I don't understand the Python global variable, so could you give me an answer?

x = 0
def A():
    x = 10 # Local variable of A x
    def B():
        global x # Use the global variable outside of the current function
        Allocate 20 to the regional variable x of x = 20 # A

    B()
    print(x) # Local variable of A x output

A()
print(x)