign to it by default
       return index

print check_index('three') # Will print 3

print check_index(100) # This will print:
                       # Item can not be found in the list!
                       # -1

Tk.mainloop()