Variable globale non initialisée

Salut! Voici un code dont vous comprendrez la tache je crois...
quand je l'execute j'ai une erreur que voici

Code:

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Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\PC\AppData\Local\Programs\Python\Python39\lib\tkinter\__init__.py", line 1892, in __call__
    return self.func(*args)
  File "C:/Users/PC/PycharmProjects/pythonProject/test1/UnCaneva.py", line 10, in draw_lines
    canvas1.create_line(x1, y1, x2, y2, width=2, fill=col)
NameError: name 'x1' is not defined

Code:

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# coding:utf-8
from tkinter import *
from random import randrange
 
 
 
def draw_lines():
    """Drawing of a line in canvas1"""
    global x1, x2, y1, y2, col
    canvas1.create_line(x1, y1, x2, y2, width=2, fill=col)
    # modification of coordinate for next line
    y1, y2 = y1 + 10, y2 + 10
 
 
def change_col():
    """random modification of line color"""
    global col
    color = ["blue", "white", "green", "red", "orange", "purple", "cyan", "maroon", "yellow"]
    i = randrange(len(color))
    col = color[i]
 
 
# widget principal
draw_frame = Tk()
 
# widgets du widget principal
canvas1 = Canvas(draw_frame, bg="black", height=500, width=500)
canvas1.pack(side=LEFT)
button_quit = Button(draw_frame, text="Quit", command=draw_frame.quit)
button_quit.pack(side=BOTTOM)
button_draw_lines = Button(draw_frame, text="New line", command=draw_lines)
button_draw_lines.pack()
button_change_color = Button(draw_frame, text="Change color", command=change_col)
button_change_color.pack()
draw_frame.mainloop()
#draw_frame.destroy()
 
# __________ Main program _____________
 
# this variable will be use global
x1, y1, x2, y2 = 0, 0, 0, 0
col = "dark green"