Python global keyword weird behaviour

def a():
    g=1
    def b():
        global g
        g=2
        def c():
            global g
            g=3
        c()
        print("c", g) #3
    b()
    print("b", g) #1 
a()
print("a", g) #3

The output is

c 3
b 1
a 3

I can’t understand why the value of g is 1 inside a() but 3 outside a()

If value of g is 3 in two places:

1. after calling c()
2. outside a().

Why is it 1 inside a()?

>Solution :

When you do g=1 in a() it creates a new local variable to that function. When you do global g in b() and c() it makes g refer to the global variable (which doesn’t yet exist when running b). So c, b and the global scope all refer to the same g (which is 3 after running c) while a refers to its local g which never changed and is still 1.

Changing global to nonlocal will make both prints of c and b to print 3, but now will make a NameError for the last print as there is no g in the global scope.

Alternatively, adding global g to the start of a, will have all three prints print 3.