Python - tkinter, canvas, entry

import tkinter as tk
#import Tkinter as tk # Python 2.7
import time

# 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 first_half():
    global canvas
    global label
    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()

    print_string(canvas_txt, prompt1)

    label = canvas.create_text(1, 72, text='', fill='white', anchor='nw', font=('Courier New', 11))

    entry = tk.Entry(root, width=32, relief='flat', font=('Courier New', 11),
                     bg='grey', fg='white', highlightthickness=0, borderwidth=0, insertborderwidth=0)
    entry.bind('<Return>', second_half) # run second_half when "Enter" is pressed
    entry.focus() # it puts cursor in entry

    canvas_entry = canvas.create_window(1, 100, anchor='nw', window=entry)

    #button = tk.Button(root, text='OK', command=second_half,
    #                    relief='flat', font=('Courier New', 11),
    #                    bg='grey', fg='white', highlightthickness=0, borderwidth=0)

    #canvas_button = canvas.create_window(1, 130, anchor='nw', window=button)

def second_half(event=None): # event=None to run it with Button(command=second_half)
    global password

    if event:
        password = event.widget.get()
        #event.widget.delete(0, 'end')
        #event.widget.insert(0, "a default value")
    else:
        password = entry.get()
        #entry.delete(0, 'end')
        #entry.insert(0, "a default value")

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

def main():
    global root

    root = tk.Tk()

    first_half()

    root.mainloop()

# --- start ---

# global variables
root = None
canvas = None
label = None
entry = None

password = ''

main()