Function closure inconsistency

[SeanMon]

I was playing around with Python functions returning functions and the
scope rules for variables, and encountered this weird behavior that I
can't figure out.

Why does f1() leave x unbound, but f2() does not?

def f1():
    x = 0
    def g():
        x += 1
        return x
    return g1

def f2():
    x = []
    def g():
        x.append(0)
        return x
    return g

a = f1()
b = f2()

a() #UnboundLocalError: local variable 'x' referenced before
assignment
b() #No error, [0] returned
b() #No error, [0, 0] returned