[Python] Python Tkinter Calculator wont evaluate text from entry widget

I am trying to make a simple calculator app using tkinter, but everytime I run the code below i get an error message saying

Traceback (most recent call last):
File "C:\Python33\Lib\site-packages\pythonwin\pywin\framework\scriptutils.py", line 326, in RunScript
exec(codeObject, __main__.__dict__)
File "C:\Users\csp\Python\Calculator App.py", line 17, in <module>
solved = eval(expression)
File "<string>", line 0

^
SyntaxError: unexpected EOF while parsing


CODE:

from tkinter import *
tk = Tk()
tk.title('Calculator')
inp = Entry(tk,text="Enter Expression Here",width=20)
inp.pack()
exit = False
def exitbtn():
global exit
exit = True
return exit
btn = Button(tk,text="Quit?",command=exitbtn)
btn.pack
canvas = Canvas(tk,width=200,height=200)
canvas.pack()
while not exit:
expression = inp.get()
solved = eval(expression)
canvas.create_text(100,100,text=expression,font=('Times', 15))
canvas.create_text(100,150,text=solved,font=('Times', 15))
if exit == True:
break
tk.destroy()


i am really new to Python and dont understand why the "solved = eval(expression)" line wont work. please help

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