Why is Python Skipping Over Tkinter Code? - python-3.x

Question

I'm in Python 3.x using Tkinter to make a button that changes a boolean variable's value from true to false then a if statement to check if that value is false. Here is my code for that:
import tkinter
import time
x = True
top = tkinter.Tk()
def helloCallBack():
x = False
print (x)
B = tkinter.Button(top, text ="Hello", command = helloCallBack)
B.pack()
if x == False:
print ('Hello')
top.mainloop()
This unfortunately didn't work, so I replaced the if statement with a time.sleep(10) and then print (x) so I would have enough time to press the button like so:
import tkinter
import time
x = True
top = tkinter.Tk()
def helloCallBack():
x = False
print (x)
B = tkinter.Button(top, text ="Hello", command = helloCallBack)
B.pack()
time.sleep(10)
print (x)
top.mainloop()
The issue with this is it skips over all of the Tkinter code and goes to the time.sleep(10), prints the value of x then brings everything from Tkinter up after. Why is it doing this?

Answer 1

The reason you see that the code does sleep() and then prints True before the tkinter windows opens is due to how the mainloop() works in tkinter.
sleep() is useful in python however due to tkinter's single threaded nature all sleep() can due is block the mainloop until it has completed sleeping. Tkinter has its own method to work around this issue called after() and you don't need it here but it is very useful for timing things in tkinter.
Tkinter does its updates in the mainloop and you have a sleep() command that occurs before your mainloop as well as a print command before the mainloop. Both of those things must finish execution before the mainloop is reached for the first time thus not allowing tkinter to start until they are complete.
What you want to do is to place this print statement into your function. As well as a few other quality of life clean ups.
See this example:
import tkinter as tk
top = tk.Tk()
x = True
def hello_call_back():
global x
x = False
if not x:
print('Hello')
tk.Button(top, text="Hello", command=hello_call_back).pack()
top.mainloop()
However in this case I think the simplest form of the code should look like this:
import tkinter as tk
top = tk.Tk()
def hello_call_back():
print('Hello')
tk.Button(top, text="Hello", command=hello_call_back).pack()
top.mainloop()
As your current use of x is redundant to the goal.