How can a function change a global variables (Tikinter example)? - python-3.x

Question

I am new in Oython. I wrote a simple calculator I can't understand How the function that we called it to "evaluate" works.
I have known that we have the local variables in the functions and the functions cant change the variables outside the function except we define it as output,
But in this function, It changes an outside variable and defines a new label.
I expect that we input ans as an input and take an output. and another strange problem is that we didn't give e as input but this function takes variable e with e.get() comment. How does the function know that we have a variable e when we didn't give it to the function,
My second question is how does "configure" work?
This is my code:
from tkinter import*
root=Tk()
label1 = Label(root,text="Enter your expression:")
label1.pack()
def evaluate(event):
data=e.get()
ans.configure(text="Answer:"+ str(eval(data)))
e = Entry(root)
e.bind("<Return>",evaluate)
e.pack()
ans = Label(root)
ans.pack()
root.mainloop()

Answer 1

The reason your function works is because Python is someone accepting of this situation.
Python will first try to edit a local variable when it cannot find ans as a local variable it then assumes it must be in the global name space and then checks there. This does not always work but can if the condition are right.
Here is a code example that shows where the function will guess correctly that your variable is in global and one function that fails at it.
import tkinter as tk
root = tk.Tk()
def print_some_number():
some_number = some_number + 5
print(some_number)
def testing():
lbl.configure(text="After button press")
some_number = 5
lbl = tk.Label(root, text="Before button press")
lbl.pack()
tk.Button(root, text="Update lavel", command=testing).pack()
tk.Button(root, text="Print some number", command=print_some_number).pack()
root.mainloop()
The print statement function will fail because we try to assign a local variable before assignment. We know it has been assigned in global but the function wont check because it looks like we are trying to assign it locally. The way to fix this is by adding the global statement like below:
import tkinter as tk
root = tk.Tk()
def print_some_number():
global some_number # global added here for this kind of fuction
some_number = some_number + 5
print(some_number)
def testing():
lbl.configure(text="After button press")
some_number = 5
lbl = tk.Label(root, text="Before button press")
lbl.pack()
tk.Button(root, text="Update lavel", command=testing).pack()
tk.Button(root, text="Print some number", command=print_some_number).pack()
root.mainloop()