a.py

x = 666
def print_x():
    print(x) # OK, you can see x

def copy_x():
    y = x # ok, you can copy x

def inc_x():
    x = x + 1 # local variable 'x' referenced before assignment?!

print("print_x")
print_x()

print("copy_x")
copy_x()

print("inc_x")
inc_x()