Trying to understand global variables

[357mag]

Just learning about using a global variable. I have this going:

x = 4

def show_value():
    print('Value of x: '+ str(x))
   # x += 3
   # print('Value of x: '+ str(x))
    
show_value()

I had to comment out those two lines in order to get the program to run. I thought that in my show_value function I could modify the value of the variable, but apparently I cannot because the compiler flagged it.

I don't quite get it. If I declare a global variable that can be seen and accessed in the show_value function, then why can't I modify it in the function as well?