Python - tkinter, canvas & threading

import time
import tkinter as tk
import threading

#
# PEP 8 - Style Guide for Python Code: https://www.python.org/dev/peps/pep-0008/
#
# PEP8: two empty lines between functions
# PEP8: lower_case_names for functions
# PEP8: more readable variables
# PEP8: all "global" in separated lines at the beginning of function, and empty line at the end
# PEP8: space after comma
# PEP8: spaces around = (but not in function's arguments)


def print_string(item, string):
    if item:
        for char in string:
            idx = canvas.index(item, tk.END)
            canvas.insert(item, idx, char)
            canvas.update()
            time.sleep(0.01)


def key_event(label, event):
    global text
    global do_it
    global checkForPassword
    global password

    string = 'abcdefghijklmnopqrstuvwxyz'
    text = canvas.itemcget(label, "text")

    if do_it is False: # PEP8: for `False`/`True` use `is` instead of `==`/ `!=`
        thread_event.set()
        print('something')
        return True

    if event.keysym == "BackSpace":
        canvas.itemconfig(label1, text=text[:-1])
    elif event.keysym == 'space':
        canvas.itemconfig(label1, text=text+' ' )
    elif event.keysym == "Return":
        thread_event.set()
        password = text
        print('password is:', password)
        do_it = False
        check_for_password=True
        print('checkForPassword =', check_for_password)
        print(bool(event))
    elif event.char in string:
        # Character is an ASCII letter
        canvas.itemconfig(label1, text=text+event.char)


def first_half():
    global canvas
    global label1
    global label2
    global entry

    # all variables
    prompt1 = """--------------------------------
WELCOME TO GLOBAL DEFENSE SYSTEM
--------------------------------
PLEASE ENTER THE PASSWORD TO ACTIVATE EMERGENCY SHIELD PROCEDURE:"""


    canvas = tk.Canvas(root, width=700, height=400, bg='black')
    canvas.pack()

    canvas_txt = canvas.create_text(1, 0, fill= 'white', anchor='nw', font=('Courier New', 11))
    canvas.update()

    label1 = canvas.create_text(1, 68, text=text, fill='white', anchor='nw', font=('Courier New', 12))
    label2 = canvas.create_text(1, 72, text='', fill='white', anchor='nw', font=('Courier New', 11))

    print_string(canvas_txt, prompt1)

    root.bind('<Key>', lambda e: key_event(label1, e))

    root.update()


def second_half():

    thread_event.wait()

    if password == 'united states':
        canvas.itemconfig(label2, text='CORRECT PASSWORD')
    else:
        canvas.itemconfig(label2, text='INCORRECT PASSWORD: ' + password)

    root.update()


def main_function_with_extra_threading():
    global root

    root = tk.Tk()

    the_first_half_threading = threading.Thread(target=first_half)
    the_first_half_threading.start()

    user_input_threading = threading.Thread(target=second_half)
    user_input_threading.start()

    root.mainloop()

# --- start ---

# global variables
root = None
canvas = None
label1 = None
label2 = None

text = ""
do_it = True
password = ''
check_for_password=False

thread_event = threading.Event()

main_function_with_extra_threading()